Efficiency of a Light Bulb

Purpose

Determine how much of the electrical energy supplied to a bulb becomes light, and how much becomes heat.

Safety

Bulbs get hot enough to burn. Let them cool before touching. Watch for water near electrical equipment.

Method

Direct measurement of light output needs equipment we do not have, so we measure the waste heat instead, by warming a known mass of water, and treat the rest as light.

$$Q=mc\Delta T$$

where $c=4.18J/(\mathrm{g}{\cdot }^{\circ }\mathrm{C})$ for water.

Procedure

1. Record the bulb’s power rating.
2. Suspend the bulb in a container holding $200\mathrm{g}$ of water without touching the sides.
3. Record the starting temperature.
4. Switch on for exactly 5 minutes, stirring gently.
5. Record the final temperature.
6. Repeat with an LED of similar brightness.

Data

	Incandescent	LED
Power rating (W)
Mass of water (g)	200	200
Initial temp (°C)
Final temp (°C)
$\Delta T$ (°C)
Heat absorbed $Q$ (J)
Electrical energy $E=Pt$ (J)
Percent wasted as heat

Analysis

1. Which bulb wasted more energy as heat? By how much?
2. This method underestimates waste heat. Give two reasons.
3. An LED costs more up front. Using your percentages and a rate of $0.13\text{textdollar}/kWh$, estimate how long it takes to pay for itself at 4 hours a day.

Hand in

• Completed table with calculations shown
• Analysis answers
• A statement of the largest source of error and its likely direction

Curriculum connection

D2.8

determine the efficiency of various electrical devices that consume or produce electrical energy, and identify the energy transformations in each device

Link to original

A1.2

apply a scientific experimentation process and associated skills to conduct investigations, making connections between their observations and findings and the scientific concepts they are learning

Link to original