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Introduction to Electric Field

Aug 27, 2022
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Key Concepts

  • The nature of electric force
  • Coulomb’s law
  • Electric field and its strength

Introduction: 

Factors affecting electric force: 

The distance between two charged objects affects the magnitude of the electric force between them. 

The closer the two charges the greater the force between the charges. 

Electric force depends on the distance between the charges 

The amount of charge on the two objects also affects the magnitude of the force. 

Force depends upon the magnitude of the charges

Explanation: 

Coulomb’s Law: 

In the year 1780, Charles Coulomb conducted a variety of experiments to determine the force between two charges. He found that the electric force of attraction or repulsion between the two charges is directly proportional to the product of the charges. 

F ∝ q1q2 

parallel

The electric force is inversely proportional to the square of the distance between them. 

F ∝ 1/r2

 The electric force between two charges

Thus, we can write: 

F = K q1q2 / r2

Here K is a constant, its value depends on the medium between the charges and the system of units chosen. In the SI system the unit of K is: 

parallel

K = Fr2 / q1q2 = newton ×metre2 / coulomb2

An electric force is a vector quantity. So, the electric force has a magnitude and direction. The electric force between two charged objects always acts along the line that connects their centers of charge. 

Properties of electric force: 

  • The electric force is exerted by one object on another even though there is no physical contact between the two objects such force is known as noncontact force or a field force. 
  • Electric force can be attractive or repulsive depending upon the nature of the charges (positive or negative). 
  • The electric force is much stronger than the gravitational force. 

Electric field: 

A charged object set up an electric field in the region around it. When a second charged object enters this field, an electric force starts acting on it. 

The strength of the electric field, E, at a location around a charge Q is determined with a small positive test charge q.  The direction of electric field strength depends upon the direction of force F acting on the positive test charge. 

The electric field strength E at any point in an electric field is a vector quantity whose magnitude is equal to the force acting on the unit positive test charge and whose direction is along the direction of the force. 

Direction of the force on a test charge

Question: 1 

Why do we choose a test charge having a charge of very small magnitude? 

Answer:

The electric field around the charged sphere could be explored by placing a positive test charge in a variety of places near the charged sphere.  

A test charge placed near a charged sphere

However, when the magnitude of the test charge [Figure: 6B] is great enough to influence the charge on the sphere, the test charge will cause a redistribution of the charge on the sphere due to induction, which changes the electric field strength. 

Test charges of different magnitudes placed near a charged sphere

To overcome this problem, we always choose the test charge [Figure: 6A] small enough to have a negligible effect on the distribution of charges on the sphere and not create its strong field and interact with the field to be tested. 

Question: 2 

Why do we choose a test charge as always positive? 

Answer:

In magnetism, we choose the direction of a magnetic field as the direction in which the north pole of a magnetic needle moves. Similarly in electricity, the direction of an electric field is taken as the direction in which a small positive charge moves. It is taken conventionally. It can be taken as negative too but, in that case, the direction of the field will be just the opposite.  

Effect on the direction of the force on changing the nature of test charge

Similarities between Electric Force and Gravitational force: 

Difference between Electric force and Gravitational force: 

Comments:

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