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7.1 Electric Field And Charge

How charges are produced?


There are two kinds of charge which produce opposite effects are called negative charge and positive charge.

Like charges always  repel each other and unlike charges always attract each other.

An object becomes negatively charged when it gains an excess of electrons. Similarly , when an object has some electrons removed, the deficiency of electrons makes the object positively charged.

The symbol of charge is Q and the unit is coulomb (C).

The charge in an electron is , e = 1.6 x 10-19C.

The Van de Graf generator built a machine in which charge continuously delivered to the inside of a hollow metal dome by means of a rotating rubber belt. So the generator can produce a high voltage, usually in the megavolt range.

The definition of current, I


Current ,I is the rate of flow of charge, Q




The unit of current, I , is   Ampere (A)

                                                1A= 1 C s-1


 The charge in an electron is , e = 1.6 x 10-19C.

  If  the number of electrons is n,


Example 1

A charge of 600 C  flow through a metal conductor in every 5 minutes. What is the electric current in the conductor?





Example 2

An electric current of  3.2 mA  flows through a bulb for  4 minutes.

(charge in an electrons ,e =1.6 x 1019C)


(a)           the electric charge

(b)           the number of electrons





The experiment to investigate the relationship between the rate of flow of  electric charge and the current


The  current increases  as  the rate of flow  of electric charge increases

Aim of the experiment :

To investigate the relationship between the rate of flow of  electric charge and the current

Variables in the experiment:

Manipulated variable:  rate of flow  of electric charge

Responding variable: the current

Fixed variable: the number of charge

List of apparatus and materials:

Van de Graf generator, crocodile clip, galvanometer, micro ammeter, metal plate, nylon, ping-pong ball coated with a metallic paint and metre rule

Arrangement of the apparatus:


The procedure of the experiment which include the method of controlling the manipulated variable and the method of measuring the responding variable.

By using a metre rule , the distance between two metal plate  is measured = x

The motor of the Van de Graff is turned on for a  few  minutes.

The ping-pong ball is touched to metal plate.

The reading of micro ammeter is recorded = I

The experiment is repeated 5 times for with different  distances between two metal plates

Tabulate the data:















Analysis the data:

 Plot the graph  I against  x


Electric field

Electric field is a region  around a charged object which  any other charged body  experience a force.

Electric field can be represented by arrow lines called electric field line or electric lines of force.

The electric field is strongest where the electric lines of force are close together.

The electric field lines of force never cross over and they start on positive charges and end on negative charges.

The electric field lines of force is  a vector quantity as it has both  force and  direction.

Making the shape of an electric field visible



Semolina powder is dispersed evenly on the surface of the oil.

The high voltage sup[ply is switched on.

The semolina powder on the oil surface become

aligned in the direction of electric field and thus maps

out the electric field lines due the action of the force of the electric field.


Some patterns of the electric fields

The pattern of electric field lines of  force depends on the shape and the number of charged objects.

The following figure shows the some examples of the electric fields:





Effect of an Electric Field on a charge



A  ping-pong ball is coated with a metallic paint.

The high voltage supply is switched on.

The ping-pong ball is touched  the negative plate and released.

The ping-pong oscillates between the two plates.

It is because when the ball touches the negative plate,it receives negative charges. So the ball is attracted by the positive plate. When the ball touches the positive plate ,its charged is neutralised and immediately receives positive charge.


Hence the ball is attracted by the negative plate. The process is repeated caused the ball oscillates.

The speed of the ball increases as

(1)     the distance between two electrode decreases

(2)     the mass of the ball decreases

(3)     the voltage of the power supply increases

Effect of an Electric Field on candle flame



A lighted candle is placed between to metal plates.

The voltage supply is switched on.

The flame is seen flatten and spreads out and more to the negative plate.

It is because the heat of burning candle produces positive and negative ions.

The positive ions which are heavier is pulled towards negative plate with a large force occupy a larger proportion of the flame.


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