How Does a Flashlight Work?

Problem

How does a flashlight work?

Materials

• flashlight that holds 2 size D batteries
• 16-inch (40-cm) aluminum foil strip
• duct tape
• 2 size D batteries

Procedure

1. Unscrew the top section (which holds the bulb) from the flashlight.
2. Wrap one end of the foil strip around the base of the bulb holder.
3. Tape the two batteries together with the positive terminal of one touching the negative terminal of the other.
4. Stand the flat, negative terminal of the battery column on the free end of the foil strip.
5. Press the metal tip at the bottom of the flashlight bulb against the positive terminal of the battery, as shown in the diagram.

Results

The light glows.

Why?

The bulb glows when an electric current flows through the circuit, which includes the battery, foil strip, and fine wire filament inside the flashlight bulb. The movement of the current through the wire filament causes the wire to get hot enough to give off light.

Let's Explore

WARNING: Other 1 ¹/₂-volt batteries such as size AAA, AA, or C may be used, but do not use more than three 1 ¹/₂-volt batteries. Electricity can be dangerous.

1. Does the number of batteries affect the results? Repeat the experiment using only one battery. It is not advisable to increase the number of batteries unless you are willing to risk burning out the bulb with excessive current.
2. Does the size of the flashlight bulb affect the results? The type of bulb a flashlight needs depends on the number of batteries used. Repeat the original experiment using different types of bulbs. Compare and record the illumination of the bulbs as bright, medium, or dim.

Show Time!

1. Make a model of a flashlight by asking an adult to strip away about 1 inch (2.5 cm) from each end of a 12-inch (30-cm) piece of 18-gauge wire. Wrap about 1 inch (2.5 cm) of one end of the wire around the metal base of a flashlight bulb. Lay two size D batteries on a 4 ³/₄-inch (11.9-cm) square piece of paper with the positive terminal of one battery touching the negative terminal of the other. Bend the wire so that the end of the bulb touches the positive terminal of the connected batteries. Lay the rest of the wire along the side of the batteries, with the excess coiled and bent to fit under the negative terminal of the batteries. Wrap the paper tightly around the batteries and wire, and tape the paper together. Stand the batteries on the coiled wire. Make sure the bottom of the bulb is pushed tightly against the top battery and that the coiled end of the wire touches the bottom of the battery.
2.  

   

   1. Study different types of flashlights, and make drawings of their parts. Display the different flashlights along with the drawings.
   2. Redesign this homemade flashlight so that it has an "ON/OFF" switch. Display your flashlight models.

Check it Out!

The shiny metal reflector under the bulb in a flashlight acts like a mirror and has a special shape called a parabola. Use a physical science text to find out more about a parabolic mirror. Where and what is the focal point of this mirror, and why is the flashlight bulb placed there?

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Janice VanCleave's Electricity
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