Magnetism And Electromagnetisms

Question 1

Which way does the magnetic field point?

Answer 1

From the north to the south

In through the south and out through the North

Question 2

Do field lines ever cross eachother?

Answer 2

Never

Question 3

What do compass needles do when near field lines?

Answer 3

Compass needles always lie along field lines.

Question 4

Define a magnetic field.

Answer 4

An area where magnetic material would feel a force.

Question 5

What do the poles on a magnet do?

Answer 5

Always seek the direction of the poles they represent.

Question 6

Like poles……

Answer 6

Repel.

Question 7

Unlike poles…

Answer 7

Attract.

Question 8

What materials attract to a magnet?

Answer 8

Iron, steel, nickel and cobalt.

Question 9

What materials do not attract to a magnet?

Answer 9

Lead. Gold, plastic, glass.

Question 10

What is the difference between magnetically hard and magnetically soft materials? Give examples.

Answer 10

Magnetically soft materials: easy to magnetise, but lose magnetic properties quickly, e.g. SOFT iron.

Magnetically hard materials: remain magnetised long after being magnetised, become permanent magnets, e.g. HARD steel.

Question 11

What is a uniform magnetic field and how can you create one?

Answer 11

Magnetic field lines that are the same distance apart and are travelling in the same direction with the same field strength everywhere.

To create one, place two flat magnets parallel to each other, with the opposite poles facing.

Question 12

What is produced when current flows through a wire?

Answer 12

A magnetic field around it.

Question 13

How can you tell which way the magnetic field lines are travelling through a wire.

Answer 13

Right hand grip rule.

Make a thumbs up or down, depending on the direction of the current through the piece of wire (thumb must follow the current) once your thumb has found this, the direction your four fingers are facing is the direction the magnetic field lines are travelling in (either clockwise or anti-clockwise).

Question 14

What shape are magnetic field lines?

Answer 14

Circular.

Question 15

What is the real current?

Answer 15

The fact that electrons actually flow from - to +.

Question 16

What is the ‘conventional’ current?

Answer 16

The assumed, and commonly used current, that electrons flow from + to -.

Question 17

What is a solenoid?

Answer 17

A long coil.

Question 18

What would happen to a magnetic field if you used a larger current, used more coil or added an iron core?

Answer 18

The strength of the magnetic field would increase.

Question 19

What happens to an electromagnet when you switch off the current?

Answer 19

It looses its magnetism.

Question 20

What are the symbols and units for each of the following

a) current
b) voltage
c) power?

Answer 20

a) current, I, measured in amps, A.
b) voltage, V, measured in volts, V.
c) power, P, measured in watts, W.

Question 21

Define a magnetic field line.

Answer 21

A magnetic field line shows the direction of force of the magnetic field.

Question 22

Describe an experiment to investigate the magnetic field pattern for a permanent bar magnet and that between two bar magnets.

Answer 22

• place bar magnet or magnets under a piece of paper.
• sprinkle iron filings on top of the paper over the bar magnet(s).
• tap the paper to guide the iron filings.
• record the magnetic field pattern.

Question 23

Describe the construction of electromagnetic induction (Dynamo effect).

Answer 23

• connect an ammeter to a conduct
• move the conductor through a magnetic field
• the ammeter will show the magnitude and direction of the induced current.
• if direction of movement is reversed, then the induced voltage or currents is also reversed.

Question 24

What will magnetic field lines around a straight wire look like?

Answer 24

Depending on direction if current, field lines will make simple circles around straight wire, either travelling clockwise or anti-clockwise (can be figured out using right hand grip rule).

Question 25

What will magnetic a flat circular could look like?

Answer 25

Imagine a circle shape of coil, like a ring, magnetic field lines will travel straight through the centre of the ring of coil, travelling from north to South, and they will also make flat circle shapes around each side of wire on the circular coil (see page 64 CGP).

Question 26

What will magnetic field lines around a solenoid look like?

Answer 26

The same as magnetic field lines around a bar magnet look like. Travelling from North to South, in through the south and out through the North.

Question 27

What happens to a charged particle when it moves through a magnetic field?

Answer 27

It experiences a force.

Question 28

What happens when a current carrying wire in a magnetic field?

Answer 28

It will experience a force.

Direction of force: use Fleming left hand rule.

• first finger (index) = direction of field lines.
• second finger (middle) = direction of current.
• so thumb shows direction of force.

Question 29

How is the fact that a current carrying wire will experience a force in magnetic field lines applied to loudspeakers?

Answer 29

• when a current flows the coil it creates a magnetic field.
• magnetic field in coil either attracts or repels the permanent magnet.
• coil moves in and out and makes diaphragm vibrate.
• vibrations in diaphragm make air vibrate, making sound.

Question 30

How is the fact that a force is exerted on a current carrying wire in a magnetic field applied to simple d.c. electric motors?

Answer 30

• when a current flows through the coil a magnetic field is created and the wire experiences a force.
• force in different directions on each side of the coil makes the coil rotate.
• commutator ensures the current always flows in same direction as the magnetic field.
• reference to left hand rule to predict direction of force.

Question 31

How would you predict which way a wire will move when it is perpendicular through a magnetic field?

Answer 31

Using Flemings left hand rule, the direction of the force (thumb) is the same way as the direction that the wire will go in.

Question 32

How would you increase the force on a current carrying wire in a magnetic field?

Answer 32

By increasing the current or increasing the magnetic field.

Question 33

What happens if a wire is moved through a magnetic field or experiences a changing magnetic field? And how would what happens be increased?

Answer 33

A voltage is induced in the wire, as cutting field lines produces electricity.

You can increase the voltage by:

• putting more turns on the coil
• using stronger magnets
• moving the wire faster

Question 34

Describe the generation of electricity by the rotation of a magnet within a coil of wire and describe the factors that affect the size of the induced voltage.

Answer 34

A dynamo is an example of the rotation of a magnet within a coil of wire generating electricity. The magnet rotates (so is moving through a magnetic field, making electricity.

More loops on coil = increased voltage
Faster speed of rotation = increased voltage.

Question 35

Describe the generation of electricity by a coil of wire within a magnetic field and describe the factors that affect the size of the induced voltage.

Answer 35

A turning coil of wire moves within a uniform magnetic field, cutting field lines and so producing electricity.

More loops on coil = increased voltage
Faster speed of rotation = increased voltage.

Question 36

Describe the structure of a transformer and how it works.

Answer 36

• Magnetic field around the transformers soft iron core.
• primary coil on the AC input side of the transformer has different number of turns to the secondary coil on the AC output side of the transformer.
• ac passes through primary coil
• this induces alternating magnetic field in coil and core
• secondary coil experiences changing magnetic field
• so ac and a voltage is induced in secondary coil.
• only work on ac not dc because you need a changing magnetic field
• soft iron core strengthens magnetic field, and is only temporarily magnetised

Question 37

What is the equation for the relationship between the primary input and the secondary output regarding the amount of voltage and number of coil turns at each?

Answer 37

Vp / Vs = Np / Ns

Primary voltage / secondary voltage = number of turns on primary coil / number of turns on secondary coil.

Question 38

What is the relationship between input power and output power?

Answer 38

Vp x Ip = Vs x Is

Primary voltage x primary current = secondary voltage x secondary current.

Question 39

Explain the use of step up and step down transformers in the large scale generation and transmission of electrical energy.

Answer 39

Step up transformers: used between power station factories and power lines as power lines need high voltage so low current, to reduce energy loss due to heating of wires.

Step down transformers: used between power lines and homes as homes need low voltage and current to decrease risk of electrocution, as voltage coming directly from power lines would fry homes.

Question 40

What is the motor effect?

Answer 40

When a current carrying wire is put in a magnetic field, there is a force on the wire and this can cause the wire to move, the motor effect.

Question 41

What is an electromagnet?

Answer 41

A material that can become temporarily magnetised when an electric current flows through it.

Question 42

What is the Dynamo effect?

Answer 42

Using electromagnetic induction to transform kinetic energy into electrical energy.