//Electric Forces Explained
Electric Forces Explained2018-06-16T11:02:48+10:00

There are many different kinds of forces, such as gravity that can make things fall down, friction that can slow things down and magnetism that can stick things onto your fridge. However, it is electric forces that we will look at in this chapter, because it is electric forces that hold atoms together, both within atoms and between atoms. You might be familiar with electric forces when you rub a plastic ruler with a cloth, and then make a piece of paper jump onto and stick to the ruler. These electric forces come from the heart of the ruler’s atoms and are so strong that you can see them work in front of your eyes.
We will use a simple pendulum, which is a kind of swing, to examine how electric forces work. A pendulum consists of an object tied to the bottom of a string, which is fixed at the top, allowing the object to swing back and forth. This is a useful way of showing that a force is a push or a pull, because the pendumum can swing either left or right, thus showing the direction of the force. You can think of the pendulum as a kind of force detector. It tells us both how strong the force is, and also which direction it is pointing. (Some physical quantities like force are directional, and some are not. Time is an example of a non-directional quantity. For example, we do not say that the time is 11o’clock south-east. Mass is another non-directional quantity. You don’t buy 1kg sugar west, for example.)

Electric Charge

Have you experienced your hair stand on end after sliding down a plastic ramp? This is because your hair strands were carrying an electric charge, causing them to repel each other.

Have you noticed that batteries purchased from supermarkets have a + sign on one end, and a – sign on the other? This is because there are two kinds of electric charge; positive and negative. This is called the polarity of the electric charge. The reason why scientists called them positive and negative, rather than, say, “alpha” and ‘beta”, or “red” and “green” is because they cancel each other out if they are combined. When charges are cancelled out, an object is no longer charged, so is called “neutral”, as though they have no charge at all.

Teacher Notes: When an object is electrically neutral, it still possesses lots of electric charge within its atoms. However, the number of positive charges (carried by its protons) is exactly equal to the number of negative charges (carried by its electrons, so they cancel out. When you slide down a surface like the plastic ramp, negative electrons are rubbed off your clothes and get stuck to the other plastic ramp. Note that the electrons are always the charge carriers, as they are on the outside of the atoms and relatively accessible and mobile, whereas the protons are locked up within the atoms’ nuclei, making them immobile. When the electrons transfer from the ramp, your clothes (and you) now have fewer electrons than protons, so you carry a net positive charge. The ramp now has more electrons than protons, so it carries a net negative charge. The size of the net electric charge is due to the difference in numbers of positive protons and negative electrons.

What Affects the Stength of the Electric Force?

To discover how electric charges work, we will do a series of tests (see below). We will examine how (1) the polarity of the charges (2) the separation distance of the charges and (3) the amount of charge affects the strength of the electric force.

1. Effect of separation distance
In the series of diagrams on the next page, the ruler is brought progressively closer to the pendulum. The top diagram shows the situation in which they are far apart, whereas the bottom diagram shows the situation in which they are very close. What happens to the strength of the electric force as the ruler is brought closer to the pendulum? Does the pendulum swing more or less? You can see that the closer charges get, the more the pendulum swings. This means that teh electric force gets stronger as the charges get closer, or the separation distance gets smaller. Scientists say that there is an inverse relationship between the electric force and the separation distance, because as the distance gets smaller, the force gets bigger.
This relationship is very important for atoms, because they are constantly changing the distance between each other as they mover about, and as they change from solid, to liquid to gas. This means that the electric forces between them change according to the distance between them. For example, the atoms in solids are close together, so the forces between them are strong. On the other hand, the atoms or molecules in gases are far apart, so the forces between them are weak.

The electric force gets stronger as the charges get closer.

2. The effect of polarity
In the first, we have a positive charge on the bottom of the pendulum, and a positive charge also on the end of a ruler.
A positive and a negative charge

Before: The positive charge and a negative charge are far apart.

After: The negative charge on the ruler is brought close to the positive charge on the pendulum.
You can see that the pendulum swings to the right when the ruler is brought close. This means that the positive charge and negative charges are attracted to each other.

A negative and a positive charge

Before: The positive charge and a negative charge are far apart.

After: The negative charge on the ruler is brought close to the positive charge on the pendulum.

You can see that the pendulum swings to the right when the ruler is brought close. This means that the negative charge and positive charges are attracted to each other, again.

Two positive charges

Before: Two positive charges are far apart.

After: The positive charge on the ruler is brought close to the positive charge on the pendulum.
The pendulum moves left, which means that the two positive charges repel each other.

Two negative charges

Before: Two negative charges are far apart.

After: The negative charge on the ruler is brought close to the positive charge on the pendulum.

The pendulum moves left again, which means that two negative charges also repel each other, just like two positive charges do.
To summarise the polarity relationships, when the charges were opposite to each other (positive & negative, or negative & positive) they attracted each other. On the other hand, when the charges were like each other (positive & positive, or negative & negative) the charges repelled each other.

Opposite charges attract.
Like charges repel.

3. Effect of amount of charge
The final test looks at whether the amount of charge affects how strong the electric force is. This would result if youi rubbed the pendulum ball harder and longer with the cloth, leaving a bigger charge on it. The pendulum in the first diagram below has a small charge compared to ball in the second diagram. Has the pendulum swung more or less?

Before:

After:

You can see that the pendulum has swung more to the left, which means that it is more repelled by the ruler. This would also happen if we had increased the charge on the ruler instead. The conclusion is that the electric force increases with an increased amount of electric charge.

The electric force increases as the amount of charge increases.

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