Section 7- Fields

Question 1

What is meant by the strength of a gravitational field?

Answer 1

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

Question 2

Define radial field and uniform field.

Answer 2

Radial field: field lines look like spokes of a wheel

Uniform fields: perpendicular field lines towards mass

Question 3

What is gravitational potential?

Answer 3

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

Question 4

Where would an object have to be placed for its gravitational potential energy to be zero?

Answer 4

At infinity

Question 5

How does gravitational attraction vary with distance?

Answer 5

Proportional to 1/r^2 and the mass of each object. It is always an attractive force.

Question 6

Describe the shape of a graph of g against r for points outside the surface of a planet.

Answer 6

Inverse square law curve.

Increases with straight line before decreasing exponentially.

Question 7

What is significant about the gradient of the V against r graph?

Answer 7

At any point it is equal to -g

Question 8

What is the condition needed for a satellite to be in a stable orbit?

Answer 8

E=- GMm/ 2r

Question 9

What happens to the speed of a satellite if it moves closer to earth?

Answer 9

V decreases

Question 10

Why must a geostationary satellite be in an orbit above the equator?

Answer 10

It will have a time period of 24h so that it will stay at exactly the same place in relation to earth.

Question 11

How do you charge a metal object?

Answer 11

The metal must be isolated from the earth then can be charged by direct contact with a charged object.

Question 12

What does the direction of an electric field line show?

Answer 12

The path of a free positive test charge.

Question 13

Describe how to measure the strength of an electric field.

Answer 13

E=F/Q measure the force and charge

Question 14

Is electric field strength a scalar of vector?

Answer 14

A vector from positive to negative.

Question 15

What is electrics potential at a certain position in an electric field defined as?

Answer 15

The work done per unit positive charge on a positive test charge when it is moved from infinity to that position.

Question 16

The electric field strength is equal to…

Answer 16

The negative of the potential gradient.

Question 17

How does the force between two point charges depend on distance?

Answer 17

F is proportional to 1/r^2 because F=kQ1Q2/r^2

Question 18

State the equation that gives the electric field strength near a point charge.

Answer 18

F=(1/4piEo)*(Qq/r^2)

Question 19

Why is E equal to zero inside a charged sphere?

Answer 19

Because the charge is assumed to be in the centre of the sphere.

Question 20

What are the main similarities between gravitational and electric fields?

Answer 20

The line of force/ field lines are found with a free test mass/ charge.
In uniform fields g and E are the same everywhere and the field lines are parallel and equally spaced.

Question 21

What are the main differences between gravitational and electric fields?

Answer 21

The force attracts only in a gravitational field whereas the charges can attract or repel in an electrostatic field.

Question 22

What happens when a capacitor charges?

Answer 22

A capacitor is connected to a battery and on conductor will gain electrons from the battery, the other conductor will lose electrons to it.

Question 23

Relate the pd across the plates of a capacitor to the charge on its plate.

Answer 23

It is a straight line through the origin- it is proportional.

Question 24

What are capacitors used for?

Answer 24

Smoothing circuits, backup power supplies, timing circuits, pulse producing circuits, tuning circuits, filter circuits.

Question 25

Why does a capacitor store energy as it's being charged?

Answer 25

Because electrons are forced into one plate and taken off the other. The energy is stored as electric potential energy.

Question 26

What circuit components would you change to make a capacitor charge/ discharge slower?

Answer 26

Increase the resistance or capacitance.

Question 27

How does a dielectric affect a capacitor?

Answer 27

More charge is stored on the plates because the positive side of the dielectric attracts more electrons from the battery on the negative plate and the negative side pushes electrons back to the battery.

Question 28

What is the relative permittivity?

Answer 28

The ratio of charge stored with the dielectric to the charge stored without the dielectric.

Question 29

Describe the action of a simple polar molecule rotating in an electric field.

Answer 29

Polar dipoles rotate with the negative end towards the positive plate. In an alternating electric field the dipoles with rotate back and forth.

Question 30

What factors do the magnitude of a current carrying wire depend on?

Answer 30

The angle of the wire to magnetic field (greatest at right angles). The current The length of the wire

Question 31

What happens to a charged particle in a magnetic field?

Answer 31

They experience a force and follows a circular path.

Question 32

Why does a force act on a wire in a magnetic field when a current flows along the wire?

Answer 32

Because the electrons moving along the wire are pushed to one side by the force of the field. If the electrons are confined to the wire then the wire itself must move.

Question 33

What equation is used to find the force of a moving charge?

Answer 33

F=BQv

Question 34

Why do moving charges move in a path that is circular?

Answer 34

Because the magnetic force is always perpendicular to velocity.

Question 35

What factors affect the radius of the circular path of a charge in a magnetic field?

Answer 35

R decreases if: B is increased v is decreased Particles with a larger specific charge are used

Question 36

What must happen to a conductor for electricity to be generated?

Answer 36

A magnet is moved near a wire and a current passes through it. This causes an induced emf which forces electrons around a circuit.

Question 37

What factors increase induced emf?

Answer 37

Moving the wire faster Using a stronger magnet Make the wire into a coil

Question 38

What is lenz's law?

Answer 38

The direction of the induced current is always such as to oppose the change that causes the current.

Question 39

What is meant by the rms value of an alternating current?

Answer 39

The value of direct current that would give the same heating effect as the alternating current in the same resistor.

Question 40

Explain the purpose of transformers.

Answer 40

Transmission of electrical power over long distances is much more efficient at high voltage than at low voltage.

Question 41

How is the efficiency of transformers improved by design?

Answer 41

Low resistance windings to reduce power lost from heat. Laminated core- layers of iron separates by insulator to reduce eddy currents (induced currents in the core). Soft iron is easily magnetised and demagnetised- stops energy being wasted.