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. We’ll try to explore this magic a little more and try to explain what is going on. You’ll see that a lot of other mysterious things are connected to this phenomenon. If you rub certain objects together they gain special properties.

 

Purpose

 

Students will get familiar with the basic electrostatics phenomena. They’ll get to know the notion of electric charge, charging an object, attraction and repulsion. They’ll discover that electrostatic forces depend on charge and distance (Coulomb’s law). They’ll also experience that different materials have different electric properties. In the last part of the activity they’ll 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 friend’s hair. What happens?

The hair is attracted to the tube.

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

The paper pieces are attracted to the tube.

 

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?

When the distance between the rods gets small enough, the hanging rod turns around. It is repelled by the other rod.

 

 

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

Yes, if the rods are far apart then no effect can be observed.

 

 

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

The effect is the same as between the white rods. The force between the clear rods is also repulsive.

 

 

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?

Instead of repulsion, there is attraction between the clear and the white rod.

 

Without rubbing the rods don’t have any effect on each other. However, after rubbing, although the two rods didn’t 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 don’t 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?

The negative charges inside the soda can are repelled by the golf tube so they move to the far side of the can. The positive charges remain on the close side and they are attracted to the golf tube. As the electrostatic forces get weaker as distance increases the attraction between the unlike charges is greater than the repulsion between like charges. The soda can is attracted to the golf tube.

 

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?

The two rods attract each other.

 

 

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

 

It doesn’t matter which type of rod you use, your observations should be the same.

The two rods attract each other. If you get a repulsion, the white rod wasn’t discharged fully, try rubbing your hands over it some more.

 

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 molecules in the rods or in the hair get polarized in the presence of a charged object. In the case of the negatively charged golf tube and the hair, for example, the polarized molecules in the hair are going to be oriented with their positive side towards the golf tube. The polarized charges inside the hair cancel each other but there is going to be some charge on the surface. The close surface is positive and the far surface is negative. Since the electrostatic forces depend on the distance, the attraction between the unlike charges is greater than the repulsion between the like charges.

  

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.

 

 

Comments:

 

- When you charge an object by rubbing it you should hold it with the cloth tight and move your hand relatively fast. Experiment with the rods how to charge them. Friction, the same thing that gets things hot, helps move charges around. Think of how you would rub a rod to make it get hot.

- Make sure that students use the right cloths for charging the rods. Otherwise they might end up with charging the rods oppositely and discovering some ‘new’ physics rules.

- If the cloths get damp from being held they won’t work so have plenty of spares. I usually take the rods I want to charge to the cloth section of the store and try out various remnants to see what works best. For the clear rods cotton, cotton denim or silk work great. I get the cheapest remnant and cut it up with pinking shears so the edges don’t ravel. Felt squares for crafts work well with the PVC or white plastic. Just cut them in half.

- Oven roasting bags are the best kind of plastic bag for charging rods. (You can wash them with Dawn when they get greasy.) However, some other plastic bags work as well. You can experiment.

- Only the part of the rods that they rub are going to be charged. Make sure they use the charged ends for the observations.

- Do not touch the charged part of the rods because you might end up discharging them.

- To discharge a rod you need to rub it all over with your hand. Make sure whoever does this is not wearing lotion, or it will never charge up again until it is cleaned.

-Dawn dish detergent works very well at cleaning things so they charge well. Dish soaps will inhabit the charging process, just like lotion.