Gravitational and Electric Fields (Unit 4)

Question 1

Newton’s Law of Gravity

Answer 1

an attractive force between two point masses

proportional to the product of their masses and inversely proportional to their separation squared

Question 2

Concept of a force field

Answer 2

the region in which a body experiences a force

Question 3

Representation of gravitational field lines (radial and uniform fields)

Answer 3

See sheet

Question 4

Equipotential

Answer 4

Line joining points of equal potential

No work is done moving an object along an equipotential

Question 5

Gravitational field strength, g

Answer 5

the force acting per unit mass

Question 6

Gravitational field strength units

Answer 6

N kg-1 VECTOR

Question 7

Gravitational potential, V, (at a point)

Answer 7

work done per unit mass to move a small mass from infinity to that point.

Question 8

Units of gravitational potential

Answer 8

J kg-1 SCALAR

Question 9

Gravitational potential, V, at infinity

Answer 9

zero

Question 10

Gravitational potential difference, V, between two points

Answer 10

work done per unit mass to move a small mass from one point to the other.

Question 11

Graphical variation of magnitude of g with r

Answer 11

See sheet

Question 12

Graphical variation of V with r

Answer 12

See sheet

Question 13

Area under graph of gravitational field strength against r

Answer 13

work done moving a unit mass between the two points

Question 14

Gradient of graph of gravitational potential against r

Answer 14

gradient = -g (gravitational field strength); g= -(deltaV/delta r)

Question 15

Derivation of Kepler’s Law

Answer 15

1. Gravitational force = centripetal force
2. GMm/r2 = mv2/r or GMm/r2 = mr(omega)2
3. substitute for v (v = (2pi x r)/T) or omega( omega = (2pi)/T)
4. re-arrange to get T2 = ((4pi2)/GM) x r3

Question 16

Energy considerations of an orbiting satellite

Answer 16

Total satellite energy = kinetic energy + grav. potential energy
Total satellite energy = 1/2mv2 - GMm/r

Question 17

Features of a geosynchronous orbit

Answer 17

1. orbits over equator
2. maintains a fixed position relative to surface of Earth
3. period is 24 hours (same as the Earth)
4. offers uninterrupted communication between transmitter and receiver
5. steerable dish is unnecessary

Question 18

Escape velocity of an object from a planet

Answer 18

loss of kinetic energy = gain in grav. potential energy
(to get to infinity, need to provide grav. potential energy)
= 1/2mv(escape)2 - GMm/r
gives v(escape) = Square root of 2GM/r

Question 19

Coulomb’s Law

Answer 19

magnitude of force between two point charges
is proportional to the product of their charge and inversely proportional to their separation squared
the force is ATTRACTIVE with un-like charges and REPULSIVE with like charges.

Question 20

Representations of electric field lines

Answer 20

See sheet

Question 21

Electric field strength, E

Answer 21

force acting per unit charge on a positive charge.

Question 22

Electric field strength units

Answer 22

N C-1 or V m-1 VECTOR

Question 23

Electric potential, V, (at a point)

Answer 23

work done to move a small unit positive charge from infinity to the point

Question 24

Units of electric potential, V

Answer 24

J C-1 or V (Volts) SCALAR

Question 25

Electric potential, V, at infinity

Answer 25

Zero

Question 26

Electric potential difference, V, between two points

Answer 26

work done to move a small unit positive charge from one point to the other.

Question 27

Graphical variations of E with r (radial and uniform fields)

Answer 27

See sheet

Question 28

Graphical variations of V with r (radial and uniform fields)

Answer 28

See sheet

Question 29

Area under graph of electric field strength against r

Answer 29

See sheet

Question 30

Path of charged particle in a uniform electric field

Answer 30

Path is parabolic, because,

Magnitude of force is constant and always in the same direction

Question 31

Speed of charged particle accelerated across a potential difference, V

Answer 31

Loss of electrical potential energy = gain in kinetic energy

QV = 1/2mv2

Question 32

Similarities between electric and gravitational fields

Answer 32

field strengths are both inversely proportional to separation squared
potentials are both inversely proportional to separation
(see table on Pg 89 of A2 text book.)

Question 33

Differences between electric and gravitational fields

Answer 33

masses always attract but charges may attract or repel

see table on Pg 89 of A2 text book