Comparison between Electric and Gravitational fields

Question 1

State the similarities between gravitational and electric fields

Answer 1

1) The line of force (field line)
2) Inverse square law of force
3) Field strength (Force per unit mass/charge)
4) Unit of field strength (Nkg⁻¹ and NC⁻¹)
5) They are uniform fields
6) Potential (Potential per unit mass/charge)
7) Unit for potential (Jkg⁻¹ and JC⁻¹)
8) Radial fields (due to point mass/charge)

Question 2

State the differences between gravitational and electric fields

Answer 2

1) Action at a distance
2) Force (attracts only for gravity)
3) Constant of proportionality in force law

Question 3

State the laws and give the equations for the inverse square of the force

Answer 3

Gravitational:
▪ Newton's law of gravitation
▪ F = Gm₁m₂ / r²
Electrical:
▪ Coulomb's law of force
▪ F = Q₁Q₂ / 4πε₀r²

Question 4

Give the equations for field strenght

Answer 4

Gravitational: force per unit mass, g = F / m
Electrical: force per unit +ve charge, E = F / q

Question 5

Give the symbol and similarities between the uniform fields

Answer 5

Gravitational: g is the same everywhere, field lines are parallel and equally spaced
Electrical: E is the same everywhere, field lines are parallel and equally spaced

Question 6

State the potentials and units for the 2 forces and

Answer 6

Gravitational potential energy per unit mass (Jkg⁻¹)

Electrical potential energy per unit +ve charge (V = JC⁻¹)

Question 7

Give the equation for the potential energy between 2 point masses/charges

Answer 7

Gravitational: E = -Gm₁m₂ / r
Electrical: E = Q₁Q₂ / 4πε₀r

Question 8

State the radial fields for both forces

Answer 8

Gravitational: due to point mass or a uniform spherical mass M
▪ g = GM / r²
▪ V = -GM / r
Electrical: due to point charge Q
▪ E = Q / 4πε₀r²
▪ V = Q / 4πε₀r

Question 9

Give the constant of proportionality in the force law for:

i) Gravity
ii) Electricity

Answer 9

i) G

ii) 1 / 4πε₀

Question 10

Describe the action at any distance for:

i) Gravitational forces
ii) Electrical forces

Answer 10

i) Between any two masses

ii) Between any two charged objects

Question 11

Describe the direction/s of the force for:

i) Gravity
ii) Electricity

Answer 11

i) Attracts only

ii) Unlike charges attract, like charges repel