Circuits
Switched lamp
Flip the switch — the bulb lights, brightness ∝ power
Control a lamp with a switch in a simple circuit. See how opening and closing the switch affects current flow — a hands-on intro to circuit continuity.
Switch control
A switch in series controls whether current reaches the bulb. When closed, the lamp lights; brightness depends on power.
Opening the switch breaks the loop and stops current. Closing it completes the path so power P = I·V is delivered to the bulb filament.
- P = I·V
- Open switch → I ≈ 0
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 9 V battery lights a bulb through a closed switch. If you swap in a bulb with lower resistance, does it glow brighter or dimmer?
Predictions to weigh
- A lower-resistance bulb glows dimmer.
- A lower-resistance bulb draws more current and dissipates more power — it glows brighter.
- Brightness doesn't depend on the bulb's resistance.
Variable roles
What you set:
- Bulb resistance R (Ω)
What you measure:
- Current I (A)
- Power dissipated P (W)
How the investigation runs
- Open the switched-lamp preset and press Reset. The switch starts closed, so the 9 V battery lights the lamp.
- Enable the power readout on the lamp.
- Set the lamp's resistance for each trial and record the power dissipated.
Governing equation
Electrical Power (from I and R) — P = I²·R
Power dissipated as heat and light in a resistive load is P = I²R. At fixed voltage, a lower-resistance load draws more current, and power grows even faster since it depends on the square of current.
What the printable worksheet asks students to work out
- For one trial, compute I = E/R using E = 9 V, then P = I²R (or equivalently E²/R). Compare to the table.
- Explain why a lower-resistance bulb, which draws more current at the same voltage, ends up dissipating more power — brightness in a real bulb scales with power.
Where this shows up beyond the lab
- A '100 W' light bulb at 120 V draws about 0.83 A and has a resistance of about 144 Ω when hot. Using P = E²/R, explain why a 'higher wattage' bulb rated for the same voltage must have LOWER resistance, not higher.
- Incandescent filaments often fail right when switched on, when they're cold and their resistance is lowest. Using your power relationship, explain why the current (and power) surge is highest at that moment.
- 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 Switched Lamp
- Select the bulb
- Press Play
- Flip the switch
- 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.