Resistance (Edexcel GCSE Physics)

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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 & Variable Resistor, downloadable IGCSE & GCSE Physics revision notes

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:

Ohm's law, downloadable AS & A Level Physics revision notes

  • This equation can be rearranged with the help of the following formula triangle:

VIR triangle (3), IGCSE & GCSE Physics revision notes

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.

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 Tip

Remember that 'voltage' and 'potential difference' are the same, either wording will be accepted in your exam answers

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.