Electrostatic and Gravitational Forces
You'll be able to describe the electrostatic force between two charged objects.
Rub a balloon on your hair and it sticks to the wall. Drop the same balloon and it falls to the floor. Two invisible forces, both acting across a gap, are pulling the strings.
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Q1 · You rub a balloon on your jumper and it sticks to a wall without any glue. What do you think happened to the balloon when you rubbed it?
Q2 · The Moon stays near Earth, and Earth stays near the Sun, even across enormous empty distances. What invisible force do you think holds them in place?
● Know
- That objects can become electrically charged when materials rub together
- That there are two kinds of charge, positive and negative
- That every object with mass exerts a gravitational pull on every other object
● Understand
- Why like charges repel and unlike charges attract
- Why a force that acts across a gap must get weaker with distance
- How Earth's gravity holds the Moon, and the Sun's gravity holds the planets
● Can do
- Describe the electrostatic force between two charged objects
- Describe the gravitational force between two masses
- Compare electrostatic and gravitational forces and explain how they are alike and different
When you rub a balloon on your hair, you are not adding magic, you are moving tiny charged particles. Rubbing two materials together transfers some of these particles (called electrons) from one material to the other. This is how an object becomes electrically charged.
There are exactly two kinds of charge:
- Positive charge, written with a plus sign (+)
- Negative charge, written with a minus sign (−)
The key rule of the electrostatic force is simple:
- Like charges repel, two positives push apart, and two negatives push apart
- Unlike charges attract, a positive and a negative pull together
Two details about charge are worth having straight, because they explain things the rule alone does not. First, rubbing does not make charge, it moves it. Every object already contains vast numbers of positive and negative particles in equal numbers, which is why ordinary objects seem to have no charge at all: the two kinds cancel out. Rubbing simply scrapes some electrons off one surface and onto the other, so one object ends up with a few too many and the other with a few too few. That is why charging always happens in pairs, and why the balloon and your hair end up attracted to each other.
Second, this is why a charged balloon sticks to a wall that was never rubbed at all. The wall is uncharged overall, but the balloon's charge pushes the wall's own like charges slightly away from the surface and pulls its unlike charges slightly toward it. The nearest part of the wall now has the opposite charge to the balloon, so the two attract. You are watching one charged object rearrange the charges inside an uncharged one, which is a real effect and a common exam trap: "it stuck, so the wall must be charged" is the wrong conclusion.
You meet this force every day:
- A balloon rubbed on your hair sticks to a wall
- A static shock jumps from your hand to a metal doorknob
- Dust clings to a television or computer screen
- Lightning is a giant electrostatic spark between clouds and the ground
Australian context: on a hot, dry day in the outback, static builds up easily because the air carries little moisture. That is why you are more likely to feel a static shock stepping out of a car in dry inland towns than on a humid day at the coast.
Every object that has mass pulls on every other object with mass. This pull is called the gravitational force, and it is always an attraction, it only ever pulls objects together and never pushes them apart.
Two things control how strong that pull is, and it is worth naming what each one is doing. The first is mass: gravity is the pull between amounts of matter, so the more matter there is on either side, the stronger the pull. That is why you notice Earth's pull on you constantly and never notice your desk's pull on you, even though the desk has mass and is far closer. The second is distance: the further apart two objects are, the weaker the pull becomes, and you will see in the comparison card below that this happens for a reason that applies to every force acting across a gap.
Earth has an enormous mass, so it pulls every object on or near it toward its centre. This pull toward Earth's centre is what we call weight, and you'll explore weight fully in the next lesson. A dropped pencil, a falling leaf, and rain all move downward because of Earth's gravitational pull.
Gravity has one property the electrostatic force does not, and it is the reason gravity runs the Solar System even though it is by far the weaker of the two. Charge comes in two kinds that cancel each other, so a large object such as a planet, made of enormous numbers of positive and negative particles mixed together, has almost no overall charge and exerts almost no electrostatic force. Mass has no opposite. It only ever adds up. So the more matter you pile together, the stronger the pull gets, with nothing to cancel it, and that is why the objects with the strongest gravity are simply the biggest ones:
- Earth's gravity holds the Moon in orbit, even though they are about 384,000 km apart
- The Sun's huge gravity holds Earth and all the planets in their orbits
- Gravity from the whole galaxy keeps billions of stars circling together
Because the Sun is far more massive than Earth, its gravitational pull reaches all the way across the Solar System. The further a planet is from the Sun, the weaker the pull it feels, which is part of why distant planets take so much longer to orbit.
This is the card where the two forces are put side by side, so it is where the property they share gets explained rather than just named. Both are non-contact forces: neither needs the objects to touch, and both reach across a gap with nothing in between. Both also get weaker as the objects move apart, and the reason is the same in each case. Picture the influence of the charge or the mass spreading outward in every direction at once, like light leaving a bulb. At twice the distance it has to cover a much larger surrounding area, so much less of it lands on any given object. Nothing is being used up; it is being spread thinner. That single picture explains both of this lesson's examples at once: why a charged balloon has to be brought right up to a wall before it will hold, and why Neptune, so far out that the Sun's pull on it is spread extremely thin, takes 165 years to go round once while Earth manages it in one.
The crucial difference is that the electrostatic force can either attract or repel, while gravity can only ever attract, and by now you have the reason for that too. Charge comes in two kinds, so two charged objects can be alike or unlike, and the force goes one way or the other. Mass comes in only one kind, so there is no "unlike mass" for gravity to push away from. A rule you can state a reason for is a rule you will still have in an exam.
| Feature | Electrostatic force | Gravitational force |
|---|---|---|
| What causes it | Electric charge (positive or negative) | Mass |
| Contact needed? | No, acts across a gap | No, acts across a gap |
| Can it attract? | Yes, unlike charges attract | Yes, always attracts |
| Can it repel? | Yes, like charges repel | No, gravity never repels |
| Effect of more distance | Force gets weaker | Force gets weaker |
| Everyday example | Balloon stuck to a wall | A dropped pencil falling |
So when you see a balloon stick to a wall and then watch it fall after it slowly loses its charge, you are watching both forces in the same minute, the electrostatic force holding it up, and gravity always ready to pull it down.
You rub two balloons on your hair and hang them side by side on strings. Both balloons pick up the same kind of charge. Predict what the balloons will do, will they swing toward each other, swing apart, or hang straight down? Explain why.
How close was your prediction?
Well done, you remembered that like charges repel across a gap.
Key insight: same charge means repel, opposite charge means attract.
Below are 10 everyday situations. For each one, write whether the force involved is electrostatic (E) or gravitational (G), then write whether the objects will attract (A) or repel (R). Remember, gravity always attracts.
| # | Situation | E or G? | Attract or Repel? |
|---|---|---|---|
| 1 | Two balloons that both carry a negative charge | ||
| 2 | Earth pulling a falling apple toward the ground | ||
| 3 | A positively charged rod brought near a negatively charged rod | ||
| 4 | The Sun pulling Earth along its orbit | ||
| 5 | Dust sticking to a charged television screen | ||
| 6 | The Moon being held near Earth across 384,000 km | ||
| 7 | Two combs that have both been rubbed on wool | ||
| 8 | A charged balloon sticking to a wall | ||
| 9 | Rain droplets falling toward the ground | ||
| 10 | Two protons (both positive) pushing each other away |
Draw two simple force diagrams, each showing two charged spheres with arrows for the electrostatic force between them. You should include:
- Diagram 1, two spheres both labelled with a minus sign, with arrows showing the force pushing them apart
- Diagram 2, one sphere labelled with a plus sign and one with a minus sign, with arrows showing the force pulling them together
- A clear label on each diagram saying repel or attract
After drawing (in your book or on paper), answer this here, why do the arrows point in opposite directions in your two diagrams?
Q1. Explain how an object such as a balloon becomes electrically charged when it is rubbed on hair. (2 marks)
Q2. Describe the gravitational force that acts between Earth and the Moon. In your answer, say what causes it, which way it acts, and what happens to its strength as the distance changes. (3 marks)
Q3. Compare the electrostatic force and the gravitational force. Give one way they are alike and one way they are different. (3 marks)
Answers
▾MCQ 1
B, an object becomes charged when materials rub together and tiny charged particles (electrons) are transferred from one material to the other. Heat, colour and weight do not give an object an electric charge.
MCQ 2
A, unlike (opposite) charges attract, so a positive and a negative object pull toward each other. They do not need to touch because the electrostatic force acts across a gap.
MCQ 3
C, the gravitational force depends on the mass of the objects and the distance between them. Larger mass means a stronger pull, and greater distance means a weaker pull.
MCQ 4
D, gravity always attracts and acts across a gap between masses. It never repels, it does not need contact, and it gets weaker (not stronger) as objects move apart.
MCQ 5
B, both forces are non-contact forces that act at a distance and both get weaker as objects move apart. Only the electrostatic force can repel, and only gravity depends on mass alone.
Short Answer 1
Model answer: When the balloon is rubbed on hair, the rubbing transfers tiny charged particles called electrons from one material to the other. This leaves the balloon with an overall electric charge (it gains extra negative charge), so it is now electrically charged and can attract or repel other charged objects.
Short Answer 2
Model answer: The gravitational force between Earth and the Moon is caused by their mass, every object with mass pulls on every other object with mass. It acts as an attraction, pulling the Moon toward Earth (and Earth toward the Moon) across the gap between them. It is a non-contact force. If the distance between them increased, the gravitational pull would get weaker, and if they were closer it would be stronger.
Short Answer 3
Model answer: Alike: both are non-contact forces that act across a gap without touching, and both get weaker as the objects move further apart. Different: the electrostatic force can either attract (unlike charges) or repel (like charges), but the gravitational force can only ever attract, it never pushes objects apart.
At the start of this lesson you saw the hook: a spark jumping to a doorknob and keys falling to the floor, both starting without anything touching. Now you know what's going on, the spark is the electrostatic force and the falling keys are the gravitational force, and both can act across a gap.
How does your new understanding compare to what you first thought? Explain one way these two forces are alike and one way they are different.
- Objects become charged when materials rub together and tiny charged particles transfer. There are two charges, positive and negative.
- The electrostatic force is a non-contact force, like charges repel and unlike charges attract, and it weakens with distance.
- The gravitational force pulls any two masses together, it is stronger for larger masses, weaker with distance, and only ever attracts.
- Both forces act at a distance and weaken as objects move apart, but only the electrostatic force can repel.