Resistance (R) – the resistance of a device is the ratio of the voltage applied to it, to the current passing through it.
The resistance of a wire or component reduces the flow of charge passing through it. The more components that you connect in series in a circuit the higher the resistance.
Resistance can be calculated using the following equation:
Where V is the potential difference measured in volts (V) and I is the current measured in amps (A).
The SI unit for resistance is the Ohm:
Ohm (Ω) – one ohm is equal to one volt per amp.
Ohm’s Law
Ohm’s Law – when physical conditions (such as temperature) remain constant, the current passing through a conductor is directly proportional to the potential difference across it.
If we rearrange the equation for resistance we can see this relationship:
As well as being directly proportional to the potential difference, the current is inversely proportional to the resistance.
Worked Examples
Example 1
A resistor has a resistance of 20kΩ. Calculate the potential difference across the resistor when a current of 15mA flows through it.
Example 2
15C of charge pass through a resistor with a resistance of 15Ω. Calculate the current flowing through the resistor when 60J of energy pass through it.
This question requires knowledge of potential difference.
Example 3
The potential difference across a resistor is 5V. 1200C of charge pass through the resistor in 60s. Calculate the resistance of the resistor.
This question requires knowledge of charge and current.
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