Activity 1...The Magic Rods

 

Introduction

 

I am sure you have experienced your hair acting weird when you are taking off a sweater. It might be attracted to the sweater or simply stands up. Weíll try to explore this magic a little more and try to explain what is going on. Youíll see that a lot of other mysterious things are connected to this phenomenon. If you rub certain objects together they gain special properties.

 

Purpose

 

To get familiar with the basic electrostatics phenomena. Youíll get to know the notion of electric charge, charging an object, attraction and repulsion. Youíll discover that electrostatic forces depend on charge and distance (Coulombís law). Youíll also experience that different materials have different electric properties. In the last part of the activity You will discover charge separation in metals by induction.

 

Apparatus

 

As you can see there are very simple everyday objects and materials prepared:

 

This is all we need to do our scientific observations.

 

 

What to do?

Take the golf tube (long, black tube) and rub it with the plastic bag. Without touching the tube bring it close to your friendís hair. What happens?

 

 

Now rub the tube again, then bring it close to little pieces of paper. What happens to the paper?

 

 

There are two types of plastic rods at your station. Take a white one, rub it with the blue cloth and hang it on the stirrup. Take the other white rod and rub it with the blue cloth. Now bring it close to one end of the other rod hanging on the stirrup. Play with the rods. What can you observe?

 

 

 

 

Does it make any difference if you move the rod farther away from the other one?

 

 

 

 

Repeat the experiment with the two clear rods. The clear rods should be rubbed with the white cloth. What can you observe?

 

 

 

Rub the white rod with the blue cloth again and put it back into the stirrup. Take the clear rod and rub it with the white cloth. Do the above experiment now using a white and a clear rod. What is different from the first time?

 

 

 

Without rubbing the rods donít have any effect on each other. However, after rubbing, although the two rods didnít touch each other, they still had an effect on each other. What has changed?

When you rub the rods with the cloths their behavior changes. We say that they become charged. There are two types of charge, we call them positive and negative. The white and the clear rods charge up differently. The white rod becomes negative and the clear rod positive. If you think back of your observations what can you tell about the charges? Circle the right answer.

 Like charges attract / repel each other.

 Unlike charges attract / repel each other.

  Try to determine what type of charge the golf tube has. Rub, charge the white rod and hang it on the stirrup. Charge the golf tube with the plastic bag and bring it close to the white rod. What do you experience?

 The charged white rod and golf tube attract / repel each other.

Now charge the clear rod and hang it up. Bring the charged golf tube near to it. What do you experience?

 The charged clear rod and golf tube attract / repel each other.

 

What is the charge of the golf tube?

The charge of the golf tube is positive / negative.

What happens to the cloths and the plastic bag when you charge the rods?

Charge the white rod and hang it on the stirrup. Charge the golf tube with the plastic bag. Now take the bag and bring it close to the white rod. What happens?

 The force between the white rod and the bag is attractive / repulsive.

Therefore we know that the bag has a positive / negative charge.

  As you rub the golf tube with the plastic bag the two types of charges separate. One type goes to the golf tube the other gathers on the bag.

  If there is equal amount of positive and negative charge on an object then we cannot observe any special behavior. We say that the object is neutral. Charges are actually present on every object but the positives cancel the negatives so the object is neutral. When we charge an object we donít create charge, we just transfer it from one object to another. (e.g. from the rod to the cloth and vise-versa)

  Play with the soda can and the golf tube. Lay the soda can on its side on the table. Charge the golf tube and bring it near the soda can. Can you roll the can across the table? Record your observations.

The force between the soda can and the golf tube is attractive / repulsive.

 

Note that the soda can is made of metal, not plastic as all of the rods are. Metals behave a little bit differently. Although the can is neutral (there is equal amount of positive and negative charge in it) it is affected by the charged tube near it. Charges are free to move around in a metal object. Can you figure out what happens inside the soda can?

 

 

 

 

 

 

 

 

 

 

  Here is the time to find out how the golf tube attracts hair and pieces of paper. Hang an uncharged rod (either white or clear) on the stirrup. Take a rod, charge it and bring it near the hanging rod. What can you observe?

 

 

 

 

Leave the uncharged rod hanging. Take the other type of rod and bring it near the hanging rod. What can you observe?

 

 

 

It doesnít matter which type of rod you use, your observations should be the same. Charged objects attract neutral objects. The explanation is just a little bit different from that of the behavior of the soda can. Ask your teacher how this works.

The magic is solved! It is nothing mysterious, just physics. It is the electric charges you discovered today that make all electric equipment, like a light bulb, an electric car or a stereo, work.