ConductorsMetals and graphite are electrical conductors.Electrons are free to move in conductors. Electrons are negatively charged. The electrons carry the energy from the power supply to the appliance.A movement of electrons is called an electric current.

Total charge = current x time Q = I x t

Coulombs , C Amps, A seconds, s( Basic electrostatic expts )

Calculation

• Calculate the current flowing if 500 C of charge flow around a circuit in 5 minutes.

• I = ? Q = 500 C, t = 5 mins = 300 s• Q = I x t

At

QI 67.1

300

500

Current

• Current is a measure of the number of charges that flow per second around a circuit.

• Units are Amps, A• Instrument : ammeter, connected is series.

Potential Difference (Voltage)

• Voltage is a measure of the energy given to each charge in the circuit . The bigger the voltage the more energy is transferred.

• Units : Volts, V• Instrument : Voltmeter • connected in • parallel

• Expts on series / parallel rules for • current/ potential difference• and useful circuits.

Series Circuits

• Components are connected in ‘line’

• Current is the same AT ALL points

Series Circuits

Sum of potential differences = Supply potential difference

Vs = V1 + V2 + V3

Remember that potential difference is a measure of energy. This equation tells us that the total energy available from the supply equals the energy used to drive current through resistor 1 plus the energy used to drive current through resistor 2 plus the energy used to drive current through resistor 3

Parallel Circuits

• Components are arranged ‘above each other’

• Total current drawn from supply equals sum of currents flowing through each component

• It = I1 + I2 + I3

Parallel Circuits

• Potential difference across each branch equals supply potential difference

• Investigation of ring circuit

Household wiring

• Appliances connected in parallel• 230 V across each• Independent switching• Power circuit( plugs ) is wired as a RING

circuit

Ring Circuit

• Current splits two ways• Thinner cable can be

used• Less heating effect in

each branch• Easy to add extra

sockets

Neutral

Live

Direct Current , D.C.

• Electrons flow in one direction only• A battery produces direct current as it is

always pushing electrons in the same direction.

Voltage

Time

Alternating Current, A.C.• Alternating current the current flows back / forwards around

the circuit.• The frequency of the supply is the number of cycles per

second. Mains frequency is 50 Hz• I cycle = 1/50 s• Time for 1 cycle ( period) = 1 / frequency ( T = 1 /f )

Time

Voltage

Alternating Current, 2

• The effective value of British mains is 230 V. This is called the quoted value and is less than the peak value.

Peak value, measured on an oscilloscopeQuoted value , measured using a meter.

To Measure Peak Voltage 1. Count the number of boxes from the trough to crest

2. Divide by 2 ( this is from the 0V line to the crest )

3. Multiply the number of boxes by the Y gain setting

Ex. 4 boxes from trough to crest

Therefore from 0V line to crest

= 2 boxes

2 x 5 = 10 V

Y gain = 5V per box

0V Line

Resistance

• This is a measure of the opposition to flow of current• Units are Ohms,Ω• Symbol for resistor• If resistance increases then current decreases

( provided supply pd is constant )

Ohm’s Law

• For a constant temperature

pd ( V ) current ( A ) resistance (Ω)

Calculate the resistance of a 12 V car bulb that draws a current of 5 A. V = 12 V I = 5 A R = ? V = I x R

RxIV

AI

VR 4.2

5

12

Ohm’s law 2

The gradient of the graph is equal to which equals = resistance

Heating conductors

A the temperature increases the gradient and hence resistance increases. As temperature increases the particles vibrate more making it more difficult for the power supply to push charges around the circuit.

Series ResistorsThe total resistance is the sum of the individual resistances.

Rt = R1 + R2 + R3

Parallel ResistorsThe effective resistance is always smaller than the smallest resistor in the network.

Ex : Calculate the effective resistance of the circuit shown.

R1 = 10 Ω R2 = 15 Ω Rp = ?

65

3030

5

30

2

30

3

15

1

10

1111

21

p

p

R

RRR

21

111

RRRp

Short circuits / open circuits

Potential DividersThese divide the voltage up . The resistors are in series therefore the same current flows through each.

The bigger the resistance the bigger the share of the voltage.

2

1

2

1

R

R

V

V

Potential divider circuitsExample:

VS = V1 + V2

VS = 18 + 6VS = 24 V

V1

V2

R1

R2

=

V1

612

4=

V1x 612

4=

V1 = 18 V

4 W

12 W

6 V

V1

VS

4 W

12 W

6 V

V1

VSVS

4 W

12 W V1

4 W

12 W

6 V

V1

VS

4 W

12 W V1

VS

Potential Dividers

Calculate the voltage across the

20 Ω resistor.

V2 = ? R2 = 20 Ω , R1 = 30 Ω, Vs = 10 V

V

RR

RVV s

450

2010

3020

2010

12

22

12

22 RR

RVV S

Electronics

Systems:

All electronic systems can be simplified to the

following block diagram,

input process output

e.g. for a radio receiver,

aerial tuner speaker

decoder

amplifier

Output Devices

examples are,- speaker (electrical energy to sound energy)- buzzer (electrical energy to sound energy)- lamp (electrical energy to light energy)- motor (electrical energy to kinetic energy)- relay (causes other circuits to be switched)- solenoid (causes a straight movement)- seven segment display

Output devices are energy changers.

Light emitting diode , LED

This device changes electrical energy into light energy. It allows current only one way through it.

Flow of electrons

LED 2Unlike a lamp it produces very little heat energy

as it does not contain a wire filament.

It lights when the current is small (e.g. 10 mA).

To prevent it being damaged by too large a

current, it always has a resistor connected in

series.

LED 3example:

An l.e.d. is designed to operate at 2.0 V and 10 mA.

What size of resistor is needed when it is powered by a 9.0 volt battery?

9.0 V

2.0 V

Pd across resistor = 9 -2 = 7V

R = V/I = 7 / 0.01 = 700 Ω

10 mA = 0.01 A

Input Devices

• Two main kinds : energy changers such as a microphone or thermocouple

• Or the component changes the size of the input voltage as a physical property changes e.g. LDR or thermistor

Microphone

energy change is,

sound energy to electrical energy

Thermocouple

energy change is,

heat energy to electrical energy

Used to measure high temperatures

+-

Solar Cell

energy change is,

light energy to electrical energy

Thermistor

The resistance of a thermistor usually decreases as the temperature increases .

Light Dependant Resistor

The resistance of an LDR decreases as the light level increases.

LDR in potential divider circuit

V4.7 kW

6.0 V

Explain what happens as it gets darker:

1. Resistance of LDR goes up

2. Voltage across LDR goes up

3. Hence voltage across fixed resistor goes down

Thermistor in potential divider circuit

V

2.4 kW

9.0 V Explain what happens as it gets colder;

1. Resistance of thermistor goes up

2. Voltage across thermistor goes up

3. Voltage across fixed resistor goes down

Transistors

• Two types• NPN and MOSFET• Both act as voltage operated switches

MOSFETMetal Oxide Semiconductor Field Effect Transistor

The MOSFET has a different construction from the

NPN transistor and switches on at a higher voltage

(about 2 volts).

Gate

Drain

Source

NPN transistor

This switches on when the voltage across the base emitter is 0.7 V

Collector

Emitter

Base

Example of transistor switching

5.0 V

As the temperature increases, the resistance of the thermistor decreases.

As a result, the voltage across the thermistor decreases.

This causes the voltage across the variable resistor to increase which switches on the MOSFET, causing the l.e.d. to light.

Electrical energy 1When a current flows through a component there is an energy change.

In a resistor Ee Eh

In a loudspeaker Ee Es

Electrical energy 2Power is the rate at which energy is transferred

I W is equivalent to 1 Js-1

Power can also be calculated from :

Power = voltage x current

P = V x I

t

EP Joules, J

Seconds, sWatts,W

Electrical energy 3There are 2 other equations that can be used to calculate power :

1: Combine P = V x I and V= I x R

Substitute for V = I x R into P = V x I

P = I x R x I = I2 x R

2 : Substitute for into P = V x I

R

V

R

VxVP

R

VI

2

R

VI

R

V

R

VxVP

2

Electrical energy 4

• Four equations to calculate power :When energy ( joules ) and time ( seconds ) is

known

When current ( amps ) and voltage ( volts ) are known

When current ( amps ) and resistance (ohms ) are known

When Voltage ( volts ) and resistance (ohms ) are known

t

EP

IxVP

RxIP 2

R

VP

2

Electrical energy 5

Calculate the current flowing through a 230 V mains, 2 kW kettle.

P = 2 KW = 2000 W V = 230 V I = ?

P = V x I

AV

PI 7.8

230

2000

Electrical energy 6Calculate the voltage across a 4 Ω loudspeaker when it produces 20 W of sound power.

P = 20 W R = 4 Ω V = ?

VV

xRxPV

RxPV

R

VP

9.880

420

2

2

Electrical energy 7

Calculate the current flowing through a 2000 W, 20Ω resistor.

P = 2000 W, R = 20 Ω, I = ?

P = I2 x R

R

PI 2

20

20002 I

1002 I

AI 10100

ElectromagnetismThe term ‘electromagnetism’ comes from the fact that there is a magnetic field around a wire when there is an electric current in the wire.

The magnetic field is stronger when,- the current is higher- the wire is longer

The direction of the magnetic field reverses if the current flow reverses direction.

Electromagnetism

When a wire is moved through a magnetic field avoltage is generated (induced) in the wire.This can be when the magnet is stationary and the wire is moved, or when the wire is stationary and the magnet is moved. The induced voltage is greater when,

- the magnetic field is stronger- the movement is faster- the wire is wound into a coil