A Rheostat Dimmer: Continuously Variable Resistance
1 · Predict
A rheostat (variable resistor) is wired in series with a lamp. As you slide the rheostat toward higher resistance, does the lamp's current rise or fall?
- Current falls as rheostat resistance rises — more total series resistance means less current.
- Current rises as rheostat resistance rises.
- Current doesn't depend on the rheostat setting.
2 · Set Up
- Open the dimmer preset and press Reset. The lamp is fixed at 100 Ω; the battery supplies 9 V.
- Enable the current readout on the lamp.
- Set the rheostat's resistance for each trial and record the lamp current.
3 · Collect Data
| Rheostat resistance R_rh (Ω) | Lamp current I (A) |
|---|---|
| 0 | |
| 50 | |
| 100 |
Plot current I (y-axis) against rheostat resistance R_rh (x-axis) for your three trials.
4 · Analyze
- For one trial, compute I = E/(R_lamp + R_rh) using E = 9 V, R_lamp = 100 Ω. Compare to the table.
- The rheostat and lamp are in series, so this is the same relationship as series-resistors — except now one 'resistor' is continuously adjustable. Explain why sliding the rheostat smoothly dims or brightens the lamp.
5 · Extend
- Unlike a modern LED dimmer (which switches power on and off rapidly), a rheostat dimmer wastes energy as heat in the rheostat itself, exactly proportional to its own I²R. Explain why an old-style rheostat dimmer gets warm.
- This rheostat can be set anywhere from 0 Ω to 100 Ω. What's the maximum possible current (rheostat at 0 Ω) and minimum possible current (rheostat at maximum)? Use your formula to check both limits.
The Physics Behind This Experiment
Rheostat as a Series Resistance
A rheostat acts as an adjustable series resistor: I = E/(R_lamp + R_rh). Sliding it toward higher resistance continuously reduces the current, dimming the lamp smoothly rather than switching it fully on or off.