Section 7.3: Electric Fields

Question 1

State Coulomb’s law?

Answer 1

The magnitude of the force between two point charges in a vacuum is directly proportional to the product of the two charges and inversely proportional to the square of the distance between their centres.

(Air can be treated as a vacuum when using the formula)

Question 2

Define electric field strength

Answer 2

The force per unit positive charge experienced by an object in an electric field, symbol E.
It is constant in a uniform field and varies in a radial field.

Question 3

What direction are electric field lines?

Answer 3

They always act towards negative charges and away from positive charges, since they represent the force on a positive charge.

Question 4

Define absolute electric potential at a point

Answer 4

The work done per unit positive charge to move a positive test charge from infinity to that point

or

The potential energy per unit positive charge at that point in the field.

(Symbol V)

Question 5

Where is electric potential greatest and where is it zero?

Answer 5

It is greatest at the surface of a charge, and zero at infinity.

Question 6

How do you find electric field strength at a point graphically?

Answer 6

Draw a tangent to a potential-distance graph at that point and find the gradient.

Question 7

Define electric potential difference

Answer 7

The energy needed to move a unit charge between two points in an electric field, defined by ΔV.

Question 8

How do you find electric potential difference graphically?

Answer 8

Find the area underneath a graph of electric field strength over distance, between two given distances.