Electric Forces & Charge
Learn how electric charge works: positive and negative charges, why like charges repel and opposite charges attract, how rubbing transfers charge, and what factors affect electric force strength.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Electric Forces & Charge, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Every object around you carries electric charge, even if you can't see it. When you rub a balloon on your hair, electrons move from your hair to the balloon — and suddenly the balloon sticks to the wall. This isn't magic; it's electric force at work. Electric forces are one of the fundamental forces in nature, and understanding them helps explain everything from why your socks stick together in the dryer to how lightning forms. In this lesson, you'll discover the two types of charge, learn why some objects push each other away while others pull together, and find out exactly what determines how strong an electric force really is.
Two Types of Charge: Positive and Negative
All objects are made of atoms, which contain two types of particles that carry electric charge: protons (positive charge) and electrons (negative charge). In a neutral atom, the number of protons equals the number of electrons, so the charges balance out. But when objects rub together, electrons can transfer from one object to another. When an object loses electrons, it becomes positively charged because it has more protons than electrons. When an object gains electrons, it becomes negatively charged because it has more electrons than protons. Think of charge like a balance scale: if one side has more weight, the scale tips in that direction.
Every electric charge in the universe is either positive or negative — there is no third type. Charge is measured in units called coulombs (C), but even tiny objects can carry millions of coulombs of charge. The key insight is that charge is not created or destroyed; it only moves from one object to another. This idea is called conservation of charge. When you rub a balloon on your hair, you are not creating charge — you are redistributing it between the balloon and your hair.
Every electric charge in the universe is either positive or negative — there is no third type. Charge is measured in units called coulombs (C), but even tiny objects can carry millions of coulombs of charge. The key insight is that charge is not created or destroyed; it only moves from one object to another. This idea is called conservation of charge. When you rub a balloon on your hair, you are not creating charge — you are redistributing it between the balloon and your hair.
Like Charges Repel, Opposite Charges Attract
Now that you understand the two types of charge, the next rule is simple but powerful: like charges repel each other, and opposite charges attract each other. Two positively charged objects will push away from each other. Two negatively charged objects will also push away from each other. But a positively charged object and a negatively charged object will pull toward each other.
This is why the rubber balloon sticks to the wall. When you rub the balloon on your hair, electrons transfer to the balloon, making it negatively charged. The wall itself is made of material that becomes slightly positively charged (or is neutral but can be polarized). The negative balloon and the positive wall attract each other, and the balloon clings to the surface. If you were to rub two balloons on your hair and bring them close together, both would be negatively charged, and they would repel each other — they would push apart instead of attracting.
These forces act at a distance, meaning objects do not have to touch for the force to occur. This is similar to how gravity works (which you studied in an earlier lesson) but operates through a different mechanism called the electric field.
This is why the rubber balloon sticks to the wall. When you rub the balloon on your hair, electrons transfer to the balloon, making it negatively charged. The wall itself is made of material that becomes slightly positively charged (or is neutral but can be polarized). The negative balloon and the positive wall attract each other, and the balloon clings to the surface. If you were to rub two balloons on your hair and bring them close together, both would be negatively charged, and they would repel each other — they would push apart instead of attracting.
These forces act at a distance, meaning objects do not have to touch for the force to occur. This is similar to how gravity works (which you studied in an earlier lesson) but operates through a different mechanism called the electric field.
How Rubbing Transfers Charge
Rubbing is one of the most common ways to move electrons from one object to another. When two materials rub together, electrons from one material can be pulled into the other. Which direction the electrons move depends on the materials involved — some materials hold onto their electrons more tightly than others. A material that easily gives up electrons is more likely to become positively charged; a material that easily gains electrons is more likely to become negatively charged.
Your hair and a rubber balloon are a classic example. Rubber holds onto electrons very strongly, so when it rubs against your hair, electrons transfer from your hair to the balloon. Your hair becomes positively charged (missing electrons) and the balloon becomes negatively charged (extra electrons). This charge separation persists until the electrons have a path to move back — for example, when the balloon eventually touches a conductor like a metal object, the excess electrons can flow away.
Other familiar examples include rubbing a plastic ruler on wool (the ruler gains electrons and becomes negatively charged) or scuffing your feet on a carpet (electrons transfer to you, making you negatively charged, which is why you might get a shock when you touch a metal doorknob). The amount of charge transferred depends on how long you rub, how hard you press, and what materials you use.
Your hair and a rubber balloon are a classic example. Rubber holds onto electrons very strongly, so when it rubs against your hair, electrons transfer from your hair to the balloon. Your hair becomes positively charged (missing electrons) and the balloon becomes negatively charged (extra electrons). This charge separation persists until the electrons have a path to move back — for example, when the balloon eventually touches a conductor like a metal object, the excess electrons can flow away.
Other familiar examples include rubbing a plastic ruler on wool (the ruler gains electrons and becomes negatively charged) or scuffing your feet on a carpet (electrons transfer to you, making you negatively charged, which is why you might get a shock when you touch a metal doorknob). The amount of charge transferred depends on how long you rub, how hard you press, and what materials you use.
Factors That Change Electric Force Strength
Electric force is not always the same strength. Two main factors determine how strong the electric force between two charged objects will be:
The first factor is straightforward: the more charge an object has, the stronger its electric force. If you double the charge on one object, the force doubles. If you double the charge on both objects, the force increases four times (because both objects contribute).
The second factor follows a pattern called the inverse square law, just like gravity does. When you double the distance between two charged objects, the electric force becomes one-quarter as strong. When you triple the distance, the force becomes one-ninth as strong. Mathematically, force is proportional to , where is the distance. This means electric force drops off very quickly as objects move farther apart. Two charged balloons stuck to a wall 10 centimeters apart will exert almost no noticeable force on each other, but if they are 1 centimeter apart, the repulsive force is much stronger.
These relationships are why lightning jumps across the sky (enormous amounts of charge at a distance) and why bringing your finger close to a rubbed ruler causes it to move (even a small charge at a very short distance produces measurable force).
| Factor | Effect on Force |
|---|---|
| Amount of charge | More charge → stronger force |
| Distance between objects | Closer together → stronger force; farther apart → weaker force |
The second factor follows a pattern called the inverse square law, just like gravity does. When you double the distance between two charged objects, the electric force becomes one-quarter as strong. When you triple the distance, the force becomes one-ninth as strong. Mathematically, force is proportional to , where is the distance. This means electric force drops off very quickly as objects move farther apart. Two charged balloons stuck to a wall 10 centimeters apart will exert almost no noticeable force on each other, but if they are 1 centimeter apart, the repulsive force is much stronger.
These relationships are why lightning jumps across the sky (enormous amounts of charge at a distance) and why bringing your finger close to a rubbed ruler causes it to move (even a small charge at a very short distance produces measurable force).
Common Misconceptions About Electric Charge
One widespread misconception is that rubbing creates charge. It does not. Rubbing only moves electrons that already exist from one place to another. No new charge is invented; the total charge is conserved.
Another misconception is that all objects are either charged or uncharged. In reality, all objects always have charge — they are simply neutral when positive and negative charges balance out. Even a "neutral" object can exert an electric force on a charged object because the charges inside it can separate slightly (a process called polarization). This is why a negatively charged balloon attracts to a neutral wall: the negative charge on the balloon repels electrons in the wall, leaving a positive region near the surface that attracts the balloon.
Students sometimes also think that distance matters less than it actually does. The inverse square law is powerful: moving from 10 centimeters to 20 centimeters (doubling the distance) reduces the force by a factor of four, not two. Understanding this relationship is crucial for predicting when electric forces will be strong enough to matter.
Another misconception is that all objects are either charged or uncharged. In reality, all objects always have charge — they are simply neutral when positive and negative charges balance out. Even a "neutral" object can exert an electric force on a charged object because the charges inside it can separate slightly (a process called polarization). This is why a negatively charged balloon attracts to a neutral wall: the negative charge on the balloon repels electrons in the wall, leaving a positive region near the surface that attracts the balloon.
Students sometimes also think that distance matters less than it actually does. The inverse square law is powerful: moving from 10 centimeters to 20 centimeters (doubling the distance) reduces the force by a factor of four, not two. Understanding this relationship is crucial for predicting when electric forces will be strong enough to matter.
Key terms
- Electric charge.
- A fundamental property of matter that can be positive or negative; the quantity that determines the strength of electric forces between objects.
- Positive charge.
- The type of charge carried by protons; an object is positively charged when it has more protons than electrons.
- Negative charge.
- The type of charge carried by electrons; an object is negatively charged when it has more electrons than protons.
- Electron transfer.
- The movement of electrons from one object to another, usually caused by rubbing or contact, which creates a charge imbalance.
- Repulsion.
- The force that pushes two objects apart; occurs when like charges interact.
- Attraction.
- The force that pulls two objects together; occurs when opposite charges interact.
- Inverse square law.
- A relationship in which force decreases with the square of the distance; doubling the distance reduces force to one-quarter.
- Conservation of charge.
- The principle that electric charge cannot be created or destroyed, only transferred between objects.
Worked example
A student rubs a balloon on wool and finds that the balloon becomes negatively charged. She then brings the balloon close to two objects: a neutral wall and a metal sphere that is positively charged. In each case, the balloon moves toward the object. Explain why the balloon is attracted to both objects, and predict which attraction will be stronger if the balloon is the same distance from each object.
Start by identifying what charge each object has. The balloon is negatively charged (gained electrons from the wool). The wall is neutral (equal positive and negative charge), and the metal sphere is positively charged.
Why does the balloon attract to the wall? Even though the wall is neutral overall, the negative charge on the balloon repels electrons in the wall, pushing them away from the surface nearest the balloon. This leaves a positive charge on the near side of the wall, which attracts the negatively charged balloon. This is called polarization.
Why does the balloon attract to the metal sphere? The balloon (negative) and the sphere (positive) are opposite charges, so they attract each other directly according to the rule that opposite charges attract.
Which attraction is stronger? To compare them fairly, assume the balloon is the same distance from each object. The metal sphere has a fixed positive charge, while the wall's attraction depends only on the polarization created by the balloon's charge. The metal sphere will exert a stronger force because (1) it has a definite positive charge, and (2) the attraction is direct rather than induced. Additionally, in real situations, a metal sphere typically has a large amount of charge concentrated on its surface, which produces a very strong force even at modest distances. Therefore, the balloon will move toward the metal sphere with more force than toward the wall.
Why does the balloon attract to the wall? Even though the wall is neutral overall, the negative charge on the balloon repels electrons in the wall, pushing them away from the surface nearest the balloon. This leaves a positive charge on the near side of the wall, which attracts the negatively charged balloon. This is called polarization.
Why does the balloon attract to the metal sphere? The balloon (negative) and the sphere (positive) are opposite charges, so they attract each other directly according to the rule that opposite charges attract.
Which attraction is stronger? To compare them fairly, assume the balloon is the same distance from each object. The metal sphere has a fixed positive charge, while the wall's attraction depends only on the polarization created by the balloon's charge. The metal sphere will exert a stronger force because (1) it has a definite positive charge, and (2) the attraction is direct rather than induced. Additionally, in real situations, a metal sphere typically has a large amount of charge concentrated on its surface, which produces a very strong force even at modest distances. Therefore, the balloon will move toward the metal sphere with more force than toward the wall.
Practice questions
When you rub two rubber balloons on your hair, both balloons become negatively charged. What will happen if you bring the two balloons close together?
- The balloons will attract each other.
- The balloons will repel each other.
- The balloons will have no effect on each other.
- The balloons will rotate around each other.
Answer: The balloons will repel each other.
Both balloons have the same type of charge (negative), so they are like charges. According to the fundamental rule of electric forces, like charges repel. The electrons on each balloon's surface will push away from each other, causing the balloons to move apart.
A scientist measures the electric force between two charged objects when they are 2 centimeters apart and finds it equals 80 newtons. If the objects are moved to 4 centimeters apart (doubling the distance) without changing the amount of charge, what will the new force be?
Answer: 20 newtons
This is an inverse square law problem. When the distance doubles, you divide the force by the square of the distance multiplier. The distance multiplier is 2, so the force is divided by . Therefore, newtons. The key is remembering that the relationship involves the square of the distance, not just the distance itself.
Explain what happens when you rub a plastic comb on dry hair. Include in your answer what type of charge each object ends up with, why that charge type occurs, and what happens if you bring the comb near small pieces of paper.
Answer: When you rub a plastic comb on dry hair, electrons transfer from the hair to the comb. The comb becomes negatively charged (it gained electrons) and the hair becomes positively charged (it lost electrons). This happens because plastic holds onto electrons more strongly than hair does. When you bring the negatively charged comb near paper (which is neutral), the negative charge repels electrons in the paper, creating a positive region on the near side. Opposite charges attract, so the paper pieces are pulled toward the comb and stick to it.
A complete answer identifies both charge types and their causes, recognizes that the paper pieces are initially neutral but can be polarized, and applies the attraction rule correctly. A common incomplete answer might only say the comb becomes negatively charged without explaining why or how the paper responds.
FAQ
- Can an object have both positive and negative charge at the same time?
- Yes. Every object has both types of charge because it contains both protons and electrons. When an object is neutral, the positive and negative charges are balanced. When it is charged, one type outnumbers the other. For example, a negatively charged balloon has more electrons than protons, but it still has protons in its nucleus.
- Why do you sometimes get a shock when you touch a metal doorknob after walking on carpet?
- Walking on carpet rubs electrons onto your body, making you negatively charged. When you reach for a metal doorknob (which is conductive and connected to the ground), the excess electrons jump from your finger to the doorknob in a sudden spark. This is a small electric discharge, and the spark is what you feel as a shock. It happens because the force between your negative charge and the positive charge (or the attraction to ground potential) becomes strong enough to push electrons across the tiny gap between your finger and the knob.
- Is rubbing the only way to transfer charge?
- No. Rubbing is a common and visible way, but charge can also transfer through direct contact (touching), through a conductor (like a wire), or even through the air under extreme conditions (like lightning). The balloon-on-the-wall demo uses rubbing, but a charged object could also transfer charge if it touched another object or if the distance between them became so small that electrons jumped across.
- Why does the electric force follow the inverse square law?
- The inverse square law comes from how electric field spreads out in space. Imagine the force spreading equally in all directions from a charged object. The farther out you go, the more area the force is spread over. If you double the distance, the force is spread over four times the area (because the surface area of a sphere is proportional to ), so the force at any point is one-quarter as strong. This same logic applies to gravity, which is why both forces follow the inverse square law.
Learn this with a teacher, not a page
The Crimsora tutor teaches Electric Forces & Charge live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.