Describe charging by friction, the forces between charges, and current as the flow of charge

What this dot point is asking

SEAB wants you to describe how objects become charged by friction, to explain the forces of attraction and repulsion between charges, and to describe electric current as the flow of charge, using $Q = It$. The big idea is that charge can be moved by rubbing (static electricity) or made to flow around a circuit (current).

The answer

Electric charge

There are two kinds of electric charge, positive and negative. An atom normally has equal positive charge (in the nucleus) and negative charge (the electrons), so it is neutral. Electrons can be moved from one object to another, and this is what creates charge.

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 material that loses electrons becomes positively charged.

For example, rubbing a plastic rod with a cloth can move electrons onto the rod, leaving it negative and the cloth positive. The charges are always equal and opposite, because the electrons are simply moved, not created.

Forces between charges

Charged objects exert forces on each other:

• Like charges (both positive or both negative) repel each other.
• Unlike charges (one positive, one negative) attract each other.

A charged rod can also attract small uncharged objects, such as bits of paper, by pulling on the charges within them.

Electric current

An electric current is the rate of flow of electric charge around a circuit. In a metal wire the current is a flow of electrons. Current is measured in amperes (amps, $\text{A}$) using an ammeter connected in the circuit.

Charge is measured in coulombs ($\text{C}$). The charge that flows is linked to the current and time by:

where $Q$ is the charge in coulombs, $I$ is the current in amps, and $t$ is the time in seconds. A bigger current, or a longer time, means more charge has flowed.

Examples in context

Example 1. Static shocks. Walking on a carpet can rub electrons onto you, leaving you charged. When you touch a metal door handle, the charge suddenly flows, and you feel a small shock. Dry air makes this worse because the charge cannot leak away gradually.

Example 2. Lightning. Inside a storm cloud, moving ice and water rub together and separate charge, building up a huge negative charge at the base of the cloud. When the charge is large enough, it jumps to the ground as a lightning strike, a very large, very fast flow of charge.

Try this

• Cue. Explain why a plastic comb rubbed on a sleeve can pick up small pieces of paper. [2 marks] Rubbing charges the comb; the charged comb attracts the uncharged paper by pulling on the charges within it.

• Cue. A current of $4.0\ \text{A}$ flows for $10\ \text{s}$. Find the charge. [2 marks] $Q = It = 4.0 \times 10 = 40\ \text{C}$.

• Cue. State what happens when a positively charged rod is brought near a negatively charged rod. [1 mark] They attract each other, because unlike charges attract.