Current–Voltage Characteristics (AQA A Level Physics)

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:

• 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

Circuit for plotting graphs of current against voltage

• 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

Circuit for plotting graphs of current against voltage

• 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

I–V characteristics for an ohmic conductor (e.g. resistor), semiconductor diode and filament lamp

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