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Physics 06254.2 Electrical quantities

Ohm's law and I–V graphs

Sweep the voltage across a resistor, a filament lamp or a diode and plot the I–V characteristic live.

  • V = IR
  • Non-ohmic components
  • I–V characteristics
V (volts)I (amps)

Voltage

3.0V

Current

0.3A

Measured R = V/I

10Ω

Power = IV

0.9W

Predict

For a filament lamp, what happens to V/I as the voltage increases?

Experiment

  1. 1With the fixed resistor, check that V/I stays the same at 1 V, 3 V and 5 V.
  2. 2Switch to the filament lamp and repeat. Watch the "measured R" readout climb.
  3. 3Switch to the diode and sweep the voltage negative. Notice the current stays near zero.

Explain

Ohm’s law says that for a metallic conductor at constant temperature, current is directly proportional to potential difference. That is the straight line through the origin you get with the fixed resistor.

A filament lamp breaks the condition: as current flows the filament heats up, the ions vibrate more, the electrons are impeded and the resistance rises. The graph flattens.

A diode conducts in one direction only, and needs about 0.7 V before forward current rises sharply.