How much energy does a tiny lamp use?

Situation and prediction

Two devices run for different lengths of time. The more powerful one need not use more energy in a day: duration matters. Before calculating, predict which teaching lamp receives more energy: 0.06 W for 60 s or 0.03 W for 180 s. We are extending the notebook: we now estimate electrical energy use as carefully as we estimated room heating.

New words, step by step

Power P is the rate of energy transfer, P = E/t. Its unit, the watt (W), is a joule per second. For a circuit component P = UI: each coulomb transfers U joules and I coulombs pass each second. Energy E = Pt = UIt is measured in joules. A watt-hour (Wh) is also energy: 1 Wh = 3600 J. A kilowatt-hour (kWh) = 1000 Wh is used on electricity bills; multiply kWh by a local tariff for cost, and tariffs depend on location and terms.

Working through supplied data

Supplied data give 3 V, 0.02 A and 60 s for a lamp. P = 3 × 0.02 = 0.06 W. E = 0.06 × 60 = 3.6 J = 0.001 Wh. A second device at 0.03 W for 180 s receives 5.4 J despite lower power. These numbers do not prove that every LED operates at 3 V and 0.02 A.

A safe check

Draw energy bars for the two supplied devices. If you have a household bill, find its kWh unit, but never alter wiring or open an electrical panel. The whole activity can be done on paper.

A reasoning trap

A mistake is to give the minute’s total as 0.06 W. Watts are a rate; the total is 3.6 J or 0.001 Wh. Energy received by a lamp is also not the quantity of charge stored by its battery.

Support map

Transfer to a new case

For 3 V, 0.01 A and 2 minutes, find power and energy. Convert minutes to seconds before multiplying.

Lab notebook entry

Add a page to My World Lab Notebook with four lines: question; prediction before reading the answer; observation or calculation from supplied numbers; revised conclusion and its conditions. If you did not do an experiment, do not write ‘measured’. After the diagram, explain where charge moves, where energy is transferred, and where the model simplifies a real device.

Next lesson: Checkpoint: design lighting for a model room