Resistance (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

• Resistance is defined as:

The opposition to current

• Resistance occurs because the free electrons flowing in the circuit (current) collide with the metal ions in the wire
• These collisions slow down the electrons, or, in other words, resist their flow
  • The higher the resistance of a circuit, the lower the current
  • This means that good conductors have a low resistance and insulators have a high resistance

• The resistance of a circuit can be increased by adding resistors (or variable resistors) to it
• Every electrical component has a resistance, even wires
  • In exam questions, the resistance of the wires and batteries are assumed to be negligible

The effect of resistance on the current in a circuit

       When a circuit has a high resistance, a lower current will flow, and vice versa

Ohm's law

• Current, , potential difference, , and resistance, , all affect one another
• Changing any one of these in a circuit, changes all of them
• Current and resistance are inversely proportional 
  • If the resistance is doubled, current will halve
• This relationship is described by the following equation, known as Ohm's law

• Where
  • = resistance, measured in ohms (Ω)
  • = potential difference, measured in volts (V)
  • = current, measured in amperes or amps (A)

Extended tier only

• The relationship between current and potential difference of a component can be shown on a current-voltage (I-V) graph

• When the relationship between current and potential difference is linear:
  • the I-V graph is a straight line which passes through the origin
  • the resistance is constant
  • these are known as ohmic resistors

• When the relationship between current and voltage is non-linear:
  • the I-V graph that is not a straight line
  • the resistance is not constant
  • these are known as non-ohmic resistors

Current-voltage (I-V) graph for a resistor and a filament lamp

Linear IV graphs are straight lines through the origin, indicating a constant resistance. Non-linear IV graphs are curved, indicating a variable resistance

• Components with linear I-V graphs (ohmic resistors) include:
  • fixed resistors (at constant temperature)
  • wires (at constant temperature)

I-V graph for ohmic conductors

• The relationship between current and voltage for a wire or fixed resistor is linear, or directly proportional, which means
  • the IV graph is a straight line, so voltage and current increase (or decrease) by the same amount
  • the slope of the graph is constant, so resistance is constant

I-V graph for a a wire of fixed resistor

The current is directly proportional to the potential difference (voltage) as the graph is a straight line through the origin