Fields

Question 1

Gravitational Potential

Answer 1

Work done (against the field) per unit mass to move an object from infinity to a point in the field.

Question 2

Newtons law of gravitation

Answer 2

The force of attraction between two point masses is proportional to the product of their masses and inversely proportional to the square of the distance between them.

Question 3

Properties of geostationary orbit

Answer 3

• a type of geosynchronous orbit
• orbits over equator
• fixed position relative to earth
• time period of one day
• about 36000km from earths surface

Question 4

Faradays Law

Answer 4

The induced e.m.f is directly proportional to the rate of change of flux linkage.

Question 5

Lenz’s Law

Answer 5

The induced e.m.f is always in such a direction as to oppose the change that caused it.

Question 6

Coulomb’s law

Answer 6

force between two (point) charges is proportional to (product of) charges and inversely proportional to the square of their distance apart

Question 7

The energy of an orbiting satellite is…

Answer 7

constant

Question 8

Why is the energy of a satellite in circular orbit constant

Answer 8

Because the distance from the mass is constant

Question 9

Why is the energy of a satellite in elliptical orbit constant

Answer 9

Because as height decreases, a satellite speeds up, so its kinetic energy increases and potential energy decreases (so energy remains constant)

Question 10

Properties of low orbit satellites

Answer 10

• Below 2000km
• Used for communication
• Orbit over north and south pole

Question 11

The energy of a satellite

Answer 11

Potential + kinetic energy:

E = - GMm/2r

Question 12

Electric field strength

Answer 12

Force per unit charge on a positive test charge placed at that point, (N/C)

Question 13

E is proportional to

Answer 13

Q/A

Question 14

Electric potential

Answer 14

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

Question 15

Gravitational Field strength

Answer 15

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

Question 16

Capacitance

Answer 16

Charge stored per unit pd

Question 17

Time constant

Answer 17

Time take for capacitance to fall to 0.37 of its initial value

Question 18

Dialectrics

Answer 18

Insulating materials that increase the ability of a capacitor to store charge (increasing capacitance of the capacitor) because each molecule of the dielectric becomes polarised. The positive side of the dielectric attracts more electrons from the battery onto the negative plate. The negative side of the dielectric pushes electrons back to the battery from the positive plate.

Question 19

To increase the capacitance of a capacitor

Answer 19

Make the area as large as possible, make the plate spacing as small as possible and fill the space between the plates with a dielectric with a high relative permittivity.

Question 20

Flux linkage and Emf are

Answer 20

90 degrees out of phase

Question 21

What happens in power stations

Answer 21

magnet spins instead of a coil, (50 Hz), different phases have different emfs

Question 22

Back emf

Answer 22

The emf that is induced to oppose the current that caused it

Question 23

Opposing emf is low in a motor when

Answer 23

The motor is spinning slowly due to high load

Question 24

Opposing emf is high in a motor when

Answer 24

The motor is spinning fast when unloaded

Question 25

V supplied in a motor (IR)

Answer 25

V supplied - Back Emf

Question 26

Left hand rule

Answer 26

Motor effect - current makes force

Question 27

Right hand rule

Answer 27

Generator rule - force makes current

Question 28

North pole produces

Answer 28

anti-clockwise current

Question 29

South pole produces

Answer 29

Clockwise current

Question 30

Describe how a downwards travelling north facing magnet is affected when travelling through a coil

Answer 30

North pole created at the top of the coil (repulsion due to Lenz’s law), terminal velocity reached, halfway through, the polarity of coil switches and north tries to prevent the magnet from leaving.

Question 31

How are masses detected? (equation too)

Answer 31

Heavier atoms have a larger radius when subject to a magnetic field
r = mv/BQ

Question 32

Velocity selector equation

Answer 32

V = E/B

Question 33

Cyclotron equation

Answer 33

Need radius to increase as they cross the gap, ac current allows polarity to switch every 1/2 turn. f = BQ/2piM and T = 2piR/V

Question 34

A free electron in a magnetic field will undergo

Answer 34

Circular motion, speed and ke are constant, F is perpendicular to the displacement

Question 35

What is a cyclotron and how does it work

Answer 35

A cyclotron consists of two semicircular electrodes placed inside a uniform magnetic field. Charged particles are fired into the centre of the cyclotron where they are deflected by the magnetic field into a circular path. When they reach the edge of one semicircular electrode they accelerated by a potential difference between the electrodes until they reach the second semicircular electrode. The magnetic field again deflects the particle so it still follows a circular path in the second electrode, but the radius of the particle’s path is larger as its speed has increased. It returns to the first electrode and this process continues until the particle has been accelerated to a high speed and is then released from the cyclotron.

Question 36

If a coil is parallel to the field and is turned by a small angle, will it produce a maximum or minimum magnitude of emf and why?

Answer 36

Maximum, because when it is parallel, the flux linkage in at a minimum (as no field lines are being crossed). As soon as it turned by a small angle, there is a big change in flux linkage resulting in a high emf being produced.

Question 37

If a coil is perpendicular to the field and is turned by a small angle, will it produce a maximum or minimum magnitude of emf and why?

Answer 37

Minimum, because when it is perpendicular, the flux linkage in at a maximum (as the maximum field lines are being crossed). As soon as it turned by a small angle, there isn’t a massive change in flux linkage as most lines are still being crossed resulting in a minimum emf.