Section 11 - Magnetic Fields

Question 1

How is a magnetic field induced around a current carrying wire?

Answer 1

1. The field lines are concentric circles centered on the wire
2. The direction of a magnetic field around a current carrying wire can be worked out with the right-hand rule
3. If you loop the wire into a coil, the field is doughnut shaped, while a coil with length (solenoid) forms a field like a bar magnet

Question 2

What is magnetic field strength defined as?

Answer 2

The force on one metre of wire carrying a current of one amp at right angles to the magnetic field Tesla or weber per metre squared

Question 3

What path do charged particles in a magnetic field take?

Answer 3

• Flemming’s left hand rule says that force on a moving charge in a magnetic field is always perpendicular to its direction of travek
• This is the condition for circular motion

Question 4

How do you derive the radius of the circular path followed by a charged particle in a magnetic field?

Answer 4

1. F=mv^2/r
2. F=BQv
3. mv^2/r = BQv
4. r=mv/BQ

Question 5

How are cyclotrons used in practical applications?

Answer 5

Produce radioactive tracers or high energy beams of radiation used in radio therapy

Question 6

What are cyclotrons made up of?

Answer 6

Two hollow semicircular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes, and an alternating potential difference applied between the electrodes

Question 7

How do cyclotrons work?

Answer 7

1. Charged particles are fired into one of the electrodes
2. The magnetic field makes them follow a circular path and then leave the electrode
3. An applied pd between the electrodes accelerates the particle across the gap until they enter the next electrode
4. Because the particle’s speed is slightly higher, it will follow a circular path with a larger radius before leaving the electrode again
5. The pd is reversed so the particle is accelerated again before entering the next electrode
6. This repeats as the particle spirals outwards, increasing in speed, before exitting the cyclotron

Question 8

What is magnetic flux density?

Answer 8

A measure of the strength of a magnetic field (number of field lines per unit area)

Question 9

What is electromagnetic induction?

Answer 9

• When there is relative motion between a conducting rod and a magnetic field, the electrons in the rod will experience a force which causes them to accumulate at the end of one rod
• This induces an emf across the ends of the rod

Question 10

How can you induce an emf in a flat coil or solenoid and why does that happen?

Answer 10

• Moving the coil towards or away from the poles of a magnet
• Moving a magnet towards or away from the coil
• Caused by the changing magnetic flux that passes through the coil
• If the coil is a part of a complete circuit, an induced current will flow through it

Question 11

What does the size of the emf depend on?

Answer 11

• Magnetic flux passing through the coil
• Number of turns in the coil that cut the flux

Question 12

What is the rate of change of flux linkage?

Answer 12

A change in flux linkage of one weber per second will induce an electromotive force of 1 volt in a loop of wire

Question 13

What is Faraday’s law?

Answer 13

The induced emf is directly proportional to the rate of change of flux linkage

Question 14

What is Lenz’s law?

Answer 14

The induced emf is always in such a direction as to oppose the change that caused it

Question 15

Why is lenz’s law negative?

Answer 15

• Shows the direction of induced emf
• An induced emf will oppose the change that caused it
• This agrees with the principle of the conservation of energy
• The energy used to pull a conductor through a magnetic field against the resisitance caused by magnetic attraction, is what produces the induced current

Question 16

How can Lenz’s law be used to find the direction of an induced emf and a current in a conductor travelling at right angles to a magnetic field

Answer 16

• Using flemming’s left hand rule point your thumb in the direction of the force of ressistance which is in the opposite direction to the motion of the cinductor
• Point your first finger in the direction of the field
• Second finger will give the direction of the induced emf

Question 17

What is the graph of flux linkage against t?

Answer 17

cos graph

Question 18

What is the graph for emf against t?

Answer 18

sin graph

Question 19

How do transformers work?

Answer 19

1. An alternating current flowing in the primary coil produces magnetic flux
2. The changing magnetic field is passed through the iron core to the secondary coil, where it induces an alternating voltage of the same frequency as the input voltage

Question 20

What is the difference between step-up and step-down transformers?

Answer 20

• Step-up increase the voltage by having more turns on the secondary coil than the primary
• Step-down reduce the voltage by having ferwer turns on the secondary coil

Question 21

What are eddy currents?

Answer 21

• Looping currents induced by the changing magnetic flux in the core
• Create a magnetic field that acts against the field that induced them
• Reduce the field strength
• Dissipate energy by generating heat

Question 22

How do you reduce eddy currents?

Answer 22

Laminating the core with layers of insulation

Question 23

How do you minimise the resistance in the coils?

Answer 23

Thick copper wire is used which has a low resistance

Question 24

How are transformers used in the National Grid?

Answer 24

If you double the transmitted current, you quadruple the power lost

Question 25

When does emf change direction or stay the same?

Answer 25

* It will change direction when it passes a parallel point * It will stay the same when it cuts the magnetic field when it's always in the same direction - passes a perpendicular point

Question 26

In what circumstance is the radius taken by 2 charged particles in a magnetic field different even though the speed is the same?

Answer 26

When the masses are different eg they are isotopes

Question 27

When is a search coil suitable?

Answer 27

When it is cut by changing flux

Question 28

How do you find percentage change in flux when it's been rotated through an angle?

Answer 28

1-cosθ x 100

Question 29

Explain function of the iron core in a transformer

Answer 29

Increases linkage from the primary to secondary coil compared to an air core

Question 30

Explain the function of the secondary coil in a transformer

Answer 30

Has a changing magnetic flux which induces an emf

Question 31

How is the efficiency of a transformer increased by constructing the core in a particular way?

Answer 31

* Material has a resistivity to reduce eddy currents * Thin sheets are used so smaller emfs are induced in the core

Question 32

Why would engineers use a particular value for the potential difference in a system?

Answer 32

* If the voltage is lower, the power is transmitted at high current so power is wasted in the cable * If the voltage is too high, there will be major insulation issues

Question 33

How do you show that something is/isn't exponential?

Answer 33

F1/F2 = F2/F3

Question 34

What is the missing equation for emf?

Answer 34

E = Blv