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objectives
objectives
Electric Fields
Electric Fields
- The girl on the right has given herself an electric charge by touching the dome of a Van de Graaff generator.
- The dome can reach over 100 000 volts, although this is reduced when she touches it.
- However, the current that flows into her body (0.00002 amperes or less) is far too small to be dangerous.
- The force of repulsion between the charges on the girl’s head and hairs is strong enough to make her hairs stand up.
- If electric charges feel a force, then, scientifically speaking, they are in an electric field.
- So there is an electric field around the dome and the girl.
Electric Field Patterns
Electric Field Patterns
- In diagrams, lines with arrows on them are used to represent electric fields.
- There are some examples of field patterns below.
- In each case, the arrows show the direction in which the force on a positive (+) charge would act.
- As like charges repel, the field lines always point away from positive (+) charge and towards negative (-) charge.
Curves, Points and Ions
Curves, Points and Ions
- When a conductor is charged up, the charges repel each other, so they collect on the outside. The charges are most concentrated near the sharpest curve.
- This is where the electric field is strongest and the field lines closest together.
- If a sharp spike is put on the dome of a Van de Graaff generator, any charge on the dome immediately leaks away from the point. At the point, the metal is very sharply curved.
- Here, the charge is so concentrated that the electric field is strong enough to ionize the air (see above).
- Ionized air conducts, so the dome loses its charge through the air.
- Ions* are electrically charged atoms (or groups of atoms).
- Atoms become ions if they lose (or gain) electrons.
- A stream of ions is a flow of charge, so it is another example of a current.
- Most of the molecules in air are uncharged, but not all, as shown on the right.
- Flames, air movements, and natural radiation from space or rocks can all remove electrons from molecules in air so that ions are formed.
electric fields questions
electric fields questions
The diagram on the right shows electric field lines round a charged metal sphere (in air).
Copy the diagram.
- Draw in the direction of the electric field of each field line.
- If a positive charge were placed at X, in which direction would it move?
- If a negative charge were placed at X, in which direction would it move?
- If a sharp spike were placed on top of the sphere, what would happen to the charge on the sphere?
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