6.5 Static electricity and electric fields — Learn

Prerequisites: Section 6.1, especially electric charge and $Q = It$; the idea that a force can act without contact, from Chapter 2.

By the end of this section you can: describe how static electricity is produced by rubbing insulating materials; explain the transfer of electrons; describe the forces between charged objects; and explain the non-contact force using an electric field.

1. Charging by rubbing

When two different insulating materials are rubbed together, they become electrically charged. Electrons are rubbed off one material and on to the other.

• The material that gains electrons becomes negatively charged.
• The material that loses electrons is left with an equal positive charge.

This is why the charge is called static. The charge stays on the material and does not flow away, because the material is an insulator. A dry cloth and a plastic window are both insulators, so rubbing them together leaves charge on both.

2. Forces between charged objects

When two charged objects are brought close together, they exert a force on each other.

• Two objects with the same type of charge repel each other.
• Two objects with different types of charge attract each other.

You can show this with two polythene rods. Rub both with a cloth, then bring them together. They push apart. Bring a rubbed polythene rod near a rubbed perspex rod, and they pull together. The polythene has gained electrons and the perspex has lost them, so the two rods carry opposite charges.

Attraction and repulsion between two charged objects are examples of a non-contact force. The force acts even when the objects do not touch.

3. Electric fields

A charged object creates an electric field around itself. The field is the region where the object can exert a force on another charged object.

The electric field is strongest close to the charged object. The further away from the object, the weaker the field. A second charged object placed in the field experiences a force, and that force gets stronger as the distance between the objects decreases.

The electric field around an isolated charged sphere can be drawn as lines radiating from the surface. The lines point away from a positive sphere and towards a negative sphere. The lines are closer together near the surface, which shows that the field is stronger there.

4. Sparking

A strong electric field can cause a spark. When charge builds up on an object, the field near it becomes very strong. If the field becomes strong enough, it can ionise the air, which means it removes electrons from the air molecules. The ionised air can conduct a current, so charge jumps across the gap as a spark.

You may have felt a small spark when you touch a car door after walking on a dry carpet. Charge built up on your body, and it jumped to the metal as you touched it.

Worked example 1. A spark transfers a charge of 3.0 μC in a time of 1.5 μs. Calculate the average current.

The average current is 2.0 A. The spark lasts for a very short time, so the current is large even though the charge is small.

5. Changing the charge on a sphere

The figure shows the electric field around a small charged sphere. You can change the sign of the charge and the distance from the sphere.

Predict first. Do not press the button yet. Write down your answers.

1. For a positively charged sphere, which way do the field lines point: towards the sphere or away from it?
2. Where is the field strongest: close to the sphere or far from it?

Test. Press the button to change the sign of the charge. Watch the arrows on the field lines. Then move the distance slider and watch the strength readout.

Explain. A charged sphere can attract a small charged object without touching it. Use the idea of an electric field to explain how the force acts at a distance. Write your answer in two or three sentences.

Watch out. Static charge does not stay on every material. Charge flows away through a conductor, and damp air helps charge to leak away as well. Also keep the two rules straight: like charges repel and opposite charges attract. Check your answer against a charged balloon that is pulled towards your hair: the balloon and the hair carry opposite charges, so they attract.

6. Progress check

Return to the chapter problem. The plastic window is rubbed with a dry cloth, and afterwards it attracts dust.

1. Explain how the window becomes charged.
2. Explain why the charged window attracts the dust.

You should now be able to

• describe how rubbing two insulators produces static charge;
• explain the transfer of electrons, and which material becomes negative;
• state that like charges repel and opposite charges attract;
• describe a non-contact force between charged objects;
• describe an electric field, and state that it is strongest close to the charged object;
• draw the field pattern for an isolated charged sphere;
• explain a spark using a strong electric field.