Current–Voltage Characteristics (AQA A Level Physics): Revision Note

Ohm's law

• Ohm’s law states:

  For a conductor at a constant temperature, the current through it is proportional to the potential difference across it

• Constant temperature implies constant resistance

• Ohm's law is represented in the equation below:

$V=IR$

• Where:

  • $V$ = potential difference (V)

  • $I$ = current (A)

  • $R$ = resistance (Ω)

• The relation between potential difference across an electrical component (in this case, a fixed resistor) and the current can be investigated through a circuit such as the one below

• By adjusting the resistance on the variable resistor, the current and potential difference will vary in the circuit

• Measuring the variation of current with potential difference through the fixed resistor will produce a straight line graph, such as the one below

• Since the gradient is constant, the resistance $R$ of the resistor can be calculated by using 1 ÷ gradient of the graph

• An electrical component obeys Ohm’s law if its graph of current against potential difference is a straight line through the origin

  • A resistor does obey Ohm’s law

  • A filament lamp does not obey Ohm’s law

• This applies to any metal wires, provided that the current isn’t large enough to increase their temperature

I–V characteristics

• As the potential difference (voltage) across a component is increased, the current also increases (by Ohm’s law)

• The precise relationship between voltage and current is different for different components and can be shown on an I-V graph

  • For an ohmic conductor, the I–V graph is a straight line through the origin

  • For a semiconductor diode, the I–V graph is a horizontal line that goes sharply upwards

  • For a filament lamp, the I–V graph has an 'S' shaped curve

Ohmic conductor

• The I–V graph for an ohmic conductor at constant temperature e.g. a resistor is very simple:

  • The current is directly proportional to the potential difference

  • This is demonstrated by the straight-line graph through the origin

Semiconductor diode

• The I–V graph for a semiconductor diode is slightly different

• A diode is used in a circuit to allow current to flow only in a specific direction:

  • When the current is in the direction of the arrowhead symbol, this is forward bias

    • This is shown by the sharp increase in potential difference and current on the right side of the graph

  • When the diode is switched around, it does not conduct and is called reverse bias

    • This is shown by a zero reading of current or potential difference on the left side of the graph

Filament lamp

• The I–V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference

• This is because:

  • As the current increases, the temperature of the filament in the lamp increases

  • Since the filament is a metal, the higher temperature causes an increase in resistance

  • Resistance opposes the current, causing the current to increase at a slower rate

• Where the graph is a straight line, the resistance is constant

• The resistance increases as the graph curves

• The filament lamp obeys Ohm's law for small voltages