S.MORRIS 2006

Electricity and Magnetism

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All you need to be an inventor is a good imagination and a pile of junk.-Thomas Edison

Electricity – electrons moving through a metal wire.

The CELL

The cell stores chemical energy and transfers

it to electrical energy when a circuit is

connected.An example of a cell is a

Battery.

The cell’s chemical energy is

used up pushing a current

around a circuit.

What is an electric current?An electric current is a flow of microscopic

particles called electrons flowing through wires

and components.

+ -

In which direction does the current flow? By convention (thanks to Ben Franklin) - from the Negative terminal to the Positive terminal of a cell. (Electrons actually move in the opposite direction, from the negative terminal to the positive one.)

Lamp 1 Lamp 2

Switch 1 Switch 2

simple circuits

Here is a simple electric circuit. It has a cell, a

lamp and a switch.

To make the circuit, these components are

connected together with metal connecting wires.

cell

lamp

switch

wires

simple circuitsWhen the switch is closed, the lamp lights up. This is because there is a continuous path of metal for the electric current to flow around.

If there were any breaks in the circuit, the current could not flow.

circuit safety• If you connect a wire between the two terminals,

the electrons will flow from the negative end to the positive end as fast as they can. This will quickly wear out the battery and can also be dangerous

• To properly harness the electric charge produced by a battery, you must connect it to a load. The load might be something like a light bulb, a motor, a resistor or an electronic circuit like a radio.

circuit diagram

cell switch

lamp

wires

Scientists usually draw electric circuits using symbols;

circuit diagramsIn circuit diagrams components are represented by the following symbols;

cell or battery

switch

lamp

motorammeter

voltmeter

buzzer

resistor

variable resistor

generic resistor

types of circuit

There are two types of electrical circuits;

SERIES CIRCUITS PARALLEL CIRCUITS

The components are connected end-to-end, one after the other.

They make a simple loop for the current to flow round.

SERIES CIRCUITS

If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out.

PARALLEL CIRCUITS

The current has a choice of routes.

The components are connected side by side.

If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight.

Ohm’s Law

I = V / R

Georg Simon Ohm (1787-1854)

I = Current (Amperes) (amps)

V = Voltage (Volts)

R = Resistance (ohms)

How you should be thinking about electric circuits:Voltage: a force that pushes the current through the circuit (in this picture it would be equivalent to gravity)

Resistance: friction that impedes flow of current through the circuit (rocks in the river)

How you should be thinking about electric circuits:

Current: the actual “substance” that is flowing through the wires of the circuit (electrons!)

How you should be thinking about electric circuits:

Would This Work?

Would This Work?

Would This Work?

The Central Concept: Closed Circuit

measuring current

Electric current is measured in amps (A)

using an ammeter connected in series in

the circuit.

A

measuring current

A A

This is how we draw an ammeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUIT

measuring currentSERIES CIRCUIT

PARALLEL CIRCUIT

• current is the same

at all points in the circuit.

2A 2A

2A

• current is shared

between the components

2A2A

1A

1A

copy the following circuits and fill in the missing ammeter readings.

?

?

4A

4A

4A

3A?

?

1A

?

3A

1A

1A

measuring voltage

The ‘electrical push’ which the cell gives to the

current is called the voltage. It is measured in

volts (V) on a voltmeter

V

Different cells produce different voltages. The

bigger the voltage supplied by the cell, the bigger

the current.

measuring voltage

Unlike an ammeter a voltmeter is connected

across the components

Scientist usually use the term Potential

Difference (pd) when they talk about voltage.

measuring voltage

V

This is how we draw a voltmeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUIT

V

V

measuring voltage

VV

V

series circuit

1.5V

• voltage is shared between the components

1.5V

3V

• voltage is the same in all parts of the circuit.

3V

parallel circuit

3V

3V

measuring current & voltage

copy the following circuits on the next two slides.

complete the missing current and voltage readings.

remember the rules for current and voltage in series and parallel circuits.

measuring current & voltage

V V

6V4A

A

A

a)

measuring current & voltage

V

V

6V4A A

A

A

b)

answers

3V 3V

6V

4A 4A6V

6V

6V4A 4A

2A

2A

4A

a) b)

What is Magnetism?

Magnetism is the force of attraction or repulsion of a magnetic material due to the arrangement of its atoms, particularly its electrons.

Atoms themselves have magnetic properties due to the spin of the atom’s electrons.

These areas of atoms are called “domains”

Groups of atoms join so that their magnetic fields are all going in the same direction

When an unmagnetized substance is placed in a magneticfield, the substance can become magnetized.This happens when the spinning electrons line up in thesame direction.

An unmagnetized substance looks like this…

While a magnetized substance looks like this…

Iron

Lodestone (Magnetite)

Which of the metals below are magnetic metals?

aluminium (Al)

silver (Ag)

iron

(Fe)

gold (Au)nickel (Ni)

cobalt (Co)copper (Cu) zinc (Zn)

magnesium (Mg)

Magnetic materials

N

S

Al ZnAg

Cu

Au

Mg

Fe Ni

Co

A magnetic material is attracted to a magnet.

Magnetic materials

Only iron (Fe), nickel (Ni) and cobalt (Co) are magnetic.

ELECTRICITY MAGNETISM

• How can you tell?Compass needle moves when near electrical current - creating an magnetic field

Electric currents cause magnetism

MAGNETISM ELECTRICITY

• How can you tell?

Electromagnetic inductionmoving a magnet in and out of a coiled wire created an electrical current without a battery

A moving magnet can generate electricity

Magnets have two ends, usually marked "north" and “south“

Magnets attract things made of steel or iron

Fundamental law of all magnets: Opposites attract and likes repel.

An electromagnet works in the same way, except it is "temporary" the magnetic field only exists when electric current is flowing.

An Electromagnet

                               

This magnet is able to pick up small steel things like paper clips, staples and tacks.

Simple electromagnet

Strength of Electromagnets

Depends on three main things:• Number of turns in the coil

– The more turns in a coil, the stronger the magnet

• Amount of current in the coil– If the voltage is increased, so will the

current, which will increase the strength of the magnetic field