# Ohm’s Law & Lamp Power Lab - current vs voltage and resistance, V–A method, lamp power

> Virtual school electricity lab: current vs voltage and resistance, measuring resistance and lamp power, with wiring mistakes and faults explained.

- URL: https://qilingtools.com/en/physics/ohm-law-lab
- Category: Physics Tools (https://qilingtools.com/en/physics)
- Tags: [Physics](https://qilingtools.com/en/tags/physics), [Education](https://qilingtools.com/en/tags/education)
- Last updated: 2026-10-29
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

A virtual lab bench for the core middle-school electricity experiments, with a battery, switch, ammeter, voltmeter and rheostat. Investigate how current depends on voltage and on resistance, measure an unknown resistor with the voltmeter–ammeter method, and find a lamp’s power. Reversed meter terminals, wrong ranges, a rheostat wired to both top or both bottom terminals, or a slider not at maximum before switching on are all flagged in red with an explanation. Meters read to their real division size, and recorded data is plotted as I–U or I–1/R graphs automatically.

## How to use

1. Pick an experiment: current vs voltage, current vs resistance, measuring R, lamp power or circuit faults
2. Set the supply voltage, resistor, rheostat and meter ranges, then close the switch
3. Drag slider P, watch the meter needles and readings, and press Record to add a row to the table
4. Compare the graph and conclusion, then try the randomized Practice questions with instant marking

## Features

- Realistic ammeter and voltmeter dials (0–0.6 A / 0–3 A, 0–3 V / 0–15 V) read to one division
- Wiring mistakes highlighted: reversed terminals, off-scale, range too large, rheostat both-top or both-bottom, meters swapped, short circuit
- The current-vs-resistance experiment checks the voltage is held constant, says which way to move the slider and the minimum rheostat size
- Lamp filament resistance rises with temperature, giving a curved I–U graph, and too much voltage burns it out
- Fault-finding puzzles from lamp brightness and meter readings: open circuits, shorts and wiring errors
- Practice: Ohm’s law, meter reading, range choice, rated power, average resistance and more

## FAQ

### In the current-vs-resistance experiment, which way should the slider move after fitting a bigger resistor?

In series, a bigger resistance takes a bigger share of the voltage. After fitting a larger resistor its voltage rises, so to bring it back the rheostat must take more voltage too, which means increasing its resistance. The tool tells you which end that is for your wiring (bottom terminal A or B).

### How big must the rheostat be?

Use the largest fixed resistor. The rheostat voltage U₀ − U and the resistor voltage U are in the same ratio as their resistances, so R_rheostat = R(U₀ − U)/U. With a 4.5 V supply, 2 V held across the resistor and a 25 Ω resistor you need at least 31.25 Ω. A 20 Ω rheostat can never bring the voltage down to 2 V.

### Why can I average a resistor’s value but not a lamp’s?

A fixed resistor’s value hardly changes, so differences between readings are measurement error and averaging reduces it. A lamp filament’s resistance rises a lot as it heats up, so it really is different at each voltage and an average means nothing. The same goes for the lamp’s power at different voltages.

### The lamp is off, the ammeter reads zero and the voltmeter shows about the supply voltage. What is wrong?

Zero current means there is a break somewhere. A voltmeter reading near the supply voltage means its own path back to the battery is complete, so the break is in the part it is connected across: the lamp is open (broken filament or loose contact). A voltmeter wrongly placed in series gives the same symptoms.

### How do I measure a 3.8 V lamp when the 0–15 V range is too coarse?

The 0–15 V range has 0.5 V divisions and can’t read 3.8 V well. Instead, put the voltmeter on its 0–3 V range across the rheostat. When it reads the supply voltage minus 3.8 V (0.7 V for a 4.5 V supply), the lamp is at its rated voltage. You can try this wiring on the Lamp power tab.

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