3rd/4th form – Electric circuits

Conductors and insulators

• An electric current is a flow of charge.• These charges are often electrons.• Electrons carry a negative charge.• Electrons are usually bound to atoms, but some

more strongly than others• In a conductor some charges are able to flow so

we can use them in an electric circuit.• In an insulator the electrons are held tightly in

position and so are unable to move. These materials do not conduct electricity.

Conductors and insulators

• Generally, most metals are good conductors and most non-metals are not.

• An electric cable makes use of both types of material.

• There is a third category, semi-conductors, which are also very useful (not GCSE)

Metal wires to carry electricity

Plastic insulation to isolate metal wires

Conduction electrons

• Metals have lots of ‘free’ electrons– weakly bound outer electrons

• so they are good conductors.

• Normally they move around randomly

• When an electric field is applied they move in a common direction

Counting charges

• The charge on a single electron is tiny

• We define a “useful” quantity of charge which allows sensible measurements, called the Coulomb.

• 1C is equal to the charge on 6.24151×1018 electrons– That’s 6,241,510,000,000,000,000 electrons!– Ammeters need to go in the circuit so the

current can flow in and out of them.

Electric current• An electric current is a flow of electric

charge (pushed along by a voltage).

• Where do these charges come from?

• In a circuit, all the wires and components are full of electrons– so as soon as a power supply is connected a

current starts flowing– No time delay with long wires!– (Electrons move quite slowly)

Electric Current

• Current is a flow of charge

• We can define current as the number of coulombs flowing past a point in 1 second

• When 1C of charge flows through a wire in 1s, the current is 1A.

t

QI

Current (A)

Charge (C)

time (s)

Examples

1. What is the current when 4 C of charge flows for 2 seconds?

2. What is the charge flowing through a wire in5 s if the current is 3 A?

Measuring currentAMMETER

• Ammeters need to go in the circuit so the current can flow in and out of them.

• To just indicate the flow of current without measuring it we can use an indicator light.

An ideal ammeter does not affect the circuit, no energy is transferred to it: it has zero resistance

Conventional Current• Charges come in two flavours, + and –• Conventional current is defined as the

direction in which + charges flow• so in a wire, the conventional current is in

the opposite direction to the electron flow

Circuits: a reminder

• Circuit diagrams are simplified drawings

• Circuit diagrams are drawn with a ruler!– Connections must connect– There are symbols for each component

Circuit symbols

• You’ve just got to learn them!

• Full list is on p. 242

Electric current

• When a circuit with a battery is competed, the battery “pushes” the charges around.

• Electric current is never “used up” as electrons flow around a circuit. – The current is the same through all

components in series circuit– All ammeters read the same:

– (note connect in series)A

Electrical Energy

• Electricity is useful because it can be easily converted into other types of energy.

• A battery or power supply gives electrical energy to the electrons in a circuit.

• Other circuit components then convert this to different forms of energy.

Potential Difference (“Voltage”)

• The voltage between two points in a circuit is a measure of how much energy is transferred to or from the charges as they pass between those points.

• The voltage of a power supply is a measure of how hard it “pushes” charge– So a larger voltage supply means a

larger current will flow in a circuit

Voltage

• Voltage is a measure of the energy change experienced by charges

• We define voltage as the number of joules transferred per coulomb of charge

• When charge passes through a p.d. of 1V, 1J of energy is transferred per coulomb.

Q

EV

voltage (V)

energy (J)

charge (C)

Examples

1. When 2C of charge pass through a battery they acquire 24J of energy. What is the voltage of the battery?

2. The p.d. across a lamp is 3V. How much energy is transferred when 5C of charge pass through it?

3. (harder) How much energy is transferred when a current of 2A flows through a lamp with a p.d of 3V across it for 1 minute?

Combining cells

• For batteries in series, the supply voltage is the sum of the individual batteries– Adding more batteries in series

increases voltage and therefore increases current

• For batteries in parallel, the supply voltage is the same as a single branch, but the battery capacity is increased (battery lasts longer)– Adding more batteries in parallel

does not affect voltage or current

V

7.5 V

V

3 V

Measuring voltage

• To measure the voltage between two points we connect a voltmeter in parallel across those two points

Here we are measuring the voltage across the resistor R

An ideal voltmeter does not affect the circuit, no energy passes through it: it has infinite resistance

Resistance

• Resistance is the opposition to current flow displayed by components– for a fixed voltage, the larger the

resistance, the smaller the current

• Resistance of connecting wires is usually so small it is ignored

• Resistors dissipating energy get hot!– e.g. lamp filament

Resistance• Resistance is caused by collisions between the

free charges and the lattice of atoms which makes up the conductor

• Each collision transfers energy to the atoms of material – material heats up

• A high current means more collisions – resistor gets hotter

Resistance

• Resistance is a measure of the opposition to current flow

• We define the resistance of a device as the voltage needed to push a given current through it

• When a p.d. of 1V causes a current of 1A to flow through a device, its resistance is 1

I

VR

resistance ()(Ohms)

voltage (V)

current (A)

Examples

• If a lamp has a current of 3A when there is a p.d. of 12V across it, what is the resistance of the lamp?

• What is the current through a 100 resistor with a p.d. of 5V across it?

• A real ammeter has a resistance of 0.5. What will the p.d drop across it be when a current of 5A is flowing?

Ohm’s Law

• “The current through a conductor is proportional to the current across it, provided the temperature remains constant”– Generally true for metals– Not true for all components

0 5 10 15 20 25 30 35 40 Current (mA)

6 5 4 3 2 1 0

Voltage (V)

Voltage, Resistance & Current

• We have

• So for a given circuit:– What happens to the current if we increase the

voltage of the power supply?– What happens to the current if we increase the

resistance of the components?

I

VR

resistance ()

voltage (V)

current (A)

Resistors limit current

• They can be used to protect vulnerable circuit components in case of a fault

• Variable resistors can be used to control things

Series and Parallel Circuits

• A series circuit has only one path for the current to flow, all the components are joined together in a continuous line.

• A parallel circuit contains branches, the current splits and recombines.– Each branch is unaffected by the

other branches

AA

Current in series circuits

• Remember, current is never “used up” as electrons flow around a circuit.

• The current is the same through all components in series circuit– All ammeters read the same

AA

AAAA

Voltage in a series circuit

• The energy transferred to the charge by the battery = the energy dissipated by all the components in the circuit

Vbattery

V1 V2 V3

321 VVVVbattery

• The largest resistance has the largest voltage across it (most energy transferred)

– If all resistances are equal, the battery voltage is divided equally

Think about this circuit…• What happens to V1

as the resistance is increased?

• What happens to V2?

• What happens to the current?

• What effect will this have on the lamp?

Vbattery

V1 V2

A

Voltage in a parallel circuit

• All components connected to a battery in parallel have the same voltage across them.

Vbattery321 VVVVbattery

• The current through each component is the same as if the other components weren’t there.– We can use V=IR on each branch

in turn

Vbattery

AA

Current in a parallel circuit

• The total current through the battery is equal to the sum of the currents through each parallel branch

321 IIIIbattery • The smallest current flows

through the branch with the highest resistance.

Ibattery

I1

I2

I3

Characteristics of circuits

• Series:– simple– one switch affects all components– one broken component affects all

components– voltage is shared between components

• Parallel:– components can be switched individually– one broken component only affects its

own branch– all branches receive the full supply

voltage