 Light Bulb Efficiency and Hand Crank Energy Generation Printer-friendly version

What does it really take to power a light bulb?

Big Ideas:
• Energy can be converted from one form to another (e.g., mechanical to electrical).
• By comparing the amount of power used to create electricity with the amount of electrical power actually generated, we can estimate how efficient we are at making electricity.
• Different types of light bulbs (incandescent, LED) need different amounts of electrical power to generate the same amount of light. Some are more efficient than others.

Purpose

The purpose of this experiment is to get a feel, literally, for how electrical power is generated, and to develop an appreciation for how this electricity is used. Students will investigate the power consumption of different light sources, while exploring the conversion of mechanical energy to electrical energy and making measurements on a simple electric circuit.

Introduction

We use electricity every day. We use it to light our homes, cook our food, communicate with other people, and provide entertainment. We use all of this electricity to make our lives easier, but we seldom think about where it comes from or how much we actually need.

For example, let us consider the standard 60W incandescent light bulb. The packaging might tell us that it takes 60 watts of power to run that light bulb, but what does that actually mean? Well, that would be the equivalent amount of mechanical power that a 30 kg child would have to generate walking up a flight of stairs at one step a second. If you were twice as heavy, walking up a flight of stairs at the same speed would be equivalent to powering two light bulbs. If you were twice as heavy and also jogged up the stairs at two steps per second, you would be generating as much mechanical power as the electrical power required for four light bulbs. Now imagine the effort that would go into powering four 60 W bulbs in one room!

To create electricity, we harness different sources to generate mechanical power that is in turn converted into electrical power. This power doesn't normally come from people carrying their weight up stairs, but more likely comes from the heat generated from burning coal, or the kinetic energy in wind. In the experiment we perform here, a hand-crank generator harnesses the mechanical power from our hand turning the crank, converting that into electrical power. The resulting electrical power in the circuit is used by the light bulb to create light.

However, not all light bulbs are created equal. Some light bulbs need less electrical power than others to generate the same amount of light, and thus are more "efficient". This experiment will investigate the relative efficiency of different types of bulbs what this looks like by comparing the power required by an incandescent light bulb and an LED light bulb using a simple hand crank generator-powered circuit.

Materials

• Hand crank generator/ gearbox (We purchased the Tamiya 3-Speed Crank-Axle Gearbox Kit - set to the highest gear ratio, 204:1)
• 1.5V incandescent light bulb (We purchased ours from Digikey)
• White LED (We purchased ours from Digikey)
• Two multimeters (voltmeter, ammeter)
• Short wire
• Small luggage scale*
• Ruler*
• *For enrichment activity

Teachers' Toolbox and Tips

• This experiment involves the measurement and comparison of light bulb power consumption at a similar light bulb brightness. It may be ideal to have an even number of groups, assigning half the groups to work with incandescents and the other half with LEDs; the groups can pair up during the comparison task. We suggest that students work in groups of 2 or 3.
• For younger students, it is an option to omit the calculations and still benefit from the experiment. For example, instead of referring to the equation , it may be sufficient to explain that the electrical power consumed by the light bulb is proportional to the quantities measured by the two multimeters, current and voltage. This means that, given the same unit settings on the multimeters, larger numbers on the display mean greater power consumption.
• Before building the circuits, it may be beneficial to give students time to explore the conversion from mechanical to electrical energy. Using a breadboard, they could connect the hand crank generator to the light bulb without any other connections, and explore what happens when they turn the crank on the generator.
• It is important to note that LEDs work only with one voltage direction. Troubleshooting may require switching the direction of the LED.

Steps

1. Connect the hand crank generator to a light bulb using the breadboard provided. Explore what happens when the crank is turned on the generator. Observe that this is the conversion of mechanical to electrical power.

Note that when using LEDs, the lights work only in one voltage direction. If needed, flip the orientation of the LED.

2. Build the following circuit to measure the power consumption of the light bulb: The connections should resemble the following photograph: One of the multimeters, connected in series with the lightbulb, will measure the electric current through the lightbulb (wires labelled 'A'). Set this multimeter to DCA. The second multimeter, connected in parallel with the lightbulb, will serve measure the electrical potential difference, or voltage, across the lightbulb (wires labelled 'V'). Set this multimeter to DCV. Start by setting the ammeter and voltmeter to 200m and 20, respectively, and adjusting as necessary. 3. Measure (roughly) the power going through the lightbulb when turning the crank at a steady rate. Do this by recording the approximate readings on the two multimeters and using the relationship ,

which states electrical power as a product of electric current and voltage. Remember to take into account the units of the multimeter display; forexample, a reading of 50.7 on a multimeter set to 200m DCA is a measurement of 50.7 milliamperes (mA).

4. Replace the light in circuit with the second type of light bulb and repeat the measurements to find electric power through the new light bulb. Find a rate at which to turn the crank so that the brightness of the light is about the same as that of the earlier bulb in previous measurements, and keep this rate steady when measuring. Alternatively, construct two separate circuits for the two types of bulbs and conduct the power measurements simultaneously for easier comparison of brightness.
5. Compare the values obtained for the two light bulbs. Which one uses more power for the same amount of light? Which one uses less? What does this mean in terms of efficiency, and do the results match your expectations? How many more times is one light bulb more efficient than the other?

Enrichment option: How efficient was the hand crank generator system used?

To explore the question "how efficient were we at making electricity?", it may be interesting to investigate the efficiency of the hand crank system powering the light bulb, which requires measuring the power input as well as output of the generator.

1. Find the power output from the generator through the light bulb, as in the main part of the experiment. This time, however, take note of how fast the crank is being turned. Record how many seconds it takes for one full turn.
2. To find the power input, first roughly measure the force needed to turn the crank. In particular, consider the force that is required in the direction of motion (rotation), perpendicular to the radius arm. This can be done using a small luggage scale, as demonstrated in the photograph below: Each kilogram measured corresponds to roughly 10 newtons . Use the ruler to measure the handle of the crank, to obtain the circumference of rotation.

3. Obtain the work applied to turn the crank one full rotation, by taking the product of the force in the direction of motion and the circumference of the handle's rotation (the distance this force is applied). Dividing that by the time it takes for one full turn will give the rate at which work is being applied, or power, to turn the crank. In the form of an equation, this is: where is the mechanical power input, is the force applied in the direction of motion, is the radius of crank rotation, and is the time it takes for one full rotation.

4. Compare the values for power input and output. What is the ratio between the two? How efficient is the system?