Electrical ResistanceElectrical resistance is a measure of how difficult it is for electricity (electrons) to flow through a material.

The units of resistance are ohms and the symbol is Ω.

Measuring Resistance

A

V

The ammeter measures the current flowing through the bulb. It is connected in series with the bulb.The voltmeter measures the potential difference (p.d.) across the lamp. It is connected in parallel with the lamp.

We can calculate the resistance of a component if we have measured the current and the voltage

Resistance =

(ohms)

Potential difference (volts)

Current (amps)

Using symbols R =

VI

eg R =4.5V0.5A

= 9Ω

Ohm’s law

The current through a conductor is directly proportional

to the potential difference across it provided the temperature remains constant.

A 1 Amp

A 2 Amp

*If the voltage doubles then the current doubles”

V = I R

Eg 1 Calculate the voltage across the resistor :

2 Amp

V

4 ohm

V = I R V = 2 x 4 V = 8 Volt

Resistance PracticalSet up the circuit below.

Measure the current through and the voltage across the resistor.Record your results.Calculate the resistance of the resistor.

A

V

A

Current (amps)

Potential difference (volts)

Resistance (ohms)

Component resistor bulb

Repeat the experiment with

different components

Resistance

Resistance is a measure of how difficult it is for electrons to flow through something.

The high resistance filament converts more energy into heat and light than the low resistance cable.

More bulbs,more resistance,

less current

Circuit Y has twice the resistance of circuit X. However, there is no extra voltage and so the current is halved.

atoms block the pathsof the electrons

electrons

long thin wires have a high resistance

Long thin wires have a high resistance so less current flows

Wide wires have many more routesfor the electrons so have LOWERRESISTANCE

electrons

The moving electrons give the atoms someextra kinetic energy, so the wire heats up.

This is used in kettles, hair dryers toasters,irons.

Doubling the voltage doubles the current !

Ohm’s law

The current flowing in a conductor is directly proportional to the potential differenceacross its ends, provided that the temperature remains constant.

• Potential difference, current and resistance are related by the equation:potential difference current resistance (volt, V) (ampere, A) (ohm, Ω )