4.2 Electric and Gravitational fields

Question 1

What is Newton’s Law of gravitation?

Answer 1

The gravitational force between two particles is propertional to the product of their masses, M1 and M2, and inversely proportional to their separation squared, r²
F = GMm/r²

Question 2

What does the symbol G represent?

Answer 2

The gravitational constant
= 6.67 x 10^-11Nm²kg^-2
units = Nm²/kg²

Question 3

What is coulomb’s Law?

Answer 3

The electrostatic force, F, between two small bodies is proportional to the product of their charges, Q1 and Q2, and inversely proportional to their separation squared, r².

F = Qq/4πεr²

Question 4

What does the symbol ε represent?

Answer 4

The permittivity off free space = 8.85 x 10^-12Fm^-1

Question 5

What is electric field strength, E?

Answer 5

The force experienced per unit charge by a small positive charge placed in the field.
E = Q/4πεr²

Question 6

What is the gravitational field strength, g?

Answer 6

The force experienced per unit mass by a mass placed in the field.
g = GM/r²

Question 7

What is electric potential, V?

Answer 7

It is the work done per unit charge in bringing a positive charge from infinity to that point

V = Q/4πεr

Question 8

What is gravitational potential, V?

Answer 8

The work done per unit mass in bringing a mass from infinity to that point.
V = -GM/r

Question 9

What signs (positive/ negative) represent an attractive or repulsive force or field?

Answer 9

Positive = repulsive
Negative = Attractive

Question 10

What is the gradient equal to on an V-r (Electrical/ gradational potential- distance) graph?

Answer 10

The gradient is the electric/ gravitational field strength, E/ g.

Question 11

How do you calculate the net field strength?

Answer 11

Field strength is a vector so when multiple charges/ masses act at a point, the total field strength is the vector sum of the individual fields- i.e. sum the horizontal and vertical components.

Question 12

How do you calculate the net potential?

Answer 12

The potential is a scalar quantity so the potential at a point due to multiple charges/ masses is the sum of the potential due to the individual charges/ masses.

Question 13

How do you find the field strength from a graph of potential (V) against distance away from source of field (r)?

Answer 13

The negative gradient of the graph is the field strength (E or g).
V = -gr
so -g or -E is the gradient