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Electrical Resistance
Electrical 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
The ammeter measures the current
flowing through the bulb. It is
connected in series with the bulb.
A
V
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 =
Potential difference (volts)
Current (amps)
(ohms)
Using
symbols
eg
R=
R=
V
I
4.5V
0.5A
= 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
4 ohm
V
V= I R
V= 2x4
V = 8 Volt
Resistance Practical
Set up the circuit below.
Measure the current through and the voltage
across the resistor.
Record your results.
Calculate the resistance of the resistor.
Component
Current
A
A
V
Repeat the experiment
with different
components
(amps)
Potential difference
(volts)
Resistance
(ohms)
resistor
bulb
Resistance
Resistance is a measure of how difficult it is for
electrons to flow through something.
More bulbs,
more resistance,
less current
The high resistance filament converts more energy
into heat and light than the low resistance cable.
Circuit Y has twice the resistance of circuit X.
However, there is no extra voltage and so
the current is halved.
atoms block the paths
of 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 routes
for the electrons so have LOWER
RESISTANCE
electrons
The moving electrons give the atoms some
extra 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 difference
across 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 )