Chapter 5

ElectricityThe last chapter with formulas

What happens that causes the glass rod to become charged by the silk?

• Rubbing causes some particles to leave the rod– These particles are called electrons

• When rubber rods are rubbed with cat fur, electrons leave the fur and go to the rod.

• The glass rod is said to have a negative charge– Benjamin Franklin said it first– Too bad he didn’t call it positive

Properties of electric charges

• When the ball touches the rod, the ball always repels the rod, no matter how the rod was charged.

• When the ball is charged with the glass rod, it is attracted to the plastic rod.

• When the ball is charged with the plastic rod, it is attracted to the glass rod.

Properties of electric charges

• There are two types of charges– Positive – Negative

• Unlike charges attract• Like charges repel• Many properties of electricity are

explained by this fact.

Is the Van de Graaff positive or negative?

• What experiment can we do to determine this?

• It is Positive.• So what does the belt do to the

electrons– It removes them from the aluminum

ball

What are electric charges?

• What causes things to become charged?

• Charges are pieces of atoms.• So what are atoms?• The building blocks from which all

things are made.• There are 92 naturally occurring

types of atoms, these are called the 92 elements.

Elements

• Substances that cannot be broken down into simpler substances by a chemical reaction.

• There are 92 naturally occurring elements– All things are made by combinations of

these elements– Examples of elements are: hydrogen,

oxygen, carbon,. . .

Atoms• The smallest subdivision of an

element that retains the properties of that element.

• Composed of– Electrons (outer shell) Negative

charge• Adding or subtracting electrons makes

ions (charged)

– Protons( Positive charge)– (determine what the element is)

• 1 proton = hydrogen• 8 protons = oxygen

– Neutrons – the glue that holds the nucleus together

NUCLEUSProtons (+)

Electrons (-)

Neutrons

THE ATOMThis model of an atom is far from reality but is the one usually seen in elementary science books.

The orbits of the electrons in real atoms are much more complex, they are wave-like.

Electrons are easy to remove from or add to the atom

Atoms join to make compounds

Roles of Electrons

Electrons move between atoms to enable them to join into molecules, when this happens we call it a chemical reaction.

Electrons moving along a metal are the reason it conducts heat and electricity.

Two hydrogen atoms join with one oxygen atom to make a water molecule

H = hydrogen (atom)

O = oxygen (atom)

H2O = water (molecule)

Chemical formula (H2O)

More on the atom

• The part of the atom that moves and is responsible for almost all of the electrical effects is the Electron

• The protons and neutrons are fixed in the nucleus.

• A normal (neutral) atom has the same number of electrons (-) and protons (+)– This means it is electrically neutral.

More on the electron• It is much bigger than the proton,

but weighs much less.– It would take 1,821 electrons to weigh

as much as a proton.– Some substances like to lose

electrons and become positive.– Some like to gain electrons and

become negative.

Induction• An object can be charged in two ways:• Charged directly if touched by a charged

object and charge is transferred.– A spark is also charge being transferred directly

• An object can be charged by induction if a charged object is brought close to it and charge is allowed to move off the object.– No charge leaves the original (charged) object.

» 3q

Charging by induction

+ + + + + + + + + + + + + + +

- -

++

If this sphere is connected to the Earth (the Prof touches it), electrons will flow from ground to neutralize the positive charge and leave the sphere negative.

The electrons are free to move and attracted by the positive charge and move as far as they can toward it.

Conductivity

• If electrons move easily from atom to atom in a substance it is called a conductor.– It conducts electricity (electrons move

though it).

• A non-conductor does not allow electrons to move from atom to atom– Its electrons are bound tightly.

6q

Electrical signals move very quickly through conductors, the electrons move slowly down the wire.

When one electron enters the wire,

another leaves the other end. Not the same electron that entered but a different one.

Force between electrical chargesCoulomb's Law

• The force between two charges, q1 and q2 which are a distance “r” from each other is given byF = k q1 q2/r2

Where k is a constant of value =9x109 [m2/coulombs2]N

The unit of electrical charge is the coulomb.

Find the force between a 3 coulomb charge and a 2 coulomb charge that are 3 meters apart.

Dataq1 = 3 coulombs

q2 = 2 coulombs

R = 3 metersk = 9x109 [m2/coulombs2]N

Be sure this number is on your formula sheet

Solution and Answer

F = k q1 q2/r2 =

= 9x109 [m2/C2]N •3 C•2 C/(3 m)2

= 6x109 N

Similarity between electricity and gravity

• Electricity F = k q1 q2/r2

k = 9x109 [m2/coulombs2]N• Gravity• The attraction between any two

objects of masses m1 and m2 which are a distance r apart is given by:

F = Gm1 m2/r2

G = 6.67 x 10-11 n m2/kg2

Electricity is much much much stronger than gravity

k = 9x109 [m2/coulombs2]NG = 6.67 x 10-11 n m2/kg2

So we can see that electricity is 1020 times stronger

100,000,000,000,000,000,000 Electricity holds everything together

Current

• Current is the flow of electrons• Current is measured in Amperes• One ampere is one coulomb per

second• Symbol for current = I• The letter “C” is the unit of

electrical charge (the coulomb)

Voltage

• Voltage is a measure of the amount of potential energy the electrons have– We can think of voltage as a pressure

caused by the electrons trying to get away from each other (they have the same charge).

• It is sometimes called potential• It is measured in volts• One volt = 1 joule per coulomb• Volt = joule/coulomb

Resistance• Resistance to the flow of electrons• Measured in Ohms• Symbol

– 10 ohms = 10Ω– not 10 O

• This looks too much like 100

• Resistance depends on the material from which the wire is made and on the size of the wire.– The smaller the wire, the higher the

resistance

Ohm’s law

• V = I R – Put on formula sheet!

• Voltage = current x resistance• R=V/I• One ohm = volt/amp• I = V/R

What current will flow when 10 volts is applied across 50 ohms?

DataI = ?V = 10 vR = 50ΩV = IRI = V/R = 10 V/ 50 Ω = 0.2 Amps

300,000 volts won’t always hurt you, 0.2 amps can kill.

• Voltage measures the “force” behind the electrons.

• Current gives the number of electrons passing through the circuit

• The resistance together with the voltage is what determines the current.

• Current is what is dangerous, not voltage.

• The higher the voltage the higher the current

Electrical Safety

• 0.0005 amps – can feel• 0.005 amps – painful• 0.01 amps causes – muscle

contractions• 0.018 amps – stops breathing• If conditions are right (for a high

current), 110 volts can kill.

Why can a bird sit on a wire, but if I touch the wire, I am killed?

• For electricity to flow, there must be a complete circuit– The electrons must have a place to go, a

path– Electrons like to flow to (from) the “earth”

or “ground”– The electrons have no path from the bird to

the ground– If I touch the wire and I am in contact with

the ground, the electrons flow through me to the ground and the current is sufficient that I am injured.

Why are electrical appliances so dangerous close to the bathtub?• The water in the bathtub makes a

good path for the electrons to get to ground.

• A ground fault circuit detects when current if flowing to ground, and turns off the circuit.

• All circuits in bathrooms should have ground fault plugs – These break the circuit if the electrons

flow to the ground instead of the return circuit

Lightning kills more people in the U.S. each year than any other natural force such as hurricanes and tornados.

• Lightning begins when raindrops fall though the air and gather electrons, taking them to the ground (or a cloud)

• When the electric field becomes strong enough, a spark jumps – Lightning bolt

Lightning strikes the Empire State building more than 100 times/year.

Lightning facts

• 500 – 1000 people in the U.S. are killed or injured by lightning each year.

• A stroke last 1/1000 of a second, many strokes happen in one “flash”.

• The voltage that produces lightning is millions or billions of volts

More lightning facts

• The current in a lightning stroke may be 100,000 amps– Remember that 0.018 amps kills

people

• The temperature may be 50,000 ºF• The average length of a stroke that

reaches the ground is 10 miles.• Most lightning is cloud to cloud and

does not reach the ground.

Electric Power

• Power = current x voltage• P = IV

– Put on formula sheet– As V=IR, P = I x IR = I2R– Put both of these equations on your

formula sheet.

How many appliances (power) can I put on a 20 amp, 120 v circuit?

• Data I = 20 amp, V = 120 VP = IVP = 20 amps x 120 volts = 2400 (?)P = 2400 wattsCan I put a 1200 w iron, 3 100 watt

lights, a 750 w computer and a 155 w radio on this circuit?

1200+300+750+155=2405 NO

Magnetism

• Magnetism is produced by moving electrical charges– This is the only way it is produced.

• Permanent magnets are substances that have their electrons moving in a special way – their spins are aligned.

• Magnetism was discovered in China– In a type of iron mineral called

magnetite.

Permanent Magnets• Only three elements can be made into

permanent magnets– Iron– Cobalt – Nickel– Many alloys are permanent magnets

• Permanent magnets are produced by placing a magnetic material in a magnetic field, – The magnetic field may be produced by another

permanent magnet or by an electromagnet.– The magnetization of most permanent magnets

can usually be easily reversed.

Electric and magnetic fields• Electric fields are regions of space

where an electric charge feels a force.• Magnetic fields are regions of space

where a magnet feels a force.• A stationary electric charge feels a

force in an electric field but no force in a magnetic field.

• A moving electric charge feels a force in both an electric and a magnetic field.

Electromagnetic Induction – the way magnetism is connected to electricity and electricity to magnetism

• A changing electrical field (or a moving charge) will create a magnetic field

• A changing magnetic field will create an electrical field– This is how our electricity is generated

• This is the reason we use alternating current– Because transformers only work with AC and

they depend on electromagnetic induction– And transformers easily change voltage and

current greatly reducing power losses in transmission lines from the power station to your house

Current – Alternating or Direct• Direct current

– The electrons always move in the same direction

– Batteries produce direct current– The + and – do not change.

• Alternating current– The electrons change direction (usually 60

times a second)– This is used because it is easiest to make

using generators and because it allows the use of transformers.

• Transformers allow us to reduce power and the amount of electricity lost to heat in transmission lines.

– The lights in this room are flashing on and off 120 times a second.

Transformers• A changing electric current causes

a changing magnetic field in the primary coil

• Which in turn causes a changing magnetic field in the iron core

• Which in turn causes a changing magnetic field in the secondary coil

• Which in turn causes a voltage in the secondary coil.

Transformer

Primary side

Current and Voltage enter the transformer

Secondary side

Current and Voltage leave the transformer

The transformer works with alternating current. The changing electric field on the primary side in the wire makes a changing magnetic field in the iron core.

The changing magnetic field in the core induces a changing electric field in the wires on the secondary side of the transformer.

Electromagnetic Induction – the way magnetism is connected to electricity and electricity to magnetism

• A changing electrical field will create a magnetic field

• A changing magnetic field will create an electrical field– This is how our electricity is generated

• This is the reason we use alternating current– Because transformers only work with AC and

they depend on electromagnetic induction– And transformers easily change voltage and

current greatly reducing power losses in transmission lines from the power station to your house

Transformer equation

• The voltage in the secondary coil depends on the ratio of the number of turns in the primary to the number of turns in the secondary.

• Vp/Vs=Np/Ns=Is/Ip

What will the secondary voltage be if the primary is 120 volts, the number of turns on the primary is 100, and the number on the secondary is 5000?

• Vs = ?, Vp=120 v, Np = 100, Ns=5000

• Vp/Vs=Np/Ns

• 120v/Vs=100/5000

• Vs = 120 v ·5000/100 = 6000 v

Microphones and speakers

• Microphone– Sound causes a diaphragm to vibrate– This vibrates a coil in a magnetic field

which makes an electrical current.

• Speaker– A vibrating current in a coil causes a

magnet to vibrate – This vibrates a diaphragm causing

sound

Microphone

Sound waves cause the diaphragm to vibrate, causing the wires to move in the magnetic field. This induces a current in the wires by the process of electromagnetic induction.

Sound

Television• An electron “gun” shoots electrons

through a magnetic field where they are guided to the screen in (525) lines.– High definition has twice as many lines

• Every other line is drawn 1/60 of a second, the whole picture every 1/30 of a second.

• The dot that sweeps the lines is Red, Green or Blue, the picture is drawn by varying the intensity of each of the colors.

Television Tube

A beam of electrons traces a set path in a series of lines across the screen. By making the dot different colors and different intensities, a picture is formed.

• Chapter 5 test Friday – review/problems due.

• If you have not picked them up, Chapter 4 tests and printouts are on the front desk

• Papers were due Friday, you are losing 5 points per day.

• Labs for this week and the chapter 5 reviews are on the front desk.

• Papers should be given to me as soon as they are completed and signed by the writing lab. You are to give me the paper, hard copies of the articles and the various copies that you have submitted to the learning center.

• You are then to send me an email with the paper (but not the articles) attached.

• Papers turned in on Monday, Oct. 31 will have 5 points taken from their score, and you will lose 5 more points for each day you are late.

• You will loose up to the 400 points if the paper is not well done and checked by the learning center.

Electric motors

• These are pushed by magnetic fields

• The fields change to keep the rotor moving.

The electric motor is turned by a magnetic force. The current changes as the armature reaches a magnet, making it repel and keep turning.

Electric Motor

When recording, electromagnets magnetize the tape, on playback the magnetized tape induces a current in the playback head.

Many electronic devices operate this way including the hard (or floppy) disk on your computer and VCR tapes.

Tape recorders, etc.

Electromagnetic Radiation

• Caused when electrons vibrate or oscillate.– This causes a change in both the electric

and the magnetic fields.• Electromagnetic radiation

• When the electrons are in a wire and vibrate millions or thousands of times a second– The wire is called an aerial– The waves given off are called radio waves

Types of Electromagnetic radiationfrom longest to shortest

• Radio waves– No known effect on humans

• Microwaves– Destroy tissue and can cause genetic

defects

• Infra-red– Feels like heat, not dangerous

• Visible– Electromagnetic waves we can see– Long wavelength = red– Short wavelength = violet

Types of Electromagnetic radiationfrom longest to shortest (2)

• Ultra-violet radiation– Can cause chemical reactions– Sunburn– Genetic defects

• X-rays– Can kill cells– Can cause genetic defects

The Electromagnetic Spectrum

Long WavelengthsShort Wavelength

Low EnergyHigh Energy