# What is an electric field and how is it created?

## What is an electric field and how is it created?

An electric field is a region in which an electric charge experiences an electric force.

1. Figure shows an electric field created by a positively-charged sphere.
2. Such a field can be represented by a number of lines, called electric lines of force. These lines indicate both the strength and direction of the field. Therefore, electric field is a vector quantity.
3. The direction of the electric field at a particular point is given by the direction of the force acting on a positive test charge placed at that point.
4. In Figure, a positive charge experiences a force which pushes it away from the sphere. Hence, the direction of the electric field at that point is away from the sphere.
5. When a negative charge is placed in this electric field, it experiences a force pulling it towards the positively-charged sphere.
6. Figure shows the electric field lines pointing away from a positively-charged isolated sphere and pointing towards a negatively-charged isolated sphere. In both cases, the direction of the electric field is determined by the direction of force on a positive charge, placed in each electric field.

## The Effect of an Electric Field on a Charge

1. A small polystyrene ball coated with conducting material is suspended on an insulating thread. It is then brought near to a positively-charged metal dome of a Van de graaff generator as shown in Figure (a). The near end of the polystyrene ball is negatively charged by induction.
2. Unlike charges attract and hence the coated polystyrene ball is attracted towards the positively-charged dome as shown in Figure (b). When the polystyrene ball makes contact with the charged dome, the negative charges are neutralised and it acquires positive charges.
3. Since like charges repel, it is repelled by the charged dome as shown in Figure (c).
Similarly, when a small positively-charged sphere is placed in an electric field produced by two parallel plates as shown in
4. Figure, the positively-charged sphere will experience a force, F acting in the direction of the field. In other words, it is repelled by the positive plate and is attracted by the negative plate.
5. When a small polystyrene ball coated with conducting material is placed inside the electric field produced by the two parallel plates as shown in Figure, the coated polystyrene ball will move towards the positively-charged plate when displaced slightly to the left. It sticks to the plate momentarily and then repels towards the negatively-charged plate.
6. It will stick to the negatively-charged plate momentarily and then repel towards the positively-charged plate. The process is repeated causing the coated polystyrene ball to move to and fro between the plates.

## Electric Field Experiment

Aim: To observe the pattern of an electric field.
Materials: Cooking oil, talcum powder
Apparatus: Extra High Tension (E.H.T.) power supply (0 – 5 kV), electrodes with different shapes (pointed electrode and plane electrode), Petri dish, connecting wires
Method:

1. The apparatus is set up as shown in Figure. The talcum powder is sprinkled on top of the oil.
2. The E.H.T. power supply is switched on and the voltage is adjusted to 4 kV.
3. The movement of the talcum powder and the field patterns are observed.
4. Steps 1 to 3 are repeated by using different arrangements of the electrodes as shown in Table. The patterns are also shown in the table.

Discussion:

1. Electric field lines always extend from a positively-charged object to a negatively-charged object, from a positively-charged object to infinity or from infinity to a negatively-charged object.
2. Electric field lines never cross each other.
3. Electric field lines are most dense around objects with the greatest amount of charges.
4. At locations where electric field lines meet the surface of an object, the lines are perpendicular to the surface.

Conclusion:
The pattern formed by the talcum powder is called an electric field pattern. The electric field can be represented by lines of force.

## Effect of Electric Field on Objects Experiment

The Effect of an Electric Field on a Table Tennis Ball Coated with Conducting Paint
Aim:
To observe the effect of an electric field on objects.
Materials:
A table tennis ball covered with conducting paint, nylon thread
Apparatus: Two metal plates, E.H.T. power supply (0 – 5 kV), two retort stands with clamps, connecting wires
Method:

1. A table tennis ball coated with a conducting metallic paint is hung between two metal plates with a nylon thread (as an insulator) as shown in Figure 2.12.
2. The E.H.T. power supply is switched on and the voltage is adjusted to 4 kV.
3. The table tennis ball is displaced to plate X and then the movement of the table tennis ball is observed.

Discussion:

1. When the E.H.T. power supply is switched on, plate X is positively-charged and plate Y is negatively-charged. Since the table tennis ball is neutral, it remains at the centre as the electric forces acting on it are balanced.
2. The ball is then displaced to any of the plates, as in this case plate X. When the table tennis ball touches plate X, it is charged with positive charges and repels towards plate Y. See Figure (b).
3. When the ball touches plate Y, the positive charges are neutralised by negative charges. The ball is then negatively- charged and repels towards plate X. See Figure(c). The process is repeated and the ball oscillates to and fro between metal plates X and Y.
4. The rate of oscillation of the table tennis ball can be increased by:
(a) increasing the voltage of the E.FH.T. power supply,
(b) decreasing the distance between plates X and Y.

## Effect of Electric Field on Candle Flame Experiment

Aim: To observe the effect of an electric field on Candle Flame.
Material: Candle
Apparatus: Two metal plates, E.H.T. power supply (0 – 5 kV), two retort stands with clamps, connecting wires
Method:

1. The table tennis ball is replaced with a lighted candle as shown in Figure.
2. Step 2 in section A is repeated.
3. The shape of the candle flame is. observed.

Discussion:

1. When the E.H.T. power supply is switched on, the candle flame splits into two portions in opposite directions. The portion that is attracted to the negative plate Y is very much larger than the portion that is attracted to the positive plate X. See Figure.
2. The hot flame of the candle ionises the air molecules in its surrounding into positive and negative ions. The positive ions are heavier and move much slower. Thus, a larger portion of the flame follows the positive ions, moving towards the negative plate.
3. The negative ions are electrons and are very much lighter than the positive ions. They move faster towards the positive plate, X. Thus, the portion of the flame that follows the negative ions are much smaller and thinner.