# 3.1 Electric Field

### Electric Field

1. An electric field is a region in which an electric charged particle experiences an electric force.
2. Electric field is represented by a number of lines with arrows, called electric lines of force or electric field lines.
3. The direction of the field at a point is defined by the direction of the electric force exerted on a positive test charge placed at that point.
4. The strength of the electric field is indicated by how close the field lines are to each other. The closer the field lines, the stronger the electric field in that region.
Example
5. The lines of force are directed outwards for a positive charge and inwards for a negative charge.
6. The electric line of force will never cross each other.
7. The figure shows a few examples of the field pattern that you need to know in the SPM syllabus.

### Effect of Electric Field on a Ping Pong Ball Coated with Conducting Material

1. A ping ball coated with conducting material is hung by a nylon thread.
2. When the ping pong ball is placed in between 2 plates connected to a Extra High Tension (E.H.T.) power supply, opposite charges are induced on the surface of the ball. The ball will still remain stationary. This is because the force exert on the ball by the positive plate is equal to the force exerted on it by the negative plate.
3. If the ping pong ball is displaced to the right to touch the positive plate, it will then be charged with positive charge. Since like charges repel, the ball will be pushed towards the negative plate.
4. When the ping pong ball touches the negative plate, it will be charged with negative charge. Again, like charge repel, the ball will be pushed towards the positive plate. This process repeats again and again, causes the ping pong ball oscillates to and fro continuously between the two plates.

### A Candle Flame in an Electric Field

1. Normally, with absent of wind, the flame of a candle is symmetry.
2. The heat of the candle flame removes electrons from the air molecules around it, and therefore ionised the molecule. As a result, the flame is surrounded by a large number of positive and negative ions.
3. If the candle is placed in between 2 plates connected to a Extra High Tension (E.H.T.) power supply, the positive ions will be attracted to the negative plate while the negative ions will be attracted to the positive plate.
4. The spreading of the flame is not symmetry. This is because the positive ions are much bigger than the negative ions; it will collide with the other air molecule and bring more air molecule towards the negative plate.