Section 6 - Fields

Question 1

What is a gravitational field?

Answer 1

The force field round a mass.

Question 2

What is a field line (gravitational)?

Answer 2

The path a smaller mass would follow if pulled towards a larger mass.

Question 3

Define the strength of a gravitational field (g)

Answer 3

The force per unit mass on a small test mass placed in the field.

Question 4

Describe a radial field

Answer 4

The field lines are like the spokes of a wheel, towards the centre.

Question 5

What happens to g in a radial field?

Answer 5

Decreases with increasing distance from the massive body.

Question 6

Describe a uniform field

Answer 6

Field lines are parallel and equally spaced.

Question 7

What happens to g in a uniform field?

Answer 7

Magnitude and direction of g is constant.

Question 8

Define gravitational potential energy

Answer 8

The energy of an object due to its position in a gravitational field.

Question 9

Define gravitational potential

Answer 9

Gravitational potential at a point is the work done per unit mass to move a small object from infinity to that point.

Question 10

What are equipotentials?

Answer 10

Surfaces of constant potential.

Question 11

Define potential gradient (gravitational)

Answer 11

Potential gradient at a point in a gravitational field is the change in potential per metre at that point.

Question 12

What is Kepler’s Third Law?

Answer 12

For all planets r^3/T^2 is the same.

Question 13

What are the assumptions in Newton’s law of gravitation?

Answer 13

Always attractive
Proportional to the mass of each object
Proportional to 1/r^2 (r is distance between objects)

Question 14

What is the name for G?

Answer 14

Universal constant of gravitation

Question 15

Define escape velocity

Answer 15

The minimum velocity an object must be given to escape from the planet when projected vertically from the surface.

Question 16

What is the formula to remember for escape veloctiy?

Answer 16

v(esc)=sqrt(2gR)
g is gravitational field strength
R is the radius of the planet

Question 17

What is the height of a geostationary satellite above the Earth?

Answer 17

35600km

Question 18

What do electrical conductors contain a lot of?

Answer 18

Free electrons

Question 19

What do electrically insulating materials not contain?

Answer 19

Free electrons

Question 20

What is a field line (electric)?

Answer 20

The path a free positive test charge follows.

Question 21

Describe a field between two oppositely charged plates

Answer 21

Field lines run parallel
At right angles to the plates
At the edges of the field the lines are slightly curved

Question 22

What kind of a field is it between two oppositely charged plates?

Answer 22

Uniform field

Question 23

Define electric field strength at a point in the field

Answer 23

The force per unit charge on a positive test charge placed at that point.

Question 24

Electric field strength between plates is proportional to what?

Answer 24

Q/A
Q - charge
A - surface area

Question 25

Define electric potential at a point in an electric field

Answer 25

Work done per unit positive charge on a positive test charge when it is moved from infinity to that point.

Question 26

Define potential gradient (electric) at a point in an electric field

Answer 26

The change of potential per unit change of distance in a given direction.

Question 27

How is potential gradient effected if the field is non-uniform?

Answer 27

Varies according to position and direction. | Closer equipotentials means greater potential gradient at right angles to them.

Question 28

How is potential gradient effected if the field is uniform?

Answer 28

Equipotentials between the plates are equally spaced parallel to the plates. Potential gradient is constant.

Question 29

Define electric field strength

Answer 29

The negative of the potential gradient.

Question 30

In Coulomb's law, force is proportional to what?

Answer 30

Proportional to the product of the charges | Inversely proportional to the square of the distance between them

Question 31

What is a point charge?

Answer 31

An expression for a charged object in a situation where distances under consideration are much greater than the size of the object.

Question 32

If two forces are in the same direction, what is the resultant electric field strength formula?

Answer 32

E(1) + E(2)

Question 33

If two forces are in opposing direction, what is the resultant electric field strength formula?

Answer 33

E(1) - E(2)

Question 34

If two forces are at right angles to each other, what is the resultant electric field strength formula?

Answer 34

sqrt(E(1)^2 + E(2) ^2)

Question 35

What is the constant of proportionality in electrostatic fields?

Answer 35

1/4πε(0)

Question 36

What is a capacitor?

Answer 36

A device designed to store charge.

Question 37

How does a capacitor work?

Answer 37

Two parallel metal plates placed near each other. When connected to a battery, electrons move through the battery and are forced onto one of the plates. On the other plate, an equal number of electrons leave that plate.

Question 38

What is the symbol for a capacitor in a circuit?

Answer 38

Two equal length parallel lines.

Question 39

How can a capacitor's pd be measured when it is charging?

Answer 39

Use a stopwatch and measure the voltmeter at set times. | Use a data logger.

Question 40

What would a charge against pd graph for a capacitor look like?

Answer 40

A straight proportional line passing through the origin.

Question 41

Define capacitance

Answer 41

The charge stored per unit pd.

Question 42

How is the energy stored in a capacitor obtained from a graph?

Answer 42

Charge against pd graph | Area under is a triangle of QV/2

Question 43

In what proportionality does current, charge and pd of a capacitor decrease?

Answer 43

Exponentially

Question 44

What is the time constant of a circuit?

Answer 44

RC

Question 45

When t=RC in a capacitor's discharge, what percentage charge remains?

Answer 45

37%

Question 46

When t=2RC in a capacitor's discharge, what percentage charge remains?

Answer 46

14%

Question 47

At what point, is a capacitor considered discharged?

Answer 47

When t=5RC