Magnetic fields

Question 1

What is a magnetic field?

Answer 1

A region where a magnet exerts a force on objects made from magnetic materials or on other magnets

Question 2

What happens when a current-carrying wire is placed in a magnetic field?

Answer 2

A force is exerted on the wire however if it is parallel the force is 0N

Question 3

How is a magnetic field induced?

Answer 3

By passing a current through a wire

Question 4

What are flux lines?

Answer 4

Lines that represent the direction of a force experienced by the north pole of a magnet at any point in the magnetic field

Question 5

How does a diagram show areas of stronger flux density?

Answer 5

When the flux lines are closer together

Question 6

What will happen to a freely suspended magnet in a field?

Answer 6

It will align itself with the field

Question 7

What is magnetic flux density?

Answer 7

The strength of a magnetic field or
the number of magnetic flux lines that pass through an area of 1m²

Question 8

What is the unit of magnetic flux density?

Answer 8

Tesla

Question 9

What is a Tesla?

Answer 9

The flux density that causes a force of 1N on a 1m wire carrying a current of 1A at right angles to the flux

Question 10

What is a solenoid?

Answer 10

A current carrying coil of wire that produces magnetic flux.
(This is how an electromagnet works)

Question 11

What do the poles in a solenoid depend on?

Answer 11

The direction of the current

Question 12

How do you measure magnetic field strength?

Answer 12

using a hall probe

Question 13

How does a hall probe work?

Answer 13

It contains a slice of semiconducting material. If a current flows in the semiconductor when it is perpendicular to the magnetic flux, a potential difference is generated across the side of the semiconductor and this potential difference s directly proportional to the flux density.

Question 14

What does each finger represent in Flemings left hand rule?

Answer 14

Thumb-Force
pointer- magnetic field
middle- Current

Question 15

What happens to a particle moving in a magnetic field?

Answer 15

It experiences a force

Question 16

What direction does conventional current flow?

Answer 16

Positive to negative

Question 17

What direction does a negatively charged particle flow?

Answer 17

from negative to positive / opposite to conventional current

Question 18

How do charged particles move in a magnetic when at right angles to the uniform flux?

Answer 18

In circular motion

Question 19

What is a cyclotron?

Answer 19

A particle accelerator that accelerates charged particles through a spiral path using a fixed magnetic field and an alternating potential difference

Question 20

What happens to the radius of a particle and time spent in the D as it accelerates?

Answer 20

It always spends the same amount of time in each D but the radius of the path increases as it accelerates

Question 21

What is a synchrotron?

Answer 21

A particle accelerator with an increasing magnetic field as the particle accelerates. The radius remains the same even though they speed up.

Question 22

What is magnetic flux?

Answer 22

The magnetic field lines passing through an area

magnetic flux density x cross-sectional area perpendicular to field direction

Question 23

What is the unit of magnetic flux?

Answer 23

Weber ( Tesla metre squared)

Question 24

What factors increase the number of flux line cut per second?

Answer 24

1. The length of wire increases
2. The speed of the wire increases
3. The magnetic flux density increases

Question 25

Can flux lines be "cut" when a conductor is moving parallel to the field lines?

Answer 25

No. Only when perpendicular so for parallel magnetic flux is 0Wb

Question 26

What is magnetic flux linkage?

Answer 26

Magnetic flux x number of turns in a coil

Question 27

What is the unit of magnetic flux linkage?

Answer 27

Weber-turns

Question 28

What factors does the flux linkage depend on?

Answer 28

1. Flux density 2. Orientation of coil and flux lines 3. coils cross sectional area 4. number of turns on the coil

Question 29

State Faradays law

Answer 29

The induced emf equals the rate of change of magnetic flux linkage

Question 30

Describe electromagnetic induction

Answer 30

When a magnet is moved into a coil. When it is stationary inside, the current is 0. An emf is induced if there is relative movement between the coil and the magnetic field or if the flux linkage changes. This causes one end of the coil to become positively charged and the other negative

Question 31

How can you change the flux linkage and generate an emf?

Answer 31

relative movement between the magnet and coil rotating the coil at a constant frequency in the perpendicular plane to the field as it changes the cross sectional area through which the flux passes

Question 32

State Lenz's law and explain the effect

Answer 32

The direction of the induced emf causes effects that oppose the the change producing it. when a magnet is moved north face in to the coil, the end of the coil becomes the north pole to oppose this motion, when it is removed from the coil, this end becomes a south pole to attract it/ oppose the motion.

Question 33

What are eddy currents?

Answer 33

They are circulating Electric currents flowing in the plane of the metal. They are caused by the change of flux linkage when the metal moves perpendicular to the field

Question 34

How are eddy currents induced?

Answer 34

By the alternating magnetic field in the primary coil

Question 35

How can the effect of eddy currents be reduced?

Answer 35

By using a laminated iron core because the eddy currents cannot pass through the insulator and so their amplitude is reduced. By using a core made of a material with a high resistivity

Question 36

Why is a soft iron core used?

Answer 36

to allow easy magnetisation and demagnetisation

Question 37

Name some things that use electromagnetic induction

Answer 37

Electric guitars and microphones

Question 38

What frequency is the power supply from the national grid?

Answer 38

50Hz

Question 39

What is the household rms power supply from the national grid?

Answer 39

230V

Question 40

What is the peak value of the voltage?

Answer 40

The greatest magnitude in either the positive or negative direction with respect to 0. Also half the peak-to-peak

Question 41

What is the peak-to-peak value?

Answer 41

The voltage measured from one peak to the next

Question 42

What is an oscilloscope?

Answer 42

A visual calibrated voltmeter where the operator can alter how the waveform is displayed

Question 43

What is the timebase on an oscilloscope?

Answer 43

The time per division

Question 44

What is the use of a transformer?

Answer 44

To increase or decrease the voltage of an alternating voltage supply.

Question 45

How are transformers used in the national grid?

Answer 45

They are used to increase the the voltage generated in power stations up to 40000V so that energy can be saved as electricity is transmitted around the country. Transformers are then used to reduce this high voltage for safe use in the home.

Question 46

What is the structure of a transformer?

Answer 46

It consists of two coils of wire linked by a soft iron core. The AC in the primary coil creates a changing magnetic field in the core. As the magnetic flux in the core changes, the magnetic flux linkage to the secondary coil changes and an emf is induced in the secondary coil.

Question 47

Which supply do transformers work with?

Answer 47

AC only

Question 48

Which coil has more turns in a step-up transformer?

Answer 48

the secondary coil

Question 49

Which coil has more turns in a step-down transformer?

Answer 49

the primary coil

Question 50

How can energy be lost in transformers and what can be done to prevent it?

Answer 50

Heat is produced in the coils when a current flows ( can be reduced by low resistance wire ) some magnetic flux produced by the primary coil doesn't pass through (use coils that are close to each other or wound on top of each other)

Question 51

Is a particles speed in a cyclotron affected by the magnetic field?

Answer 51

No

Question 52

What happens when a conducting rod moves relative to a magnetic field?

Answer 52

The electrons in the rod will experience a force and build up on one side of the rod causing an emf to be induced

Question 53

What does the variation of flux linkage depend on?

Answer 53

The angular speed of the coil

Question 54

What does the waveform on an oscilloscope look like for an alternating current?

Answer 54

Sinusoidal

Question 55

What is the root mean square voltage?

Answer 55

the average of all the squares of the possible voltages

Question 56

What value does the root mean squared voltage give?

Answer 56

The average voltage of the voltage output by the supply

Question 57

What is the peak voltage of UK mains?

Answer 57

325V (330 to 2 s.f.)

Question 58

Why do transformers use alternating currents?

Answer 58

To change the size of their voltage