Circuits
Dimmer (rheostat + bulb)
Drag the rheostat’s slider to change its resistance and watch the bulb brighten or dim.
Dimmer (rheostat + bulb) — interactive Circuits simulation. Drag the rheostat’s slider to change its resistance and watch the bulb brighten or dim. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Variable resistance
A rheostat in series acts as a dimmer: more resistance means less current and a dimmer bulb.
The rheostat slider changes the resistance in the loop. Total current drops as R_rheostat rises.
- I = ε / R_total
- P_lamp = I²·R_lamp
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
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?
Predictions to weigh
- Current rises as rheostat resistance rises.
- Current falls as rheostat resistance rises — more total series resistance means less current.
- Current doesn't depend on the rheostat setting.
Variable roles
What you set:
- Rheostat resistance R_rh (Ω)
What you measure:
- Lamp current I (A)
How the investigation runs
- 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.
Governing equation
Rheostat as a Series Resistance — V = I·R
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.
What the printable worksheet asks students to work out
- 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.
Where this shows up beyond the lab
- 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.
- AP Physics 2 — Unit 11: Electric Circuits
- AP Physics C: Electricity and Magnetism — Unit 11: Electric Circuits
- IB Physics — B.5 Current and circuits
- General High School Physics — Electricity & DC circuits
- NGSS High School Physics — Electric current and magnetic fields
- Welcome to Dimmer
- Select the bulb
- Press Play
- Slide to dim
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.