electromagnetism

ELECTROMAGNETISM1. Define the following terms & phrases (charge, static electricity, charging by friction/contact/induction,

conductor, insulator, uniform/non-uniform charge distribution, earthing and electrical discharge)

2. Describe the behaviour of like and unlike charges.

3. Name and give symbols for the following: DC power supply, cell, battery, switch, lamp, resistor, variable resistor, wires joined, wires crossing, ammeter, voltmeter & fuse

4. State the symbol and metric unit for: charge, current, voltage, resistance & power

5. Define the following terms and phrases: Ohm’s law, DC electricity, series, parallel, current rules and voltage rules.

6. Describe the differences and similarities in the way ammeters and voltmeters are used.

7. Draw and interpret DC circuit diagrams

8. Solve problems using V = IR, P = VI, P = E and Rtotal = R1 + R2 t

9. Describe the magnetic field patterns around permanent magnets, the earth, currents and coils.

10. State the symbol and unit for magnetic field.

11. Use the right hand grip rule to determine relative field (B) and current (I) directions.

12. Describe how the magnetic field due to a current in a straight wire varies with the size of current and the distance from the wire.

13. Solve problems using B = µ0I

2πd

Thursday, 4 November 2010

LANGUAGE


THE LANGUAGE OF ELECTRICITY

Term Definition Word list

Charge an electrical quantity based on an excess or deficiency of electrons

static electricity a form of electricity where charge does not flow continuously


NOTES


ELECTROSTATICS


The process

1. Charge transfer - When two objects are in contact with each other, one object can transfer electrons to the other object. Protons are not transferred because they are “locked” into the nucleus.

2. Charge imbalance - When the two objects are moved away from each other the process of charge transfer is unable to be reversed.

• Positively charged objects have had electrons removed• Negatively charged objects have gained electrons

Oppositely charged objects attract each other. Those with like charges repel.

Proton (+ve charge)

Neutron

Electron (-ve charge)

Empty space

Based on atomic structure

CHARGING OBJECTS


+

+

-

-

+

+

+

+

-

-

-

-

Oppositely charged objects attract each other

Objects with the same charge repel each other

Demo: The Van der Graaf Generator

CHARGE INTERACTION


Static electricity around us

Read p52 and 53 (Y10 Pathfinder) and then offer some examples to the class discussion:

The History of Electricity Generation

EXAMPLES


keynote:/Users/enricoluca/a.Work%20files/a.Teaching/10%20Science/3.Electricity/Presentations/History%20of%20Power%20generation.key










LIGHTNING Warm air currents ascend. Ice crystals descend removing electrons from cloud particles in this zone.

A zone of positively charged cloud particles results

At ground level the air becomes ionized (by losing electrons) and these positively charged particles are attracted to the negatively charged base of the cloud to give rise to a lightning bolt (an upstrike)


CLOUD TO GROUND


CLOUD TO CLOUD


BLUE JETS AND SPRITES


CHARGING OBJECTS

Methods of chargingInduction - the object being charged is not in contact with the object doing the charging (usually a rod or a ruler). It involves charge transfer to or from the earth to generate the charge imbalance in the object being charged. A charge imbalance is a non-uniform charge distributionEg.

++++- - - -

++

++

This symbol represents a connection to the earth

- -

Contact - the object being charged is contacted by the other object and charge is transferred directly from one object to another.

Friction - the object is rubbed by a material that has a greater or lesser affinity for electrons and a transfer takes place. It is more the contact than the friction that is necessary for the charging to take place.

The problem with moisture in the air:

Moisture prevents objects from holding a charge because it transfers charge to or from the object resulting in a neutral object.


The importance of the material

Conductors - are materials that allow charges to flow through them. They do not hold a static charge because any charge imbalance is easily conducted away.

Insulators - are materials that doe not allow charges to flow through them. They will hold a static charge because the charge imbalance is not easily conducted away.

Example -

ESA: Ex 15A Q.1 ESA: Ex 15B Q.1, 2 & 3


SHOCKS AND SPARKS

Flow of charge through the body causes the shockIonisation or flow of electrical energy from the charged object causes the sparkExamples

Climbing out of a car seat at the gas station. The seat becomes negatively charged because the electrons are moving from the person’s clothing to the seat. When the person is out of the car and in contact with the ground the electrons from the car seat are able to be transferred to the air particles causing these particles to become ionised. It is these negative ions that can move across a gap between the seat and the person. This results in the production of a spark. The shock is caused by the ions losing electrons to the person’s body and these electrons flowing through the body and into the earth (which is charge flowing into the person’s body. These charges

Explanation of discharging in terms of electron transfer to/from an object to result in neutrality.


CURRENT VOLTAGE


INTRODUCTIONThursday, 4 November 2010

WHAT IS ELECTRICITY?


THE ELECTRICAL CIRCUIT - introducing the idea of the electron pump


+ -

A conducting path

Power Supply

1. What is an electric current?

2. What are the two requirements necessary for an electric current to exist?


http://regentsprep.org/Regents/physics/phys03/bsimplcir/default.htm

1.Which part of the model represents the power supply?

2. Which part of the model represents the component?

3. What type of current is being modelled?

A

B

THE GRAVITY MODEL

















2. Which part of the model represents the conducting path?

3. Which part of the model represents charge?

THE WATER MODEL



2. Which part of the model represents the conducting path?

3. Which part of the model represents charge?

A

B

C

THE BIKE MODEL


THE BIKE MODEL

one link

THINK OF A LINK AS REPRESENTING A COULOMB OF CHARGE

1. In terms of this model, what do you think is meant by the term “current” ??



A conducting path

Power Supply

+ -Requirements: a power supply conducting path around which charge (electrons or ions) can flow.components (and sometimes meters)

A component

Current is a flow of electrons through a circuit

Two types:AC - Alternating current (electrons vibrate back and forth in wires) DC - Direct current (electrons flow in wires in one direction only)

Conventional current - the direction in which positive charges would flow in wires if they could.Conventional current is from positive to negative in a circuit (see diagram above)

ELECTRIC CIRCUITS



















Current - is the rate of flow of electrical charge - it is the number of coulombs of electrical charge that passes a point in one second. A coulomb is 6.25 x 1018 charges - I = electric current (measured in amps, A) by an ammeter:

Connecting an ammeter

I

+ -

A

V

I

Red

Red Black

Black

For charge to flow around an electrical circuit there is a need for a voltage source and a conducting path that is continuous and connects the positive to the negative terminal of the power supply.

+ -

Example

charge flows through the circuit as indicated by the arrows

ELECTRIC CURRENT


VOLTAGE

Voltage• Voltage (V) is a measure of the energy lost or gained between two points in a

circuit.• It is measured in the units volts , (V)

Unit of Voltage: Joule per Coulomb or Volt (JC-1) or (V)

V = ∆Ep

q

where V = potential difference or voltage (Volts, V)∆Ep = change in potential energy that a charge experiences when it moves from one side to the other side of a component (Joule, J)q = the unit of charge (Coulomb, C)

If V = 6V then a coulomb of charge has 6J more electrical potential energy at point A than it does at point B

1A

V

A B

+-Consider the voltage across a lamp:

Example


+ - V

A •

Two wires joined Two wires crossing

Cell Lamp

Battery (two cells in series)

Switch

Battery (several cells)

Diode

Voltmeter Ammeter

Resistor Power supply

fuse variable resistor(rheostat)

demo of circuit components ->

Notes CIRCUIT SYMBOLS


RESISTANCE

& OHM’S LAW


INTRODUCTIONThursday, 4 November 2010

RESISTANCE & ELECTROCUTION - 1


RESISTANCE & ELECTROCUTION - 2


FACTORS AFFECTING RESISTANCE


FACTORS THAT AFFECT RESISTANCE

Also, some materials conduct electricity better than others Eg. Copper is better than ironThursday, 4 November 2010

In an insulator, electrons are fixed

In a conductor, electrons are free to flow

Label the materials that the arrows are pointing to

_______________

_______________

CONDUCTORS & INSULATORS


THE VOLTAGE-CURRENT RATIO

1. Consider a lamp in an electrical circuit:12V

2A

12V represents the energy difference across the lamp. This drives electrons through the lamp at the rate (or “speed”) of 2A. The voltage:current ratio is _____

2. Consider a different lamp in an electrical circuit:

12V

1A

This lamp has higher resistance because 12V across this lamp can only drive electrons through the lamp at a rate of 1A. The voltage:current ratio is _____

This example shows that the greater the voltage:current ratio then the greater the resistance is. Resistance is the voltage:current ratio


In an insulator, electrons are fixed

In a conductor, electrons are free to flow

Label the materials that the arrows are pointing to

_______________

_______________

STOP OR GO?


Definitions

1. Resistance, R is a measure of the “electrical friction” in a conductor. (the opposition to the flow of current)

2. It is the ratio of the voltage across a conductor to the current through it.

Resistance = Voltage Current R = V

IUnit of resistance is the ohm, Ω

Resistance is given by the slope or gradient of a voltage - current graph

Example

In an experiment, the voltage across a lamp is measured and recorded as the current is increased 1 A at a time. Calculate the resistance of the lamp.

V (V)

I (A)0 1 2 3 4 5 6

2420161284

RESISTANCE

V

RI


1

2

3

A conductor that retains a constant temperature as the current is increased:

A conductor that is allowed to heat up as the current is increased

A conductor that is cooled progressively as the current is increased

V (V)

I (A)0 1 2 3 4 5 6

2420161284

V (V)

I (A)0 1 2 3 4 5 6

2420161284

0 1 2 3 4 5 6

V (V)

I (A)

2420161284

WHATS HAPPENING TO THE RESISTANCE AS THE CURRENT INCREASES?


SHAKING

MODELLING TEMPERATUREINCREASE IN A WIRE


For most conductors, as the temperature increases the increased vibration of particles impedes the flow of electrons. Resistance in the conductor will therefore increase. The graph slopes upwards.

V (V)

I (A)0 1 2 3 4 5 6

2420161284

0 1 2 3 4 5 6

V (V)

I (A)

2420161284

When a temperature of a lamp increases its resistance increases

The resistance of a thermistor decreases as its temperature decreases

LIMITATIONS OF OHM’S LAW


BASIC RESISTANCE PROBLEMS

6. What voltage is needed it a current of 5A is to flow through a resistance of 3Ω?

1. What is the resistance of a bulb it a 240 V supply causes a current of 2 A to flow through it?

2. What current flows through a heating element of 40Ω resistance when the element is plugged into a 240 V supply?

5. A current of 2 A flows through a 6Ω resistor. What is the voltage across it?

3. If a current of 3 A is flowing in a resistor across which there is a voltage of 6 V, what is the resistance?

4. What current must be flowing through a lamp of 0.5Ω resistance if there is a voltage of 6V across it?


Resistors which are connected end to end are in series with one another

The total resistance of the series combination, Rs is the sum of the resistances R1 and R2.

For two or more resistors in series: Rs = R1 + R2 + ...........

Resistors which are connected side by side are in parallel with each other.

The total resistance of the parallel combination, Rp is less than any individual resistor in the combination.

For two or more resistors in parallel the total resistance,Rp is given by:

1 1 1 + .... RP = R1 + R2

RESISTANCE CALCULATIONS

R1 R2

R1

R2


EXERCISES1. Read p52 to p53 QUIETLY

2. Answer questions 1 to 8 on p54 and 55 in your exercise books. There is no need to use full sentences


Study the pictures of the appliances shown below and in the table record the materials in each appliance that are conductors and insulators.

Material Conductor or Insulator?

Material Conductor or Insulator?

INSULATORS


SERIES & PARALLEL


Draw the following circuit diagrams in the spaces provided AND when you have finished, assemble them:

1.

2.

3.

Voltmeters are connected _________ components, ammeters are connected _____ a circuit

CIRCUITS: diagrams & assembly

A

+ -

V

+ -

+ -


CHARACTERISTICS OF SERIES & PARALLEL CIRCUITS

+ -

Series

+ -

Parallel

+ -+ -

1._

2._

3._

1._

2._

3._

components connected one other the other.

Single pathway

No junctions

each component has its own connection with the power supply

more than one pathway

One or more junctions


http://phet.colorado.edu/simulations/sims.php?sim=Circuit_Construction_Kit_DC_Only

CIRCUIT CONSTRUCTION

+ -

1

A

+ -3

A

+ -

2

A

+ -

1

A

+ -

2

A

+ -3

A

1. Enter the URL (above) into the address bar of your internet browser.2. Use the simulation tools to construct each of the following 3 circuits (ensure that you use

identical lamps and an the same power supply for each circuit).3. Record the current in each circuit and explain your observation.4. Repeat this exercise for the second set of 3 circuits.








DRAW THE CIRCUITS AND SET THEM UP


CIRCUITRULES


Current in series is constant I1 = I2 = I3

Voltage in series is shared VT = V1 + V2

NoteVoltage is shared in proportion to the size of the resistance

+ -

VTA1 A3

V1 V2

A2

I1 I3

I2

VOLTAGE & CURRENT IN SERIES CIRCUITS


Current in parallel is shared

IT = I1 + I2

Voltage in parallel is constant

VT = V1 = V2

NoteCurrent is shared in an inverse proportion to the size of the resistance.

For example:

If R1 = 5 and R2 = 10

and IT = 3

then I1 = 2 and I2 = 1

in other words “charge splits up as it enters a junction in a circuit”

+ -

VT

V1

V2

I1

IT

I2

IT

R1

R2

“Double the resistance then halve the current”

PARALLEL CIRCUITS


Example 1

A3 = ________

V1 = ________

V2 = ________

8V+ -

A1

A2

A3

R1

R2

2A3A V1

V2

8V+ -

R1

V2V1

Example 2

3VV1 = ________

Rule used ___________________________________________________________

Rules used ________________________________________________________________________________________________________________________________________

SIMPLE CIRCUIT CALCULATIONS


Example 3

8V+ -

A1

A2

A3

R1

R2

3A4A V1

V2

A3 = ________

A4 = ________

V2 = ________

V3 = ________

3VV3

A4

Example 4

8V+ -

A1

A2

A3

3A

A1 = ________

A4 = ________

A4

4A

Rule used ____________________________________________________________

Rules used ______________________________________________________________________________________________________________


For the circuit represented by the circuit diagram above, what is the reading on: (a) V1

(b) A2

(c) V2

(d) V3

1 + 9V -

A1

A2

A3

V2 V3

V1

5Ω 10Ω

ExamplesADVANCED CIRCUIT CALCULATIONS


2

For the circuit represented by the circuit diagram above, what is the reading on: (a) V3 if V2 = 10 V

(b) A1

(c) A2

(d) A3

(e) What is the value of resistor R?

+ 15V -

A1

A2

A3

V2

V3

V1

5Ω

10ΩR


3

For the circuit represented by the circuit diagram above, what is the reading on:

(a) V1

(b) V2

(c) V3

(d) A2

+ 12V -

A1

A2

V2

V1

2Ω

3Ω

V34.8Ω1 A


Examples

1

••100 Ω 100 Ω 100 Ω

Total resistance =

2

Total resistance =

25 Ω 30 Ω 50 Ω

+ -

R1

R2

RT = R1 + R2

RESISTORS IN SERIES

As we add resistors in series the resistance increases and therefore the current drawn decreases

As we add resistors in parallel the resistance decreases and therefore the current drawn increases


OTHER STUFF


ESA: 13B Q.5 to 8


INTRODUCING “EMF” & “ELECTRICAL POTENTIAL


METERS


+ -

A

V

IT

9 V One of these meters has a very high resistanceThe other meter has a low resistance.

Which is which?

Explain your answer


+ -

A

V

IT

9 V

METERS

Ammeter

1. connected in series with other components

2. has low resistance so that it doesn’t slow the current that it is supposed to be measuring

Voltmeter

1. connected in parallel with other components

2. has high resistance so that it doesn’t allow much current to flow through it. This would reduce the current and voltage through the component. It is supposed to be measuring the voltage


POWER &

ENERGYThursday, 4 November 2010

• Power is the rate at which electrical energy is transferred into other forms of energy.

• It is the amount of work done per second

P = E t

• It can be shown that the electrical power supplied to a device is given by:

P = VI

P

V I

E

P t

P = Power (Watts, W)

E = the amount of energy converted or work done (J)

t = the time taken (s)

P = power (watts, W) (1W = 1 Js-1)

V = Voltage (volts, V)

I = Current (amps, A)

POWER


POWER & ENERGY

Total energy used by a component/appliance can be calculated from the equation:

E = P.t

When the power value of the component/appliance is known and this value does not change over time.

If power changes over time then this change can be graphed.

P (W)

t (s)0 2 4 6 8 10

2

4

6

8

10

12

14 E = Area under the graph

E = 0.5 (9 + 13) = 4.4 J 10


TOTAL POWER USAGE in a parallel circuit

PT = P1 + P2 + P3 + P4

= 12 + 12 + 6 + 6 = 36 W

(ii)

ExampleThe power usage of the 4 lamps in parallel shown in the circuit below can be calculated in two ways:

(i) Use the total voltage (supply voltage) and the total current (current drawn from the supply) to calculate power.

(ii) Add the power usage of each of the components in parallel.

(i)

Lamps 3 & 4: I = P V = 12/12

= 1 A

lamps 1 & 2: I = P V = 6/12

= 0.5 A

12 V

12 V12 W

12 V12 W

12 V6 W

12 V6 WP1

P2

P3

P4

IT = I1 + I2 + I3 + I4 = 0.5 + 0.5 + 1 + 1 = 3 A

P = VTIT = 12 x 3 = 36 A


LAMP BRIGHTNESS IN CIRCUITS

Three main points

(i) The brightness of a lamp depends on its power output since for a lamp, power is the rate at which electrical energy is converted into light (and heat)

(ii) In a circuit which has values of voltage and current, it is both the voltage and current that determine brightness.

(iii)The lamp’s resistance will determine that voltage:current ratio that it possesses

Example: The series circuit (below), shows 2 identical lamps. A third identical lamp is added to the circuit. Explain how the brightness of the lamps in the circuit changes

+ -

12 V


EXERCISES


TV Stereo Jug Heater Stove torch downlight

Voltage (V) 240 240 240 6

Current (A) 0.3 0.02 8 0.05

Resistance (Ω) 250 450

Power (W) 1000 1200 1800 0.3 140

Running time (s)

20 min 20 min 10 min 3 h

Energy (J) 1000 125000 300000

POWERING THROUGH THE QUESTIONS


9V+ -

40Ω

X

0.1 A

0.5 A

10Ω

A 9 V battery is connected in the circuit shown.

A current of 0.5 A is found to pass through the 10Ω resistor.

(e) Determine the heat energy generated per second in the whole circuit.

(a) Calculate the voltage across the 10Ω resistor.

(b) Show that the voltage across the parallel combination of resistors is 4.0 V.

(c) If 0.1 A passes through the 40Ω resistor, determine the current through resistor X.

(d) Show that the resistance of resistor X is 10Ω.

“PARALLEL WITHIN SERIES”


PARALLEL CIRCUIT IN ACTION

A car has two tail lights and two brake lights connected as shown in the diagram:

(a) Calculate the resistance of:(i) a tail light(ii) a brake light

(b) Calculate the current supplied by the battery when both S1 and S2 are closed.

(c) When the driver takes her foot off the brake S2 is opened state what happens to the size of the current from the battery and give a reason for your answer.


Y11 Sci - W & W


(a) 5V

(b) V in series is shared. The combination of the 40Ω resistor and X are in series with the 10Ω. Therefore V of the combination is 9 - 5 = 4

(c) Current through X = 0.5 - 0.1 = 0.4A because current in parallel is shared. The total current flowing from the power supply is shared out between the 40Ω resistor and X.

(d) I through X = 0.4A and V across X = 4V (since voltage in parallel is constant). R = V/I = 4/0.4 = 10Ω

(e) “Heat energy per second” is the definition of power. For the whole circuit, I = 0.5A and V = 9V. P = VI = 9 x 0.5 = 4.5W

“PARALLEL WITHIN SERIES”answers

PARALLEL CIRCUIT IN ACTION

(a) The current through a tail light needs to be calculated first: P = VI => I = P/V = 6/12 = 0.5A For a tail light, R = V/I = 12/0.5 = 24 Ω For a brake light, P = VI => I = P/V = 12/12 = 1A R = V/I = 12/1 = 12 Ω

(b) I total = 2 x 0.5 + 2 x 1 = 3 A

(c) Less current flows through the battery because there is now more resistance in the circuit because of the reduction in the number of pathways available for charge to flow.


MAGNETIC FIELDS


THE MAGNETIC FIELD AROUND A BAR MAGNET AND THE EARTH’S MAGNETIC FIELD


WHAT IS MAGNETISM?

• Magnetism is caused by moving electrons. (The smallest magnetic field is produced by the motion of 1 electron)

• When electrons move in a common direction, a magnetic field is produced (sometimes called a magnetic force field)

• A force will be exerted on an iron object placed in a magnetic field.

• A magnetic field is a region in space where a magnetic force can be detected.

The magnetic field around a bar magnet

S

N

The compass needle is itself a tiny magnet (the North pole of this magnet points towards the South end of the magnet)

Charm compass

Magnetic field lines

Strong magnetic field (high density of lines)

---> Demo c bar magnet & major magnet


• θ changes with time

• Angle of Dip - is the angle that the field lines make with the ground. At the equator, the angle of dip is zero. Near the poles the angle of dip is close to 90 degrees.

THE EARTH’S MAGNETIC FIELD

compassS

N

θ (angle of declination) = 11o

Geographic North

Earth’s axis

Magnetic South


Home to millions of species including humans, Earth is currently the only place in the universe where life is known to exist. The planet formed 4.54 billion years ago, and life appeared on its surface within a billion years. Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful solar

radiation, permitting life on land.

THE EARTH’S MAGNETIC FIELD IS ESSENTIAL FOR LIFE ON THE PLANET.

The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. The planet is expected to continue supporting life for at least another 500 million years.


http://en.wikipedia.org/wiki/Species

http://en.wikipedia.org/wiki/Species

http://en.wikipedia.org/wiki/Humans

http://en.wikipedia.org/wiki/Humans

http://en.wikipedia.org/wiki/Universe

http://en.wikipedia.org/wiki/Universe

http://en.wikipedia.org/wiki/Life

http://en.wikipedia.org/wiki/Life

http://en.wikipedia.org/wiki/Age_of_the_Earth

http://en.wikipedia.org/wiki/Age_of_the_Earth

http://en.wikipedia.org/wiki/Abiogenesis




http://en.wikipedia.org/wiki/Biosphere

http://en.wikipedia.org/wiki/Biosphere

http://en.wikipedia.org/wiki/Earth%27s_atmosphere

http://en.wikipedia.org/wiki/Earth%27s_atmosphere

http://en.wikipedia.org/wiki/Abiotic

http://en.wikipedia.org/wiki/Abiotic

http://en.wikipedia.org/wiki/Aerobic_organism

http://en.wikipedia.org/wiki/Aerobic_organism

http://en.wikipedia.org/wiki/Ozone_layer




http://en.wikipedia.org/wiki/Earth%27s_magnetic_field

http://en.wikipedia.org/wiki/Earth%27s_magnetic_field

http://en.wikipedia.org/wiki/Solar_radiation




http://en.wikipedia.org/wiki/Geophysics

http://en.wikipedia.org/wiki/Geophysics

http://en.wikipedia.org/wiki/Geological

http://en.wikipedia.org/wiki/Geological

MAGNETIC DOMAINS


MAGNETIC MAGIC --> MAGNETIC DOMAINS


MAGNETIC THEORY

Ferromagnetic materials (Iron, Cobalt and Nickel) can be permanently magnetised.

Electrons spinning in atoms have magnetic fields around them. They set up tiny North and South poles. Such an arrangement for an electron is called a dipole moment.[Illustrate a mag. dipole and mention Exchange Coupling]

For most elements: magnetic fields cancel.

N

Se

N

S

Iron, Cobalt and Nickel:

Electron structure is such that there is a resultant magnetic field produced by each atom. These atoms are sometimes called atomic magnets.)Thursday, 4 November 2010

Regions in a metal where the orientation of the magnetic dipoles is the same are called domains.

Fully magnetised

=> the orientation of the domains is the same

Unmagnetised Iron

=> Domains are scrambled

A domainHere, a large number of iron atoms (magnetic dipoles) are aligned.

Partially magnetised

DOMAINS

N

NS

S


BREAKING A MAGNET


“Lining up” the domains - Magnetising

• Stroke the object end to end with a permanent magnet , in the same direction, using the same pole of the magnet.

• Hold the object inside an D.C or A.C solenoid (Domains line up in the direction of the magnetic field)

“Scrambling” the domains - Demagnetising

Heat or hammer the magnet (This disturbs the alignment of the domains)[CAN ALSO BE DEMONSTRATED WITH THE SOLENOID]

MAGNETISING AND DEMAGNETISING [Solenoid Demo]

“Domains are induced into alignment”

- Picking up iron objects


EXERCISES


WIRES


A circular magnetic field is formed around a straight current - carrying conductor:

View from above

•

3D View

The direction of the magnetic field lines is given by the Right-hand Thumb Rule

The right hand thumb rule:

Thumb = direction of the electric current

Curled fingers = direction of the circular magnetic field

( “•” represents current directed out of the page)

I

WIRES


I

Magnetic field lines

•

“View” from above”


PARALLEL WIRES

Piece of card

field lines bunch and this leads to the wires repelling each other

Current in opposite directions

I I


B = Magnetic field strength (in Tesla,T)

I = Electric current in the wire (in Amps,A)

µo= the permittivity of free space (the ability of a material to support a magnetic field (TmA-1)

d = Distance from the wire (in metres, m)

B = µ0I 2πd

1. Reversing the direction of the current reverses the direction of the magnetic field.

2. Magnetic field strength (symbol, B) is measured in NA-1m-1 or Tesla,T.

3. As the current in the wire, I increases the strength of the magnetic field increases B α I i.e. B is proportional to I

4. As the distance,d from the wire increases the strength of the magnetic field decreases.

B α 1/d i.e. B is inversely proportional to d

Note


Example

A special meter able to measure the magnetic field strength at any given point in the vicinity of a wire is shown below (taking a reading). It measures the magnetic field strength as 8 x 10-4 T at a distance of 0.01m from the centre of the wire. The current through the wire is 5 A.

Calculate the value of the constant µo.

Exercises

B I d µo

5 x 10-5 T 2 A 20 mm 3.14 x 10-6

6 x 10-5 T 3A 0.159 m 2 x 10-5 TmA-1

7.2 x 10-5 T 3.11 A 2.2 cm 3.2 x 10-6 TmA-1

1.05 x 10-3 3A 20 mm 4.4 x 10-5 TmA-1


CURRENT CARRYING CONDUCTOR AND MAGNETIC FIELD -----> COIL


COILS


The magnetic field of a solenoid is similar in shape to that of a bar magnet:

If the current is known, the poles of the solenoid can be determined using the right hand thumb rule applied earlier to the straight wire:

Draw the field lines

Complete this:

Field lines are parallel in the core of the solenoid which --> the magnetic field in the core is uniform.

The density of magnetic field lines is greatest in the core --> the magnetic field strength is greatest in the core.

THE SOLENOID


Predicting North and South poles:

Thumb points to Northpole of the solenoidfrom inside the coil

Curled fingers indicatethe direction of thecurrent

Factors affecting the strength of the magnetic field:

1. Increasing the current increases the magnetic field strength.

2. Increasing the number of turns of wire per given length of the electromagnet increases the magnetic field strength

STRENGTH RULZ


Uses of electromagnets

1. Electromagnets in relays are able to open and close electrical circuits (eg. starter motor circuit in a car).

2. Used in scrap yards to lift car bodies.

3. Create the ringing sound in electric bells.

4. Electromagnets in the recording heads of tape recorders are used to magnetise the audio tape during recording.

A solenoid which contains an iron core is called an electromagnet.

Adding an iron core increases the strength of the magnetic field because the iron core itself becomes magnetised and adds to the magnetic field of the solenoid.

ELECTROMAGNETS


Induced magnetismAn unmagnetised object will have have its domains aligned and therefore develop a north and south pole. The object can be picked up by the magnet because opposite poles attract.

x x x x x x x x

- - - - - - - -

North end of coil South end

S NNAttraction to South of coil

Attraction to North of coil

The dipoles in the object change along the rod as the rod is drawn into the coil and it is this dipole change which pulls the rod into the coil

Cross-section of coil

THE COIL GUN


ELECTROMAGNETS IN RELAYS


PRACTICAL


STATIC


plastic rod (charged by rubbing with a cloth)

small pieces of torn paper

Observation:

Explanation:

1

2 Balloon rubbed against hair

Removed from head and then brought back to hair

PLAYING AROUND WITH STATIC ELECTRICITY

Observation:

Explanation:


3Balloon rubbed against jersey

Release

Observation:

Explanation:

4

cotton(a) Each balloon charged separately by rubbing against

the sleeve of a jersey

(b) Holding the balloons by the cotton, release them, allowing them to come close to each other.

Observation:

Explanation: Thursday, 4 November 2010

Charged plastic rod is held near a thin stream of water

http://phet.colorado.edu/new/simulations/sims.php?sim=Balloons_and_Static_Electricity

Observation:

Explanation:

6

5

(a) Straw, charged at both ends (using a woollen cloth)

(b) Straw, also charged using a woollen cloth held horizontally and brought close

(c) Repeat (b) using a silk cloth.

Observation:

Explanation:






Cap

Insulating material

Body of electroscopeLeaf

Base

Aimto charge an electroscope by both induction and by contact and to draw charge distribution diagramsMethod1. Follow the instructions below2. Write observations as you perform each step3. Complete the diagrams only after recording the observations (you may need some help with

these)

Part 1 - Charging by induction

Equipment

dry cloth/jersey perspex rod ebonite rod electroscope

1. Charge the rod by rubbing it with a dry cloth/jersey and hold the rod near the cap of the electroscope

Observation:

If the rod was positively charged the charge distribution diagram would look like this:

++++- - - -

++

++

CHARGING OBJECTS


Complete the diagram to show how charges would distribute on the electroscope should the rod be negatively charged.

- - - -

2. With the rod in this position, earth the cap with your finger.

Observation: ++++- - - -

++

++


Complete the diagram to show how charge moves when the cap of the electroscope is earthed


3. Unearth the cap of the electroscope without removing the charged rod

Observation:

Draw the charge distribution diagram (by adding to the existing diagram on the right) showing the situation once this charge movement has finished.

++++- - - -

++

++

Draw the resultant charge distribution and the new position of the leaf on the diagram (right).

4. Remove the charged rod

Observation:

Finally, complete the diagram (right).Thursday, 4 November 2010

Part 2 - Charging by contact

Method1. Follow the instructions below2. Write observations as you perform each step3. Complete the diagrams only after recording the observations (you may need some help with

these)

1. A positively charged rod is held near the cap of the electroscope.

2. The rod makes contact with the cap.

3. The rod is removed.

++++


Cap

Insulating material

Body of electroscopeLeaf

Base

Aimto charge an electroscope by both induction and by contact and to draw charge distribution diagramsMethod1. Follow the instructions below2. Write observations as you perform each step3. Complete the diagrams only after recording the observations (you may need some help with

these)

Part 1 - Charging by induction

Equipment

dry cloth/jersey perspex rod ebonite rod electroscope

ALL CHARGED UPLab 12

1. Charge the rod by rubbing it with a dry cloth/jersey and hold the rod near the cap of the electroscope

Observation:

If the rod was positively charged the charge distribution diagram would look like this:

++++- - - -

++

++

The leaf of the electroscope springs up.


Complete the diagram to show how charges would distribute on the electroscope should the rod be negatively charged.

- - - -

2. With the rod in this position, earth the cap with your finger.

Observation: ++++- - - -

++

++


Complete the diagram to show how charge moves when the cap of the electroscope is earthed

- -

- -

++++

- -

Electrons at the cap are repelled by the negatively charged rod.

Leaf of the electroscope drops

Electrons at the cap are held in position by the positively charged rod. The earth supplies electrons to the positively charged leaf and lower stem.


- -

- -

3. Unearth the cap of the electroscope without removing the charged rod

Observation:

Draw the charge distribution diagram (by adding to the existing diagram on the right) showing the situation once this charge movement has finished.

++++- - - -

++

++

Draw the resultant charge distribution and the new position of the leaf on the diagram (right).

4. Remove the charged rod

Observation:

Finally, complete the diagram (right).

The cap and leaf now have no overall charge. Electrons on the cap are still held in position.

Leaf of the electroscope remains in the “dropped” position. The charge distribution has not changed

- - - -

- - -

- ++

+++

+

- -

- - -

- -

-

Negative charge redistributes itself around the metal parts of the electroscope leaving the stem and leaf with an overall negative charge

The leaf of the electroscope springs up.

++++

++


Part 2 - Charging by contact

Method1. Follow the instructions below2. Write observations as you perform each step3. Complete the diagrams only after recording the observations (you may need some help with

these)

1. A positively charged rod is held near the cap of the electroscope.

2. The rod makes contact with the cap.

3. The rod is removed.

++++- - - -

++

++

++

++

-

Electrons migrate up into the rod

+ + +

++

++

The electroscope is now left with an overall positive charge.

Charge separation occurs. Positive repels positive at the stem/leaf


12 Physics > resources > electricity > DC electricity > videos

SPARKS


• When the generator is turned on, the electric motor begins turning the belt.

• The belt is made of rubber and the lower roller is covered in silicon tape. Silicon has a greater affinity for electrons than rubber and so it captures electrons from the belt. The belt in turn must capture electrons from the dome, leaving the dome positively charged.

Label the picture of the Van der Graaf (left) using the labels in the box below:

Lower roller

Belt - A piece of surgical tubing

Output terminal - an aluminium or steel sphere

Upper roller - A piece of nylon

Motor

Upper brush - A piece of fine metal wire

Lower Brush

______________

____________________________

______________

______________

____________________________

Reference: http://science.howstuffworks.com/vdg3.htm

THE VAN DER GRAAF - HOW IT WORKS


http://livepage.apple.com/










http://science.howstuffworks.com/vdg3.htm

http://science.howstuffworks.com/vdg3.htm

THE VAN DER GRAAF - OBSERVATIONS & EXPLANATIONS

1. Small dome held close to generator dome

2. Hair stands on end when contact is made with the generator dome

3. Aluminium foil plates flying of the top of the generator dome

Drawn observation

Drawn observation

Drawn observation

Explanation

Explanation

Explanation


4. Sparking - a result of ionisation --> thorough step by step explanation

5. The shock that is felt ------> Charge travelling from/into the earth, through the body


DC


CIRCUIT RULES


ADDING BULBS IN PARALLEL

1. Set up each of the following circuits, one after the other (making a mental note of the brightness of the lamps in the circuit.

2. For each circuit read the ammeter and record the current in the space provided.

+ -

1

A

+ -

2

A

+ -

3

A

Current = ______ A

Current = ______ A

Current = ______ A

Observation

Explanation

8 V 8 V 8 V


ADDING BULBS IN SERIES

1. Set up each of the following circuits, one after the other (making a mental note of the brightness of the lamps in the circuit.

2. For each circuit read the ammeter and record the current in the space provided.

+ -

1

A

Current = ______ A

+ -

3

A

Current = ______ A

Observation

+ -

2

A

Current = ______ A

Explanation


CURRENT IN THE SERIES CIRCUIT IS CONSTANT

Aim

to look for a pattern in the current through bulbs and resistors in a series circuit.

1. Use ONE ammeter in the three different places shown in the circuit diagram.2. Without changing the setting on the power pack or the variable resistor write the

current readings in the spaces provided (below):

Equation

8V+ -

A2

A1

A3

A1 = ______ A

A2 = ______ A

A3 = ______ A


CURRENT IN THE PARALLEL CIRCUIT IS SHARED

Aim

to look for a pattern in the current through bulbs and resistors in a parallel circuit.

1. Use ONE ammeter in the each of the four places shown in the circuit diagram.2. Without changing the setting on the power pack record your results below:

Conclusion

Current in a parallel circuit is ____________ .

Equation relating the currents

Results

A1 = ____ A

A2 = ____ A

A3 = ____ A

A4 = ____ A

8V+ -

A1

A2

A3

A4


VOLTAGE IN THE SERIES CIRCUIT IS SHARED

Aim

to look for a pattern in the voltages across bulbs and resistors in a series circuit.

Conclusion

The power supply voltage is _____________ between the components in the circuit

8V+ -

V1

V2 V4V3

A

Equation relating the voltages

1. Set up the circuit (below)2. Use ONE voltmeter in the three different places shown in the circuit diagram.3. Without changing the setting on the power pack record your results below:

Results

V1 (power supply) = __ VV2 (variable resistor) = __ VV3 (bulb) = __ VV3 (ammeter) = __ V


VOLTAGE IN THE PARALLEL CIRCUIT IS CONSTANT

Aim

to look for a pattern in the voltages across bulbs and resistors in a series circuit.

Conclusion

The voltage across components connected in parallel is __________

Equation relating the voltages

Results

V1 = ___ VV2 = ___ VV3 = ___ V

8V+ -

V1

V2

V3

1. Set up the circuit (below)2. Use ONE voltmeter in the three different places shown in the circuit diagram.3. Without changing the setting on the power pack record your results below:


VOLTAGES AND CURRENTS IN SERIES AND PARALLEL

Aim

to investigate voltage and current in a series circuit that has a parallel portion in it.

1. Set up the circuit (below)2. Use ONE voltmeter in the four different places shown in the circuit diagram

and ONE ammeter in the four different places shown.3. Without changing the setting on the power pack record your results below:

Results

A1 = __AA2 = __AA3 = __AA4 = __A

V1 = __VV2 = __VV3 = __VV4 = __V

8V+ -

V1

V3

V4

V2

A1

A2

A3

A4 V4


OHMIC CONDUCTORS


OHM’S LAW

Results

Voltage setting of Power pack (V)

2 4 6 8 10 12

Voltage, V (V)

Current, I (A)

+ -

A V

ice

beaker

water

immersion coil

Method

1. Set up the following circuit using iced water to cool the immersion coil.

2. Increase the voltage in regular increments through an appropriate range (widest possible range)

Lab 14


Notes

•The iced water was used to keep the temperature of the coil constant .• If the iced water was forgotten and the coil was allowed to heat up then the graph would curve up.

Draw a graph of Voltage against Current on the grid provided

Repeat the experiment but this time replace the coil with a lamp (that will increase in temperature as the current through it increases)

Conclusion


RESISTANCE


Resistance specified

MeasurementsMeasurements Resistance calculated

from measurements

Calculated Power output

Resistance specified Voltage Current

Resistance calculated

from measurements

Calculated Power output

R1

R2

R3

Resistance calculated (formula provided for parallel resistors)

Resistance calculated (formula provided for parallel resistors)

across combination of resistors

drawn from the power

supply

For any circuit, set power supply voltage to 8V

For any circuit, set power supply voltage to 8V

R1 & R2 in series

R1 & R2 & R3 in series

R1 & R2 in parallel

R1 & R2 & R3 in parallel

RESISTANCE AND POWER in series and parallel


POWER


CALCULATING POWER OUTPUT OF APPLIANCES WITH AN ELECTRONIC MONITOR

Appliance

UNDER CONSTRUCTION


ELECTROMAG


HANGING MAGNETS I

NS

1. Cut out the net and fold it at the dotted lines to create a cradle

+

NS

2. Suspend the magnet in the cradle

3. Use a short length of cotton to suspend the magnet from a retort stand

4. Repeat using a second magnet.


HANGING MAGNETS II1. Position your two magnets in the

orientations shown 2. For each orientation, record your

observations

NS

N

S

NS N S

NS NS NS

N

S

A B

C D

ObservationObservation

ObservationObservation


STROKING MAGNETS

NS

A. Try magnetising an iron nail using a magnet

B. Once you have finished, check for a magnetic field using a charm compass.


PLOTTING MAGNETIC FIELDS

A. Place one or more compasses around a bar magnet

Charm compasses (moved around in a variety of positions around the magnet

B. Use a pencil to mark the North pole of each magnet using a dot

C. Connect the dots using a smooth curve

D. Plot several field lines and mark the North and South poles of the magnet.

E. Wrap your magnet in glad wrap and spring iron filings over it.

Follow the instructions (below) and draw your observations


EXAMS


2005NCEA


2006NCEA


2007NCEA

ASSNT FOR 2010


A

M


2008NCEA


2009NCEA

Topic TEST FOR 2010Laid out in a form that is ready for PCopying


Earth

3000V 1.6 x 10-6 A

ElectrostaticEliminator


6


Main points:1(b) & 1(c) - need further elaboration. See revised notes.3(a) & (b)....... confusion re. magnetic and geographic poles being opposite3(c) .......... Need to appreciate that coil becomes magnetised which causes dipole and domain alignment in the piston

3 (d) ...... problems creeping in with the algebra


1 2 CombinedCombined Grade

3 3 88 A

3 3 1010 M

1 1 3E 12 E


2010PRACTICE EXAM


Worthwhile going into some detail over this.


Marks thrown

EASY


Need to learn how to do the algebra - BIG PROBLEMS HERE - can be v easily addressed

Marks thrown

EASY


EXERCISES


Y11 Sci - W & W


1. Resistance is ______________ __________ .2. It is responsible for slowing down the ___________ ___ __________ .3. When there is resistance present in a conductor or an appliance ____________ energy is

transformed into ________ energy.4. Three everyday household appliances that have high resistance are:5. __________________ , ___________________ , _____________________6. Complete the circuit diagram which shows what happens when a third identical lamp is

added in parallel to a circuit:

+ - + -

7. If we continued to add lamps in this fashion explain why the power supply would blow a fuse.__________________________________________________________________

__________________________________________________________________

QUESTIONS ON RESISTANCE


APPLYING THE WATER MODEL TO Q.6

1. Pool of water fed by a rut

a rut

channel

2. Dig a channel and water can flow out of the pool

3. Dig another channel. What effect does this have on the flow through the rut?

3. Dig a third channel ........ What effect does this have on the flow through the rut?


“ANSWERS TO EXTRA FOR EXPERT QUESTIONS”


+ -

Draw using arrows the direction of charge flow in each circuitFor each circuit, highlight the lamps that will glow.

1 2

+ -

3

+ -

4+ -

NEED FOR A VOLTAGE SUPPLY AND A CONDUCTING PATH


CONDUCTORS & METERS

1. Which of the following substances are conductors of electricity:

wood, tap water, copper, iron, glass, sodium chloride solution, rubber 2. Draw a circuit diagram that you could use to test for conductivity using a lamp, dry cell and wires.3. Explain how a fuse protects a circuit.4. Voltage is ..........5. Current is ..........

6.

Starter


In an an Ohm’s law experiment a water-cooled resistor was connected in series with a power supply and an ammeter. A voltmeter was connected to measure the voltage drop across the resistor.The readings on the two meters were recorded. Voltage (V) Current (I)

2 0.15

4 0.31

6 0.45

8 0.59

10 0.75

12 0.92

Draw a graph of V vs I.

What is the meaning of its slope and what is its unit?

OHM’S LAW

Explain the shape of the graph that would be produced should the resistor be allowed to heat up.


electromagnetism

Education

static charge

charge imbalance

charged object

process of charge transfer

sparks flow of charge

following terms phrases

neutral object

ve charge neutron electron