Chapter 23 - Magnetic Fields

Question 1

What do the arrows on a magnetic field diagram represent?

Answer 1

The direction a in which a free north pole would experience a force

Question 2

In what direction do the magnetic field lines go for a bar magnet?

Answer 2

From the north pole to the south pole.

Question 3

Right hand grip rule

Answer 3

Thumb - direction of travel for conventional current

Fingers - direction of magnetic field lines

Question 4

What creates a magnetic field?

Answer 4

Any charged particles that moves creates a magnetic field around it.

Question 5

Fleming’s left hand rule

Answer 5

Thumb - direction of force
Index finger - Direction of the magnetic field (North to South)
Middle finger - Direction of conventional current (electrons would travel in the opposite direction)

Question 6

What is magnetic flux density?

Answer 6

The strength of a magnetic field. It is a vector.

Question 7

What will happen to a charged particle moving at a right angle to a uniform magnetic field?

Answer 7

The particle will travel in a circles as the acting on it from the magnetic field will always be perpendicular to its velocity.

Question 8

How does a velocity selector work?

Answer 8

A uniform electric field and a uniform magnetic field are perpendicular to each other. The magnetic force a charged particle experiences when passing through the magnetic field is directly proportional to its velocity. If the particle is not travelling at the exact velocity it has to, it will be deflected to either side. The velocity the particle has to travel at is dependent on the strength of the fields, as v = E/B.

Question 9

An e.m.f is induced in a circuit when…

Answer 9

there is a change in the magnetic flux linking the circuit.

Question 10

State Faraday’s law

Answer 10

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

Question 11

State Lenz’s law

Answer 11

The direction of the induced e.m.f or current is always such as to oppose the change producing it.

Question 12

Which of Newton’s laws is Lenz’s law related to?

Answer 12

Conservation of Energy. As work must be done when pushing the magnet towards the coil and when pulling it away, otherwise energy would have been created.

Question 13

What happens to the polarity of the ends of a coil when a magnet is pushed and pulled near the end?

Answer 13

Due to Lenz’s law the induced e.m.f must oppose the change, therefore when the magnet is pushed towards the coil, the polarity is the same as the nearest end of the magnet. When the magnet is pulled away the polarity is the opposite.

Question 14

What is an a.c generator made of?

Answer 14

A rectangular coil rotation through in a uniform magnetic field.

Question 15

How does an a.c generator work?

Answer 15

The coil is rotated by an external force. As the coil rotates the flux linkage changes, this induces an e.m.f in the coil.

Question 16

What is the purpose of the slip rings in an a.c generator?

Answer 16

The slip rings allows the e.m.f induced in the coil to flow to an external circuit via carbon brushes.

Question 17

State 4 ways to increase the e.m.f induced in an a.c generator

Answer 17

Decrease distance between magnet and coil. (To increase magnetic field strength experienced by coil)
Use a stronger magnet.
Increase frequency of rotation of the coil. (Double freq. = double max. e.m.f. and halving T)
Increase number of turns in the coil. (Double no. of turns = double max e.m.f.)

Question 18

Magnetic flux linkage =

Answer 18

Number of turns in the coil x magnetic flux

Question 19

What is transformer made of?

Answer 19

A laminated soft iron core, a primary coil (input) and a secondary coil (output)

Question 20

How does a transformer work?

Answer 20

An alternating current is supplied to the primary coil. This produces a varying magnetic flux in the core. This changing magnetic flux induces a current in the secondary coil.