6.5 Static electricity and electric fields — Practise with help

1. $Q = 0.20 \times 30 = 6.0\,\text{C}$.
2. Positive charge and negative charge.
3. $P = VI = 230 \times 0.10 = 23\,\text{W}$.

If question 1 went wrong, check that you multiplied the current by the time. If question 3 went wrong, look again at $P = VI$.

The charge comes from electrons moving from one material to the other. Decide which material gains them.

Say what happens to the electrons, then use that to decide the sign of the charge on each material.

Rubbing the two insulators together transfers electrons from one material to the other. Electrons are rubbed off the cloth and on to the rod.

The rod gains electrons, so it becomes negatively charged. The cloth loses electrons, so it is left with an equal positive charge.

This answer is a model answer for you to compare with your own wording.

Use the two rules for charged objects. Take the two cases one at a time.

State the force for the same charges first, then the force for the opposite charges.

Two objects that carry the same type of charge repel each other. Two objects that carry opposite types of charge attract each other.

In both cases the force acts without the objects touching, so it is a non-contact force. The force gets stronger as the distance between the spheres decreases.

The balloon creates something in the space around it. What is it called, and what does it do to a charged object?

Name the field around the charged balloon, then say what a second charged object feels when it is placed in that field.

The charged balloon creates an electric field around itself. The field is strongest close to the balloon and weaker further away.

The paper is placed in the field, so it experiences a force. Because the paper and the balloon carry opposite charges, the force is attractive, and the paper moves towards the balloon without touching it. The force grows stronger as the paper gets closer.

This answer is a model answer for you to compare with your own wording.

Think about what a very strong field does to the air between the two objects.

Say what happens to the air molecules when the field is strong enough, then say why that lets charge jump across the gap.

When charge builds up on an object, the electric field near it becomes very strong. If the field is strong enough, it ionises the air in the gap. Ionising the air means removing electrons from the air molecules, so the air can conduct a current.

The ionised air gives the charge a path, so the charge jumps across the gap as a spark. The spark lasts for a very short time, so the current is large even when the charge transferred is small.

This answer is a model answer for you to compare with your own wording.

Picture the lines starting at the surface of the sphere and spreading outwards.

Say which way the arrows point for each sign of charge, then say how the spacing of the lines shows the strength of the field.

The field lines radiate outwards from the surface of the sphere, at right angles to the surface.

For a positively charged sphere, the arrows point away from the sphere. For a negatively charged sphere, the arrows point towards the sphere.

The lines are closest together near the surface and further apart further away. This shows that the field is strongest close to the sphere and weaker at a distance.

This answer is a model answer for you to compare with your own wording.

The relationship links charge, current and time. Convert both values to coulombs and seconds first.

Write $I = \dfrac{Q}{t}$ and put in $5.0 \times 10^{-6}\,\text{C}$ and $2.0 \times 10^{-6}\,\text{s}$.

The average current is 2.5 A. The spark is brief, so the current is large even though the charge is small.