Resistance (Edexcel GCSE Physics: Combined Science)

• 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

• 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

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