Resistance (Edexcel GCSE Physics)
Revision Note
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Resistance & Current
Resistors come in two types:
Fixed resistors
Variable resistors
Fixed resistors have a resistance that remains constant
Variable resistors can change the resistance by changing the length of wire that makes up the circuit
A longer length of wire has more resistance than a shorter length of wire
This therefore can vary the amount of current through the circuit:
The larger the resistance, the smaller the current
The smaller the resistance, the larger the current
For fixed and variable resistors, once the resistance is set, it will stay at this value no matter how the current changes
However, the resistance of components such as lamps, diodes (diodes and LEDs), thermistors and LDRs (light dependent resistors) changes with the current through the component
Fixed and variable resistor circuit symbols
Calculating Resistance
The current, resistance and potential difference of a component in a circuit are calculated using the equation:
This equation can be rearranged with the help of the following formula triangle:
Voltage, current, resistance formula triangle
Worked Example
Calculate the potential difference through a resistor of resistance 10 Ω if there is a current of 0.3 A through it.
Answer:
Step 1: List the known quantities
Resistance, R = 10 Ω
Current, I = 0.3 A
Step 2: Write the equation relating resistance, potential difference and current
V = IR
Step 3: Substitute in the values
V = 0.3 × 10 = 3 V
Examiner Tips and Tricks
Remember that 'voltage' and 'potential difference' are the same, either wording will be accepted in your exam answers
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