Download OHM`S LAW - Eyemouth High School

REVISION NOTES
OHM’S LAW
NATIONAL 5
A water pump can push water through a pipe. If the pipe is large, the water flows
easily and quickly. If the pipe is narrow, it is harder to push the water along the pipe.
In the same sort of way, a battery pushes electric charge around a circuit. We can
measure how easily the charge moves round by the idea of resistance. The higher
the resistance, the more difficult it is to get charge to flow.
This is summed up by Ohm’s Law:
V=IxR
V = voltage (potential difference)
I = current
R = resistance
Resistance is measured in ohms (). It is quite common to use kilo-ohms (k) and
megaohms (M).
Resistance measures how much a circuit opposes the current.
Example
A current of 200 mA passes through a 500  resistor. Calculate the potential
difference (voltage) across the resistor.
I = 200 mA = 0.2 A
R = 500 W
V=?
V = IR
= 0.2 x 500
Confirming Ohm’s Law
This circuit is used to measure the current in a resistor at
different voltages.
If we plot a graph of voltage (p.d.) against current, we
get something like this:
The gradient of this graph gives the value of the
resistance (as R = V/I)
= 100 V
REVISION NOTES
OHM’S LAW
NATIONAL 5
Resistance and temperature
Many components that we put in circuits are ‘ohmic’ – this means that they obey
Ohm’s Law:
V = IR
However, some things are ‘non-ohmic’ – they do not obey Ohm’s Law.
A good example is filament of an old-fashioned light bulb. When the bulb is switched
on, a current starts to pass through the filament. This heats up the filament. As the
temperature of the filament increases, its resistance also increases.
This graph on the left shows an ohmic component, like an ordinary resistor. The
voltage is directly proportional to the current. The gradient of the graph gives the
resistance. The gradient doesn’t change, so the resistance doesn’t change.
The graph on the right shows a non-ohmic component, like a bulb filament. See how
the graph gets steeper as the current increases and the filament gets hotter. The
steeper the gradient, the higher the resistance.
Some components behave in the opposite way.
The resistance of a thermistor reduces as the
thermistor gets hotter. This graph shows how the
temperature of a thermistor affects its resistance.