4.2 Fields

Question 1

What are some similarities between electrostatic and gravitational forces?

Answer 1

● Inverse square force laws.
● Potential concept.
● Equipotential surfaces.
● Use of field lines.

Question 2

What are some differences between electrostatic and gravitational forces?

Answer 2

The gravitational forces from masses always attract, whilst charges may repel or attract.

Question 3

What is gravity?

Answer 3

Gravity is the universal attractive force which acts between all matter.

Question 4

What is G?

Answer 4

The universal gravitational constant. Approx.
6.67x10-11 m3 kg-1 s-2

Question 5

What can field lines tell you about a field?

Answer 5

The direction of the field (shown by arrows) and the strength of the field (shown by the density of the lines).

Question 6

What is 𝘨?

Answer 6

𝘨 is the force per unit mass on a small test mass placed in a gravitational field.

Question 7

What are field lines?

Answer 7

● They show the direction of the force on a test mass placed in a gravitational field or a positive test charge placed in an electric field.
● For a point mass, the field lines point towards the centre of the mass.
● For a positive point charge, the field lines point radially outward.

Question 8

Why can a spherical mass be treated as a point mass?

Answer 8

The gravitational field around a spherical mass is essentially the same as if all of its mass was concentrated at its centre.

Question 9

What is gravitational potential?

Answer 9

The work done per unit mass in bringing an object from infinity to that point.
Denoted by Vg, with units J kg-1.

Question 11

What is gravitational potential difference?

Answer 11

Gravitational potential difference is the difference in the gravitational potentials of two points in a gravitational field.

Question 12

What is an equipotential surface?

Answer 12

• A surface in which every point on the surface has the same potential.
• Equipotentials are perpendicular to field lines, so they’re spherical for a point mass/charge.

Question 13

How much work is done when you move 1km in any direction on an equipotential?

Answer 13

No work is done when moving across equipotentials, as the potential at each point is the same.

Question 14

Why is gravitational potential a negative value?

Answer 14

Work needs to be done to move an object from inside the field to outside the field. Since outside the field’s potential is defined as 0 (at infinite distance from the object) then the potential inside the field must be negative.

Question 15

Compare the PE and KE of a lower orbit to a higher one.

Answer 15

A lower orbit (smaller r) has less potential energy and more kinetic energy than a higher orbit (bigger r).

Question 16

What symbol represents the permittivity of free space?

Answer 16

ε0

Question 17

When calculating the force between two particles, air can be treated as a —– .

Answer 17

Vacuum.

Question 19

Which is stronger: the gravitational force of subatomic particles or the electrostatic force?

Answer 19

The electrostatic force.

Question 20

Electric field lines always go from —- to —-.

Answer 20

Positive to negative.

Question 21

What is electric field strength?

Answer 21

The force per unit charge on a positive test charge placed in an electric field.

Question 22

How is electric potential related to electric field strength?

Answer 22

E = ΔV/Δr
The change in electric potential with respect to the change in radius length.