6.5 Static electricity and electric fields — Practise

Work through these on your own. Most of these questions want an explanation, so answer in full sentences and use the science words exactly.

Both the questions and the answers below come from a set that is checked by computer every time the book is built.

Express a charge of 3.0 μC in coulombs, using standard form.
Express a time of 2.0 μs in seconds, using standard form.
A spark transfers a charge of 3.0 μC in a time of 1.5 μs. Calculate the average current.
Describe how rubbing two insulating materials together makes them electrically charged, and state the sign of the charge on the material that gains electrons.
State what force acts between two charged objects that carry the same type of charge, and between two that carry opposite types of charge. Say what kind of force this is.
Explain how the idea of an electric field helps to explain the non-contact force between two charged objects.
Explain how a strong electric field can cause a spark.
Describe the electric field pattern around an isolated charged sphere. Include the direction of the field lines for a positive sphere and for a negative sphere.
Describe how the strength of the electric field around a charged object changes as the distance from the object increases.
A current of 0.25 A flows for 80 s. Calculate the charge that flows. This uses the equation from Section 6.1.
A device on the 230 V supply takes a current of 0.30 A. Calculate its power. This uses the equation from Section 6.4.
A spark transfers a charge of 4.0 μC in a time of 2.0 μs. Say which relationship you will use, then calculate the average current.

Answers and concise solutions

1. 3.0 μC in coulombs

Answer: $3.0\,\mu\text{C} = 3.0 \times 10^{-6}\,\text{C}$ . The prefix micro means one millionth.

2. 2.0 μs in seconds

Answer: $2.0\,\mu\text{s} = 2.0 \times 10^{-6}\,\text{s}$ . The prefix micro means one millionth.

3. Average current of the spark

Answer: $I = \dfrac{Q}{t} = \dfrac{3.0 \times 10^{-6}}{1.5 \times 10^{-6}} = 2.0\,\text{A}$ .

4. How rubbing produces charge

Electrons are rubbed off one material and on to the other.
The material that gains electrons becomes negatively charged, and the material that loses electrons is left with an equal positive charge.

5. The forces between charged objects

Two objects with the same type of charge repel each other.
Two objects with opposite types of charge attract each other.
Attraction and repulsion between charged objects are non-contact forces, because they act without the objects touching.

6. The electric field and the non-contact force

A charged object creates an electric field around itself. The field is strongest close to the object and weaker further away.
A second charged object placed in the field experiences a force. The force gets stronger as the distance between the objects decreases, so the objects can push or pull each other without touching.

7. How an electric field causes a spark

When charge builds up, the electric field near the object becomes very strong.
A strong enough field ionises the air in the gap, removing electrons from the air molecules so that the air can conduct.
The ionised air gives the charge a path, so the charge jumps across the gap as a spark.

8. The field pattern of a charged sphere

The field lines radiate from the surface of the sphere, at right angles to the surface.
For a positive sphere the arrows point away from the sphere. For a negative sphere the arrows point towards the sphere.
The lines are closest together near the surface, so the field is strongest there and weaker further away.

9. How the field strength changes with distance

10. Charge that flows

Answer: $Q = It = 0.25 \times 80 = 20\,\text{C} = 2.0 \times 10^1\,\text{C}$ .

11. Power of the device

Answer: $P = VI = 230 \times 0.30 = 69\,\text{W}$ .

12. Average current of the spark

Answer: Use $Q = It$ , rearranged to $I = \dfrac{Q}{t}$ . Then $I = \dfrac{4.0 \times 10^{-6}}{2.0 \times 10^{-6}} = 2.0\,\text{A}$ .