1-2: Laws of Electric Charges

Laws of Electric Charges

Charged objects can either attract each other (they pull towards each other) or repel each other (push away). We can summarize these forces with the three Laws of Electric Charges.

We can use these laws of electric charges to explain why objects either attract or repel each other. Two neutral objects will neither attract nor repel each other.

Here is an animation showing the first two laws of electric charges:

And now, a little fun with sticky tape and a Van de Graaff Generator to make some objects charged in different ways:

Induced Charge Separation

Here is the same neutral object that we have seen before.

The negative charges are somewhat free to move; we will look at this in more detail later. However, consider this: all the negative charges (electrons) have the same charge. That means they want to repel each other. That also means they want to move as far away from each other as possible, if they can.

Below is another object with a lot of charges shown. Count up the number of positive and negative charges: is this object positively charged, negatively charged, or neutral?

If you carefully count all the positive and negative charges, you'll see that there is an equal number of each. This means the object is still neutral. However, something strange must be going on, because:

• all the negative charges have moved to the left end, and

• since those negative charges all repel each other, there must be a really good reason they're all clumped together.

Here's what may be going on, if we zoom-out a bit:

The positively-charged object on the left has induced the negative charges to move to the left. We know the negative charges will be attracted to the positive object. Since they are free to move a bit, they move to the left.

A similar thing would happen if the new object was negatively charged:

Here, the negative charges on the right have been induced to move to the right, away from the new, negatively-charged object.

But, in both of these cases, the overall charge on the original object is still neutral. We say that the original object has experienced an induced charge separation: the negative charges are not evenly distributed, as you would expect. The charges have been "induced" to move; this is a word that means "to move by persuasion or influence".

Charged and Neutral Objects Attract

Induced charge separation explains Law #3 above: any charged object attracts any neutral object. Let's follow the process.

Step 1: The neutral object is on its own, being neutral.

Step 2: A charged object is brought close to the neutral object, inducing a charge separation.

Step 3: The left end of the original object, being more negative, is now attracted to the charged object.

If the charged object is the opposite charge, the attraction happens, but the original object's charge separation is flipped:

Either way, this shows how a charged object (either positive or negative) attracts a neutral object, using induced charge separation.

Try this at home: Stick a balloon to a wall

A fun experiment you can do at home is to charge a balloon by rubbing it on your hair, then stick it to a wall. (This is called "charging by friciton," which we will look at later.) Note that this works best on days in which the air indoors is very dry; a hot, humid summer day will still work, but not as well.

Here's why this works. (You only need to see the first two minutes of the video; the rest talks about something we haven't looked at yet.)

Practice Questions

The Basics

1. State the three Laws of Electric Charges.

2. Predict if a negative object be attracted to, or repelled by, (a.) a positive object, (b.) a negative object, and (c.) a neutral object.

Extensions

1. "If a neutral object's charges are moved by induction, it's no longer neutral." Explain why this is incorrect.

2. Dust often sticks to neutral objects such as TV screens. Explain why this could be an example of induced charge separation.