Purpose
Investigate the electrical charges on two materials.
Concept
If you remove dry clothes from a clothes drier you usually notice they cling
together and spark with static electricity. Some materials hold on to their
electrons more than others. When two materials rub against each other, they
can become charged by either losing or gaining electrons. If the material
loses electrons, it becomes positively charged. If the material gains electrons,
it becomes negatively charged.
If wool or fur is rubbed against rubber, the rubber will pick up extra
electrons and become negatively charged. If a negatively charged object is
brought near metal (metal is an electrical conductor) electrons in the
metal will tend to move away from the object. This is called charging
by induction.
Procedure
1. Rub the fur against the rubber balloon. Touch the rubber balloon against the knob on the electroscope. Observe the leaves on the electroscope and describe your observations.
2. Touch the knob of the electroscope with your finger. Describe what happened to the leaves on the electroscope.
3. Rub the glass rod with the silk pad. Hold the glass rod 2 to 3 cm away from the knob on the electroscope. Take the glass rod away from the electroscope. Observe the leaves on the electroscope and describe your observations.
4. Touch the knob of the electroscope with your finger. Describe what happened to the leaves on the electroscope.
5. Rub the fur against the rubber balloon again. Hold the balloon 2 to 3 cm away from the knob on the electroscope. Take the balloon away from the electroscope. Observe the leaves on the electroscope and describe your observations.
Analysis
1. Did the leaves on the electroscope move apart when the charged rubber balloon touch the knob on the electroscope? Explain why or why not.
2. What charge did the rubber balloon acquire? Explain.
3. What charge did the glass rod acquire? Explain.
4. What happened to the electrons in the electroscope when you touched the knob with your finger? Explain.
