About this experiment
Build the classic Ohm's law circuit: a cell, a switch, an ammeter in series and a voltmeter across the component. Change the supply voltage, record each pair of meter readings and plot the current-voltage graph. A fixed resistor gives a straight line through the origin, while a filament bulb gives a curve because its resistance rises as it heats up.
Learning goals
- Connect an ammeter in series and a voltmeter in parallel with a component
- Show that the current through a fixed resistor is proportional to the voltage across it
- Calculate resistance from meter readings using R = V / I
- Explain why a filament bulb does not obey Ohm's law
How to use it
- 1Keep the fixed resistor at 20 Ω. Set the supply to 0 V and press Record reading.
- 2Raise the voltage in steps of 1 or 2 V and record a reading at each step.
- 3Compare the points with a straight line through the origin, then change the resistance and record a second set.
- 4Switch to the filament bulb, repeat the readings and compare the shape of the graph.
The model
Assumptions
- The cell has no internal resistance, so the voltmeter reads the full supply voltage when the switch is closed.
- The ammeter has zero resistance and the voltmeter draws no current.
- The fixed resistor stays at constant temperature. The bulb curve is an empirical fit for a small tungsten filament lamp.
- The moving dots show the direction of charge flow. Their speed is exaggerated but stays proportional to the current.
Try this in class
- Q1Double the voltage across the 20 Ω resistor. What happens to the current?
- Q2Two resistors give two straight lines. Which line belongs to the larger resistance, and how can you tell from the slope?
- Q3Open the switch. Why do both meters read zero even though the cell still has a voltage?
- Q4Use your bulb readings to calculate R at 1 V and at 12 V. Why are the values so different?
Frequently asked questions
What does Ohm's law actually say?
For a metal conductor kept at constant temperature, the current is directly proportional to the potential difference across it. The constant ratio V / I is its resistance.
Why is the ammeter in series and the voltmeter in parallel?
An ammeter measures the charge flowing through a point, so all the current must pass through it. A voltmeter compares the energy per charge at two points, so it connects across the component.
Is the filament bulb a broken resistor?
No. It still satisfies R = V / I at every point, but its resistance changes with temperature, so the I-V graph is a curve instead of a straight line. Components like this are called non-ohmic.
Why do the electrons move the opposite way to the current arrows?
Conventional current was defined as the flow of positive charge before electrons were discovered. In metal wires the charge carriers are electrons, which drift from the negative terminal to the positive terminal.