Course: Separate Physics only | Tier: Foundation + Higher | Read the notes, then try the questions on paper.
Notes
Charging by friction
When two insulating materials are rubbed together, electrons are transferred from one to the other. The material that gains electrons becomes negatively charged; the one that loses electrons is left positively charged with an equal charge. Only electrons move – protons stay in the nuclei.
- Like charges repel; unlike charges attract. This is a non-contact force.
- If enough charge builds up, the p.d. becomes large enough to cause a spark as charge jumps across a gap.
Electric fields
A charged object creates an electric field around it. A second charged object in the field feels a force. The field is strongest closest to the charged object, so the force gets weaker with distance. Field lines point away from positive and towards negative charges; around an isolated charged sphere they point radially. Sparking happens when the field is strong enough to make the air conduct.
Questions
Q1 (F) A polythene rod is rubbed with a cloth and becomes negatively charged. Explain why.
Show answer
Electrons were transferred from the cloth to the rod. The rod gained electrons, so it has a negative charge.
Q2 (F) What charge does the cloth in Q1 have?
Show answer
Positive – it lost electrons. Its charge is equal in size to the rod’s.
Q3 (F) Two negatively charged balloons are brought close together. What happens?
Show answer
They repel – like charges repel.
Q4 (F) Why don’t protons move when objects are charged by friction?
Show answer
Protons are held in the nuclei of atoms. Only electrons, on the outside of atoms, can be transferred.
Q5 (F/H) Explain why a spark can jump from a charged object to an earthed one.
Show answer
As charge builds up, the p.d. between the object and earth increases. When it is large enough, the electric field makes the air conduct, and charge jumps across the gap as a spark.
Q6 (F/H) Describe the electric field pattern around an isolated positively charged sphere.
Show answer
Straight field lines pointing radially outwards from the sphere, closest together near the sphere (where the field is strongest).
Q7 (F/H) A small charged ball is moved further from a charged dome. What happens to the force on it, and why?
Show answer
The force gets weaker, because the electric field is weaker further from the charged dome.
Q8 (H) A charged balloon sticks to an uncharged wall. Suggest why.
Show answer
The negative balloon repels electrons in the surface of the wall slightly away, leaving that surface slightly positive. Unlike charges attract, so the balloon is attracted to the wall.
