ElectricityElectricity

N Bronks

Basic ideas…Basic ideas…Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____.

Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta.

Resistance is anything that resists an electric current. It is measured in _____.

Words: volts, amps, ohms, voltage, ammeter, voltmeter

• Flow of electrons

timepoint a Passed ChargeCurrent

tQI

CurrentCurrent

Current in a series circuitCurrent in a series circuit

If the current here is 2 amps…

The current here will be…

The current here will be…

And the current here will be…

In other words, the current in a series circuit is THE SAME at any

point

2A

2A

2A

Current in a parallel circuitCurrent in a parallel circuit

A PARALLEL circuit is one where the current has a “choice of routes”

Here comes the current…

And the rest will go down here…

Half of the current will go down here (assuming the bulbs are the same)…

SummarySummary

In a SERIES circuit:

Current is THE SAME at any point

Voltage SPLITS UP over each component

In a PARALLEL circuit:

Current SPLITS UP down each “strand”

Voltage is THE SAME across each”strand”

Advantages of parallel circuits…Advantages of parallel circuits…

There are two main reasons why parallel circuits are used more commonly than series circuits:

1) Extra appliances (like bulbs) can be added without affecting the output of the others

2) When one breaks they don’t all fail

Georg Simon Ohm 1789-1854

ResistanceResistance

Resistance is anything that will RESIST a current. It is

measured in Ohms, a unit named after me.

That makes me so happy

The resistance of a component can be calculated using Ohm’s Law:

Resistance = Voltage (in V)

(in ) Current (in A)

V

RI

An example question:An example question:

V

A

What is the resistance across this bulb?

As R = volts / current = 10/2 = 5

Assuming all the bulbs are the same what is the total resistance in this circuit?

Total R = 5 + 5 + 5 = 15 Voltmeter reads 10V

Ammeter reads 2A

VARIATION OF CURRENT (VARIATION OF CURRENT (II) WITH P.D. ) WITH P.D. ((VV))

A

V+

6 V-

Nichrome wire

VariationsVariations

(a) A METALLIC CONDUCTORWith a wire

(b) A FILAMENT BULB (c) COPPER SULFATE SOLUTION

WITH COPPER ELECTRODES(d) SEMICONDUCTOR DIODE

Done both ways with a milli-Ammeter and the a micro Ammeter

Current-voltage graphsCurrent-voltage graphsI

VI

V

I

V

1. Resistor 3. Diode

2. BulbCurrent increases in proportion to voltage

As voltage increases the bulb gets hotter and resistance increases

A diode only lets current go in one direction

Factors affecting Factors affecting Resistance of a conductorResistance of a conductor

• Resistance depends on– Temperature– Material of conductor– Length – Cross-sectional area

TemperatureThe resistance of a metallic

conductor increases as the temperature increases e.g. copperThe resistance of a

semiconductor/insulator decreases as the temperature increases e.g. thermistor.

VARIATION OF THE RESISTANCE OF A VARIATION OF THE RESISTANCE OF A METALLIC CONDUCTOR WITH METALLIC CONDUCTOR WITH

TEMPERATURETEMPERATURE

Water Wire wound on frame

Glycerol

Heat source

10ºC

Digitalthermometer

Ω

10º C

Graph and PrecautionsGraph and Precautions

PrecautionsPrecautions

- Heat the water slowly so temperature does - Heat the water slowly so temperature does not rise at end of experimentnot rise at end of experiment

-Wait until glycerol is the same temperature -Wait until glycerol is the same temperature as water before taking a reading.as water before taking a reading.

R

Factors affectingFactors affecting Resistance of a conductor Resistance of a conductor

• Material, temperature, Area cross section and length

R = L = Resistivity A Unit: ohm meter m

RESISTIVITY OF THE MATERIAL OF A RESISTIVITY OF THE MATERIAL OF A WIREWIRE

Micrometer

Metre stick

l

Bench clamp

Stand

Nichrome wire

Crocodile clips

1. Calculate the resistivity where A =

2. Calculate the average value.

Precautions Ensure wire is straight and has no kinks like ....Take the diameter of the wire at different angles

,Al

Rñ

4

2d

ρ

Resistors in series and Resistors in series and ParallelParallel

321 IIIIT

V1

I

I1

V

I2

IT

R1R1

R2 R3

R2

321 VVVVT

Resistors in series and Resistors in series and ParallelParallel

321 IRIRIRIRT

V1

I

I1

V

I2

IT

R1R1

R2 R3

R2

321 RRRRT

321 VVVVT

Resistors in series and Resistors in series and ParallelParallel

321 R

V

R

V

R

V

R

V

T

V1

I

I1

V

I2

IT

R1R1

R2 R3

R2

321

1111

RRRRT

321 IIIIT

Wheatstone BridgeWheatstone BridgeUses

– Temperature control– Fail-Safe Device (switch circuit

off)– Measure an unknown

resistance

– R1 = R3 (When it’s balanced)

R2 R4

Metre Bridge R1 = R2 (|AB|)

|BC|

I

r 1

r2

r 4

r3

AA CC

BB

DD

Effects of an Electric CurrentEffects of an Electric Current

•Heat•Chemical•Magnetic

Current-voltage graphsCurrent-voltage graphs

I

V

I

V

1. Active Electrodes

2. Inert Electrodes

e.g. Copper in Copper Sulphate

e.g. Platinum in Water

Current CarriersCurrent Carriers

Medium Carrier

Solid (Metal) Electrons

Liquid (Electrolyte) Ions

Gas Electrons and Ions

Resistance in Resistance in SemiconductorsSemiconductors

2) Thermistor – resistance DECREASES when temperature INCREASES – Due to more charge carriers being liberated by heat

1) Normal conductor like metal resistance increases as vibrating atoms slow the flow of electrons

Resistance

Temperature

Resistance

Temperature

Fuse – Safety deviceFuse – Safety device

Fuses are designed to melt when too large a current tries to pass through them to protect devices.

Prevent Fires

Modern fuse boxes contain MCB (Miniature circuit breakers)

2A5A

Other safety devices…Other safety devices…1) Insulation and double insulation

2) Residual Current Circuit Breaker

In some parts of Europe they have no earth wire just two layer of insulating material the sign is

An RCCB (RCB) detects any difference in current between the live and neutral connectors and the earth it switches off the current when needed. They can also be easily reset.

Electrical SafetyElectrical Safety• A combination of fuse and Earth

A.C. Supply

That Hurts!

The fuse will melt to prevent electrocution and the electricity is carried to earth

The casing touches the bare wire and it becomes live

Wiring a plugWiring a plug

Earth wire

Neutral wire

Insulation

Live wire

Fuse

1.

2.

3.

4.

5.

6. Cable grip

Charge & Charge & DischargeDischarge

Uses of Uses of CapacitorsCapacitors

• Storing charge for quick release – Camera Flash

• Charging and discharging at fixed intervals – Hazard Lights

• Smoothing rectified current – See Semiconductors

Parallel Plate CapacitorsParallel Plate Capacitors• The size of the capacitor depends on1. The Distance the plates are apart d

-

-

-

+

+

+

d

Parallel Plate CapacitorsParallel Plate Capacitors

2 /.The area of overlap A

-

-

-

+

+

+

A

Parallel Plate CapacitorsParallel Plate Capacitors

• 3/.The material between ()

-

-

-

+

+

+

High material

Called a

DIELECTRIC

--

--

++

++

EquationsEquations

C

d

A=

For the parallel plate capacitor

Distance in meters

Area In m2

Permitivity inFm-1

CapacitanceIn Farads

Example 1Example 1

0

C0.01m

0.04m2

=

The common area of the plates of an air capacitor is 400cm2 if the distance between the plates is 1cm and ε0=8.5x10-12Fm-1.

C

d

A=

8.5x10-12Fm-

1x=3.4x10-11F.

Capacitance experiment on the internet

EquationsEquations

C

V

Q=

Capacitance on any conductor

Potential Difference in volts

Charge in Coulombs

CapacitanceIn Farads

Placing a charge of 35μC on a conductor raises it's potential by 100 V. Calculate the capacitance of the conductor.

Info Q = 35μC and V = 100V find C=?

Using Q=VC or C = Q/V

= 35 x 10-6/100

= 35 x 10-8 Farads

EquationsEquations

C½Work Done

(V)2=

Energy stored on a capacitor

Voltage Squared

CapacitanceIn Farads

Energy Stored

Example 3Example 3Find the capacitance and energy stored of a

parallel plate capacitor with 2mm between the plates and 150cm2 overlap area and a dielectric of relative Permittivity of 3. The potential across the plates is 150V.

A = 150cm2=0.015m2, d = 2x10-3m,

ε = 3xε0 = 27x10-12Fm-1

As C = ε0A/d = 27x10-12 x 0.015/0.002 = 2.025x10-9 F

Energy stored = ½ C V2 = ½ x 2.025x10-9x (150)2

= 2.28x10-5 Joules

DC and ACDC and AC

DC stands for “Direct Current” – the current only flows in one direction:

AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz)

Find Root Mean Square of voltage by

Vrms= Vpeak/ √2

1/50th s

240V

V

V

Time

T

The National GridThe National Grid

Power Transmitted is = P = V.I

JOULES LAW gives us the power turned into heat

Power Lost = I2R

So if we have a high voltage we only need a small current. We loss much less energy

Power stationStep up

transformerStep down

transformerHomes

Joules lawJoules law

Heating coil Lagging

Calorimeter Water

A

LidDigitalthermometer

10°C

Calculation and GraphCalculation and GraphRepeat the above procedure for increasing values of current I, taking care not to exceed the current rating marked on the rheostat or the power supply. Take at least six readings. Plot a graph of ∆(Y-axis) against I 2 (X-axis).

A straight-line graph through the origin verifies that ∆ I 2 i.e. Joule’s law.

Electrical Power lost as Heat P I2 is Joules lawThe power lost (Rate at which heat is produced) is

proportional to the square of the current.

∆

I2

Coulomb's LawCoulomb's Law

• Force between two charged bodies

Force = f Q1.Q2

d2

Q1 Q2d

Put this as a sentence to get a law!

Coulomb CalculationsCoulomb Calculations

• We replace the proportional with a equals and a constant to get an equation

Force =f Q1.Q2

d2

Force = f = Q1.Q2

4d2

= permitivity as in capacitors

Coulomb's Law CalculationsCoulomb's Law Calculations

• Force between these bodies

Force = f = Q1.Q2

4d2

2C 4mCd=2m

= 3.4 x 10-11

Coulomb's Law CalculationsCoulomb's Law Calculations

• Force between these bodies

2C 4mCd=2m

Electric Field Strength = E = F/q

Electric Field Strength =

E = 7.49 x 10-15 N /2C

= 3.75 x 10-15 N /C

PrecipitatorPrecipitator

• Carbon and ash - can be removed from waste gases with the use of electrostatic precipitators

PhotocopierPhotocopier

• Charging:• Exposure: • Developing:• Transfer: • Fusing:• Cleaning:

Potential Difference (V)Potential Difference (V)

Potential difference is the work done per unit charge to transfer a charge from one point to another (also Voltage)

i.e V = W Q

Potential Difference (V)Potential Difference (V)

V = W Q

Unit Volt V or J C-1

Volt is the p.d. between two points if one joule of work is done bringing one coulomb from one point to the other

Potential at a point is the p.d. between a point and the Earth, where the Earth is at zero potential

Current in a Magnetic FieldCurrent in a Magnetic Field

N S N S

Current in a Magnetic FieldCurrent in a Magnetic Field

N S

Force

CurrentMagnetic Field

A conductor carrying a current in a magnetic field will always feel a force

The force is perpendicular to the current and the field. – This is THE MOTOR EFFECTTHE MOTOR EFFECT

Fleming’s Left Hand RuleFleming’s Left Hand Rule

I used my left hand to show the direction the wire would move

The Size of the ForceThe Size of the Force

Force = F = B.I.lForce = F = B.I.lWhere B = Magnetic Field Density in Tesla (T)

I= Current in Amps (A)…………………………… L = length if the conductor in metres…

Example What is the force acting on a conductor of length 80cm carrying a current of 3A in a 4.5T magnetic field?

Using Force = F = B.I.l = 4.5x3x0.8

= 10.8N

The AmpereThe Ampere

• Basic unit of electricity

F=2x10-

7N/m

1m

The current flowing is 1A when the force between two infinitely long conductors 1m apart in a vacuum is 2x10-7N Per metre of length.

Moving ChargeMoving Charge• When any charged particle moves it is like a small

current of electricity• It feels the same force• The crosses show a magnetic field into the screen

e-Velocity

Force

e -

VelocityForce

e -

Velocity

Forcee-

e-

Moving ChargeMoving Charge

• A positive will move the other way

e-Velocity

Force

+

All charged All charged particles particles moving in moving in magnetic magnetic fields always fields always have a force have a force at right angles at right angles to their to their velocity so velocity so follow a follow a circular path circular path due to FLH due to FLH RuleRule

Force 0n a ParticleForce 0n a Particle

Force = F = B.q.vForce = F = B.q.vWhere B = Magnetic Field Density in Tesla (T)

q=charge on the particle (C) v=velocity of the particle…

ExampleExample What is the force acting on a particle travelling at What is the force acting on a particle travelling at 80m/s carrying a charge of 0.1C in a 10T magnetic field?80m/s carrying a charge of 0.1C in a 10T magnetic field?

UsingUsing Force = F = B.q.v= 10x.1x80

= 80N

InductionInductionis where changes in the current flow in a circuit are caused by changes in an external field.

N

Moving Magnet

Circuit turning off and on

ElectromagnetElectromagnetic inductionic induction

The direction of the induced current is reversed if…

1) The magnet is moved in the opposite direction

2) The other pole is inserted first

The size of the induced current can be increased by:

1) Increasing the speed of movement

2) Increasing the magnet strength

3) Increasing the number of turns on the coil

Faraday’s LawFaraday’s LawBasically

1. More turns (N) more EMF

2. Faster movement more EMF

Rate of change of FLUX DENSITY is proportional to induced EMF

Induced EMF = E = - Nd ( =B.A) dt

Lenz’s LawLenz’s LawThe induced EMF always opposes the current/Motion

You get ought for nought

A version of Newton III and of energy conversion

The induction always tries to stop the motion or change in the field.

Aluminum Ring

The ring moves away as the induced current is preventing more induction

Mutual Mutual inductioninduction

• Induction in a second circuit caused by changes in a first circuit

• Main use in a transformer• As the current changes the

field changes giving a EMF in the second circuit.

TransformersTransformersThis how A.C. changes voltage up or down

V In

V Out

Turns 2

Turns 1=

Self InductionSelf Induction

• property whereby an electromotive force (EMF) is induced in a circuit by a variation of current in the circuit its self

D.C. SourceCurrent Back EMF

Another example on LENZ’S LAW

Flux DensityFlux Density• Magnetic flux, represented by the Greek

letter Φ (phi), total magnetism produced by an object. The SI unit of magnetic flux is the Weber

• Magnetic field (B) is the flux through a square meter (the unit of magnetic field is the Weber per square meter, or Tesla.)

As the flux expands the density through any square meter decreases