Magnetic Fields A2

Question 1

What is the direction of magnetic fields

Answer 1

From north to south

Question 2

What is the motor effect

Answer 2

When a wire carrying a current is perpendicular to a magnetic field, this creates a force to act on the wire

Question 3

What is the equation for the motor effect

Answer 3

F=BIL

Question 4

What is the magnetic flux density

Answer 4

The magnetic field strength with units of B, in teslas

Question 5

What is magnetic flux

Answer 5

BA, where B is flux density and A is area. Flux has the units of Wb, in webers

Question 6

What is the equation of the force acting on a moving charge on a magnetic field

Answer 6

F=BQV

Question 7

What occurs when charged particles are travelling perpendicular to the magnetic field

Answer 7

They travel in circles due to the force from the magnetic field acting as centripetal force, towards the centre of the created circle (this changes the direction of the electron, not speed)

Question 8

What is inside an electric motor

Answer 8

A magnetic field, a coil and a commutator

Question 9

How does the electric motor work

Answer 9

When the current starts to flow through the coil, the perpendicular magnetic field creates a force clockwise. When the coil is vertical, the commutator causes the circuit to stop flow so that the coil can continue rotating around. Then the current is switched by the commutator so the force is still clockwise.

Question 10

What is a cyclotron

Answer 10

A device to produce high-energy beams of particles for radiation therapy

Question 11

What is in a cyclotron

Answer 11

Two D-shaped electrodes and an AC supply connected to both

Question 12

How do cyclotrons work

Answer 12

The protons inserted travel in circular motion around the first D and when they meet the gap between the electrodes, they are attracted to the negative D. This causes its velocity to increase. An AC supply is required to constantly switch the polarity of current in each electrode every half-turn of protons

Question 13

What is the dynamo effect

Answer 13

Using the right-hand rule for when a moving conductor in a perpendicular magnetic field produces a current in the conductor

Question 14

What occurs when you induce a current using the dynamo effect

Answer 14

Using the motor effect, a force is produced in the opposite direction of the original motion. This is Lenz’s Law

Question 15

What is Lenz’s Law

Answer 15

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

Question 16

What occurs when a magnet falls through a magnetic field of a coil of wire

Answer 16

A current is induced in the wire, and the top of the wire becomes a certain pole, as to oppose the magnet. This will slow the magnet down

Question 17

How to find the flow of current when the falling magnet creates a magnetic field in the coil of wire

Answer 17

If the top of the coil is N, this is anti-clockwise. If the top is S, this is clockwise

Question 18

What is induced emf

Answer 18

Voltage produced from the dynamo rule

Question 19

What is Faraday’s Law

Answer 19

The negative of the rate of change of flux (change in flux / change in time)

Question 20

How to calculate the emf of a wire cutting through a magnetice field

Answer 20

BLs/t , where s is change in height of wire or BLv

Question 21

What is flux linkage

Answer 21

BAN

Question 22

What is emf induced in a coil rotating uniformly in a magnetic field

Answer 22

BANωsin(ωt)

Question 23

What happens when a motor turns

Answer 23

This produces the dynamo effect due to Lenz’s Law, creating back emf

Question 24

What is the back emf when it is spinning fast

Answer 24

Emf is larger and current is lower

Question 25

What is the back emf when it is spinning slow

Answer 25

Emf is lower and current is higher

Question 26

What is the direction of current relative to an electron

Answer 26

Current is in the opposite direction to the motion of electrons