Activity 3...The Pith Ball


You have found out about charge, charging objects, sparks and some simple devices. Here is the time to play with charge a little more and discover some new properties.


Students will get a deeper understanding of the behavior of charges, how they move around and are transferred.


What to do?

Charge up a pith ball: Hang one of the pith balls on the stand. Charge the electrophorus the way you did in Activity 2. Touch the pith ball with the electrophorus. Now charge the clear rod with the white cloth and bring it near the ball. What happens? What charge do the objects have?

The clear rod repels the pith ball.

Since we know that the clear rod charges up positively and there is repulsion between the rod and the pith ball, the ball must also have a positive charge.

Touch the pith ball then recharge it with the electrophorus. What do you think is going to happen if you bring the charged golf tube near the ball? Try to predict. You should figure out the charge on the objects.

Test your prediction. Were you right?

The pith ball was charged up the same way as before. Therefore, it again must have a positive charge. We know that the golf tube has a negative charge so the pith ball must be attracted to the golf tube.

  Take both pith balls and hang them on the stand. Charge the electrophorus and touch the balls with it. Record your observations.

As soon as the electrophorus touches the balls they repel each other.

 What happens? Think of the charges again.

Since charges are free to move in a metal when the electrophorus touches one of the pith balls charges move to the ball. The two balls are touching each other so some charge moves on to the other ball. Both of the balls become positively charged, therefore, they repel each other.

The pith balls are like the leaves of an electroscope.

 Pith ball pendulum: Take a single pith ball and hang it up. Charge up the electrophorus and bring it slowly near the ball. Watch what happens. Now go ahead and touch the pith ball with the electrophorus. Hold the pan in place. What happens?

The pith ball is repelled by the electrophorus.

 Touch the pith ball to discharge it. Charge up one of the electrophoruses and now take both the charged and the uncharged one and bring it near the ball on both sides as you see it on the picture.

Let the charged electrophorus touch the pith ball. What happens? Play with the pans making the separation smaller and larger.

The pith ball, like a pendulum, moves back and forth between the two pans. If the pans are close to each other then the ball moves faster.

 Try to explain how this pendulum works. Remember the surface of the pith ball is painted with metallic paint.

When the pith ball touches the charged electrophorus it gets charged, repelled and hits the other electrophorus. When the other pan and the ball get in contact most of the charge from the ball moves to the pan. Since now the pan and the ball have the same charge the ball is repelled. Once again it touches the first electrophorus and the whole process starts again. The pith ball carries charge from one electrophorus to the other.


- The pith balls should be hung on an approximately 15 cm long string.

- When you hang both balls they should touch each other.

- Make sure students understand how charges are distributed in metals. If you charge a metal object and it is in contact with another then both get charged.

- You might want to tape the thread on the stand so that the pith balls are fixed in the right position.