Chapter 3 - Understanding Electromagnetism

Question 0

Magnetic poles RULE.

Answer 0

Like magnetic poles REPEL each other; unlike magnetic poles ATTRACT each other.

Question 1

Define Magnetic Field.

Answer 1

The space around the magnet which is affected by the presence of the magnet.

Question 2

Why is the poles labelled as North and South Pole?

Answer 2

Because when a magnet is suspended it is free to rotate horizontally and it will always align itself in a north-south direction.

Question 3

What happens when a magenta is free to swing vertically?

Answer 3

If it is in the Southern Hemisphere the north pith end will point upwards as well as northwards. In the northern hemisphere, the north end points downwards.

It is as though the earth itself is acting as a huge magnet, with it’s South Pole to the geographic north and it’s North Pole to the geographic south.

Question 4

What properties of magnets that may remind us of the forces between electric charges?

Answer 4

Like charges repel and opposite charges attract and the force of attraction or repulsion increases as the distance between the charges decreases.

Question 5

What are the differences between Magnetism and Electricity?

Answer 5

Magnets are more or less ‘permanent’, whereas it is hard to keep an electric charge on an object, such as a rubbed plastic comb, for much more than 10 of 15minutes. Magnetic poles do not run away through metal wires to ground as an electric charge will do.

Question 6

Can we obtain separate north and south poles?

Answer 6

We could try cutting a magnet in half, but all we get is two smaller magnets, each with it’s own north and south poles. No matter how often we keep cutting magnets, we always get more little magnets with two opposite poles.

Question 7

Dipolar RULE.

Answer 7

Magnets are Dipolar and the field around a magnet is called a Dipole Field.

Question 8

What are the two types of irons?

Answer 8

• Soft iron

* Hard iron

Question 9

What is induced magnetism?

Answer 9

When a piece of iron is in the presence of a magnet it will also become a magnet.

Question 10

Why does a permanent magnet attract another piece of iron?

Answer 10

Because it induces the other piece of iron to be come a magnet with the opposite pole closest, and this they attract. Induced magnetism is a temporary phenomenon. As soon as the permanent magnet is removed, the sort iron loses it’s magnetism.

Question 11

What are Magnetic Field Lines?

Answer 11

They are represented as arrows that flow out from the North Pole of a magnet into the surrounding space and return to the South Pole. Internally the magnetic field flows from the South Pole to the North Pole, completing the loop. If a compass was placed at any point in the space around a magnet, it’s needle would align with the field line.

Question 12

Define the direction of the magnetic field.

Answer 12

It is the direction in which the magnetic force is exerted on the North Pole of another magnet placed in the field. The force on a South Pole, then, is always in the direction opposite to the field direction.

Question 13

Define magnetic flux.

Answer 13

It is the term used to describe strength of the magnetic field. It can be likened to the gravitational field strength around a planet.

• The closer you are to the poles of a magnet, the greater the flux density.
• The magnetic field lines around the pole of a magnet are denser than the same field lines further from the pole.

Question 14

Define conventional current.

Answer 14

It is a flow of positive charge around a circuit of through space.

Question 15

Define Electron Current.

Answer 15

It is a drift of negative charge in a particular direction. In electromagnetism it is the convention to use the direction of the conventional current as the direction of current flow.

Question 16

What is the Right-Hand Grip Rule.

Answer 16

If you were to grasp the conductor with your right hand in such a way that your thumb points in the direction of the conventional electric current, your fingers curl around he direction of the field.

Question 17

If a long wire is made into a circle and a current is passed through it, the magnetic field is enhanced inside the circle and somewhat diminished outside. Explain why.

Answer 17

This is because inside the circle the field from all parts of the wire is pointing in the same direction, whereas at any point outside the circle the contribution to the field from opposite sides of the loop is in opposite directions, and so will tend to cancel.

Question 18

How is it possible to form a much stronger magnetic field?

Answer 18

This can easily be done by winding many turns of wire into a coil (solenoid).

Question 19

Explain the structure of the magnetic field around a solenoid?

Answer 19

The field around the solenoid is like the field around a normal bar magnet, just as Ampere said. This can be checked either by exploring the field with a small compass, or by putting iron filings on a cars around the solenoid.

Question 20

What is a dipole field?

Answer 20

It’s a type of field in which lines appear to converge towards two ‘poles’ at either end of the solenoid or bar magnet.

Question 21

List the properties of the magnetic field lines.

Answer 21

1. They have closed pathways from south to north inside the magnet and north to south outside the magnet.
2. Their direction represents the direction of the magnetic field at any point.
3. Their number of lines represents the strength of the magnetic field of any point.
4. They never cross over each other because it is impossible for the magnetic field to have 2 directions at the same point
5. They are working as stretched rubber bands, they like to shorten tier length and this is the reason for the attraction between different poles and repulsion between identical poles.

Question 22

How can we create a magnetic field with straight magnetic field lines?

Answer 22

By using a circular coil/ring.

Question 23

What are electromagnets?

Answer 23

An arrangement which occurs when a piece of soft iron is put right inside the solenoid. When the current is turns on, the field of the current induces magnetism in the iron. The field from the iron is now added to that of the current, making the total field very much stronger than that due to the current alone; in fact, it can be 1000 times greater.

Question 24

What is described to be ferromagnetic?

Answer 24

In ferromagnetic materials, the fields from each atom tend to align so that they add together to produce a greater field. This alignment can be permanent (hard iron) or temporary (soft iron).

Question 25

Will cutting a magnet in half give us separate poles?

Answer 25

•No matter how big or small the magnet is, cutting it will not enable us to find a single pole. This is because a normal magnet is made up of many tiny electromagnets which has 2 ‘poles’ itself, just as a solenoid does. In fact, at this level it is hardly worth speaking of poles at all; there is a continuous field threading through each little electromagnet. What we have called the poles are really just two sides of the current loop.

Question 26

In moving coil speakers, what happens when a current is moving through a coil?

Answer 26

It creates a magnetic field that interacts with the magnetic field from a permanent magnet.

Question 27

What causes a greater force of magnetic attraction or repulsion to be exerted on a magnet?

Answer 27

The larger the current that moves through the wire, the larger the induced magnetic field will be, and hence the greater the force of magnetic attraction or repulsion that will be exerted by the permanent magnet.

Question 28

What does the spring suspension do in a moving coil speaker?

Answer 28

These springs resist the force of the speaker’s movement and provide a restoring force to the cone: they return the cone to a central rest position after it has been driven forwards or backwards by an electrical signal.

Question 29

List the different sorts of moving coil speakers.

Answer 29

• Woofers 30-500Hz
• Mid-range 500-400Hz
• Tweeters 4-20Hz

Question 30

What is the difference between the right-hand force rule and the right hand grip rule?

Answer 30

• The right-hand grip rule tells us the direction if the circular magnetic field that occurs around a current-carrying conductor.
• Whereas the right hand rule tells us the direction of the force on a current in a magnetic field. This force is the result of placing a current in a magnetic field. Do not confuse it with the field created by the current itself.

Question 31

How did Ampere showed that there is a also a magnetic force between two parallel electric currents.

Answer 31

We can regard one as creating a magnetic field and the other as responding to that field.

The cause of the magnetic force in the wire is the interaction between the induced magnetic field created by the current in the conductor and the external magnetic field.

The effect is exactly the same as the attraction or repulsion that we see between to permanent magnets.