All matter is made up of tiny particles called atoms, which contain even smaller particles: protons (positive charge), electrons (negative charge), and neutrons (no charge). Objects can become electrically charged if they gain or lose electrons. There are two types of charge:
Positive (+): when an object has lost electrons.
Negative (−): when an object has gained electrons.
Electric charge is measured in coulombs (C). One electron carries a very small charge of about $−1.6 × 10^{-19}$ C.
Electric Forces
Charged objects interact with each other through a force called the electrostatic force. The rules are:
Like charges repel each other (e.g., + repels +, − repels −)
Unlike charges attract each other (e.g., + attracts −)
This is why, for example, a charged balloon can stick to a wall—it’s attracted by opposite charges induced in the wall’s surface.
Charging by Friction
When two different materials are rubbed together (like a balloon on hair or a plastic rod on a cloth), electrons are transferred from one material to the other. This process is called charging by friction:
The material losing electrons becomes positively charged
The material gaining electrons becomes negatively charged
This does not involve protons moving—only electrons are transferred during rubbing.
Charging by Conduction
Charging by conduction happens when a charged object actually touches a neutral object. Electrons are transferred directly through the contact:
Bring a charged rod (say, negatively charged) into contact with a neutral metal sphere.
Some of the excess electrons from the rod flow onto the sphere.
Remove the rod. The sphere is now left negatively charged (same type of charge as the rod).
Using this method, unlike conduction, the neutral object always ends up with the opposite kind of charge to that of the charging object.
Charging by Induction
Charging by induction means charging an object without touching it. Here’s a common demonstration:
Bring a negatively charged rod near (but not touching) a neutral metal can.
Electrons in the can are repelled and move away from the side nearest to the rod.
If you now ground the can (e.g., by touching the far end), the excess electrons leave.
Remove the ground and then the rod. The can is left positively charged.
Charging by induction can also be done using two conductors:
Place two neutral metal cans (or spheres) so that they are touching each other.
Bring a negatively charged rod near (but not touching) one end of the pair of cans.
Electrons in the cans are repelled by the rod and move to the far can, leaving the near can with a deficiency of electrons.
While the rod is still in place, carefully separate the two cans.
Remove the rod. The can nearer the rod is left positively charged, while the farther can is left negatively charged.