Electric Field

Question 1

What is Coulomb’s law of electrostatic?

Answer 1

It states that electric force is directly proportional to the product of two point charges and inversely proportional to the square of their separation

Question 2

What is electric field?

Answer 2

Electric field is a region of space where a charge experiences an electric force

Question 3

What is the direction of electric field?

Answer 3

The direction of force on a positive charge

Question 4

What does electric field lines show?

Answer 4

The direction of force acting on a stationary positive point charge in the field

Question 5

What are the properties of electric field lines?

Answer 5

• electric field lines do not cross or touch
• electric field lines are perpendicular to any conducting surface
• number of field lines entering or leaving a charge is proportional to the magnitude of charge

Question 6

What does a higher density of electric field lines mean?

Answer 6

Stronger electric field

Question 7

Can electric field lines pass through a conductor?

Answer 7

No

Question 8

Can electric field lines pass through an insulator?

Answer 8

Yes

Question 9

What do the field lines of uniform electric field look like?

Answer 9

Parallel and equally spaced

Question 10

For non-linear electric field lines, what is the direction of electric field?

Answer 10

Direction of electric field E is tangent to the electric field lines at each point

Question 11

What is electric field strength?

Answer 11

Electric force per unit charge that acts on a small stationary positive test charge placed at that point. Its magnitude equals to the potential gradient at that point

Question 12

Why must test charge be small?

Answer 12

To prevent distortion of electric field that is present due to another charge

Question 13

Why must test charge be stationary?

Answer 13

A test charge that moves in a non-uniform electric field or non-parallel magnetic field would experience varying forces at different positions

Question 14

What is electric potential?

Answer 14

Electric potential is the work done per unit positive charge in bringing a small test charge from infinity to that point by an external agent at constant kinetic energy

Question 15

What is the relationship between electric field strength E and potential V?

Answer 15

• E is always pointing towards decreasing V
• E and V are always perpendicular to each other
• E is numerically equal to the potential gradient -dV/dr

Question 16

What is the physical meaning of the negative sign in equation E = -dV/dR?

Answer 16

electric field strength is always pointing towards decreasing potential

Question 17

Why is gravitational potential always negative whereas values of electric potential can be positive or negative?

Answer 17

• Gravitational force is always attractive and thus work done by external agent is always negative when mass is brought from infinity
• Electrical force can be either attractive or repulsive and thus when charge is brought from infinity, work done by external agent is positive for charges of same sign and negative for charges of opposite sign

Question 18

Why electric field lines always intersect a charged plate at 90° to the surface of the plate?

Answer 18

• Electric potential is constant, so electric field strength is zero on its surface
• no force will be acting on the charge, hence no work is done when a positive test charge is moved along the surface
• electric field lines which represent the direction of electric field strength and force must be perpendicular to the surface of the plate

Question 19

What is electric potential energy?

Answer 19

Work done in moving a point charge from infinity to that point by an external agent at constant speed

Question 20

Two parallel plates are separated by a distance d with a potential difference of V, how to show that the magitude of electric field strength E is numerically equal to potential gradient?

Answer 20

• Electric force F acting on a charge q is qE
• work done W = external force F × displacement d = qEd
• By applying definition for electric potential, W = qV
• qV = qEd, E = V/d

Question 21

What is an electronvolt?

Answer 21

A unit of energy that equals to the energy gained by an electron when it is accelerated through a potential difference of 1 volt