Ohm's law: voltage, current and resistance
Measure the current through resistors and a bulb as you add cells, plot current against voltage, and find the resistance.
Core concept
For a resistor at constant temperature, current is proportional to voltage: V = I × R
Topic: Electricity
An ammeter, joined in series, measures the current: The flow of electricity through a wire, measured in amperes (A). All key words (I) in amperes. A voltmeter, joined in parallel across a component, measures the potential difference or voltage: The push that makes electricity flow, measured in volts (V). All key words (V) across it in volts. The resistance: How much something slows down the flow of electricity, measured in ohms (Ω). All key words (R) of the component, in ohms, is R = V ÷ I. Ohm's law says that for a metal resistor kept at a steady temperature: How hot or cold something is, measured in degrees Celsius (°C). All key words, 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
- Use the 10 Ω resistor. Record the voltage and current with 1, 2, 4 and 6 cells.
- Repeat for the 20 Ω resistor with at least two different numbers of cells.
- Replace the resistor with the bulb and record with 1 and 6 cells.
- 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.