An object becomes charged when it gains or loses electrons: gaining electrons makes it negative, losing electrons makes it positive. This appears at the start of the electricity topic, in questions about rubbing rods, cloths and balloons.
It leads straight into calculating current from charge and time, because current is simply charge on the move.
What is charge, and where does it come from?
Every atom has a positive nucleus and negative electrons. A neutral object has equal amounts of positive and negative charge, so the effects cancel. Charging means disturbing that balance.
Two rules describe what charges do to each other. Like charges repel, and unlike charges attract. These forces act without the objects touching.
How does rubbing charge an object?
When two different insulators are rubbed together, electrons are pulled off the surface of one and stick on the surface of the other. The material that loses electrons becomes positive. The material that gains them becomes negative.
Only electrons transfer, because the protons are fixed in the nuclei of the atoms. Charge is not created by rubbing. It is moved from one place to another, so the total stays the same.
Worked example
A polythene rod is rubbed with a dry woollen cloth. The rod becomes negatively charged. Explain why, and state the charge on the cloth. (Invented example, typical textbook pairing.)
Step 1, name what moves: electrons are transferred by the rubbing.
Step 2, say the direction: electrons move from the cloth to the rod, because the rod ends up negative and so must have gained electrons.
Step 3, state the cloth: the cloth has lost electrons, so it is positively charged.
Step 4, compare sizes: the rod gains as much negative charge as the cloth loses, so the charges are equal in size and opposite in sign.
Answer: electrons move from the cloth to the rod. The rod is negative and the cloth is positive, with equal amounts of charge.
The mistake to watch for
A common answer is to say that the cloth “gives positive charge to the rod” or that protons are transferred.
Mistaken answer: The rod becomes negative because it gains protons from the cloth.
Protons carry positive charge, so gaining them would make the rod positive. They also stay in the nuclei.
The correction is to start every charging explanation with “electrons move from … to …”. Then read the sign from the direction: gain of electrons is negative, loss is positive.
Check yourself
Try these, then open each answer.
1. A Perspex rod rubbed with a cloth becomes positively charged. What moved, and from where to where?
Show answer
Electrons moved from the rod to the cloth. The rod lost electrons, so it is positive, and the cloth gained electrons, so it is negative.
2. A negatively charged balloon is held near a second negatively charged balloon. Do they attract or repel? Give the rule you used.
Show answer
They repel, because like charges repel.
3. A negatively charged metal sphere is touched by a wire connected to the ground. In which direction do electrons move, and what is the final charge on the sphere?
Show answer
Electrons move from the sphere into the ground, because they repel each other and the ground can take them. The sphere ends up neutral.
Where this leads next
Once you can say which way the electrons moved, go to calculating current from charge and time. The wider charge and current module shows the full study order.
Some students can recite the rules yet lose marks because the explanation leaves out the particle or the direction. That habit is easy to spot in a live conversation, which is part of our online one-to-one Physics tuition.