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Grades 9 and 10 Intermediate About 25 min Version 1 Awaiting review Premium

Ohm's law: voltage, current and resistance

An ammeter, joined in series, measures the current (I) in amperes. A voltmeter, joined in parallel across a component, measures the potential difference or voltage (V) across it in volts. The resistance (R) of the component, in ohms, is R = V ÷ I. Ohm's law says that for a metal resistor kept at a steady temperature, the current is proportional to the voltage, so a graph of I against V is a straight line through the origin. A filament bulb does not obey Ohm's law: as it heats up, its resistance rises.

What you will learn

  • Connect an ammeter in series and a voltmeter in parallel.
  • Show that current is proportional to voltage for a resistor.
  • Calculate resistance from V and I, and explain why a bulb is non-ohmic.

Before you start

  • Build a simple series circuit.

Materials

  • Up to six 1.5 V cells
  • 5 Ω, 10 Ω and 20 Ω resistors, and a torch bulb

Equipment

  • Ammeter, voltmeter and connecting wires

Procedure

  1. Use the 10 Ω resistor. Record the voltage and current with 1, 2, 4 and 6 cells.
  2. Repeat for the 20 Ω resistor with at least two different numbers of cells.
  3. Replace the resistor with the bulb and record with 1 and 6 cells.
  4. Look at the graph of current against voltage.

How this simulation is simplified

  • Each cell gives 1.5 V; the meters are ideal and the wires have no resistance.
  • The bulb's filament resistance rises with voltage as R = 4 Ω + 3 Ω per volt.