Electromagnetism

Question 1

What is a bar magnet made of? and how?

Answer 1

Iron. The metal used to make magnets has to be iron because its atoms have a very special property - they each act like a tiny magnet, each with its own north and south pole. To make an iron bar into a magnet, the tiny atom magnets must be made to line up so that they all point in the same direction.

In an ordinary, un-magnetised bar of iron, the atom magnets are all pointing in random directions, so an overall north pole and south pole are not created.

Question 2

Why is a pull or push felt when two poles of two magnets are brought close to each other?

Answer 2

Because there is a magnetic field around each magnet.

Question 3

How can magnetic fields vary?

Answer 3

In their direction and strength. The size and direction of the forces that are felt depend on the strength and direction of the magnetic fields that are interacting.

Question 4

How do we show the effect one field has on another

Answer 4

A magnetic field is represented by magnetic field lines. Direction shown by arrows. Strength shown by how close together the field lines are.

Question 5

Where is the magnetic field the strongest

Answer 5

Around the poles (magnetic field liens are closest together), and the strength of the magnetic field gets less as the distance from the magnet increases.

Question 6

What is the direction of a magnetic field?

Answer 6

Away from a north pole and towards a south pole.

Question 7

What happens when two opposite poles come together?

Answer 7

They will pull towards each other. Diagram on pg 90

Strength of the magnetic field is relatively strong

Question 8

What happens when two like poles are brought close to each other?

Answer 8

The magnets push away from each other.

The strength of magnetic field between the two poles is relatively weak.

Question 9

Why when two like poles are brought close the repelling force gets stronger the closer they are to each other

Answer 9

Because the strength of the force depends on the strength of the interacting regions of the magnetic fields of the two magnets. The strength of the magnetic field increases as the distance from the pole decreases. As the poles get closer together the strength of the interacting fields increase and so the force increases.

Question 10

The north and south pole of a compass?

Answer 10

The north pole has a arrowhead attached. The needle is magnetised.

Question 11

Which way does a compass point?

Answer 11

Because opposite poles attract, the compass needle will always point to the south pole of any magnet that is nearby. So if a compass is put where there is a magnetic field, the needle arrow will point in the same direction that magnetic field lines point at that particular place in the field.

Question 12

What is a plotting compass used for?

Answer 12

Because of its small size, it can be used to track the changing direction of a magnetic field over a relatively small area.

Question 13

Geographical north vs magnetic north

Answer 13

The compass always points to the geographical north pole, however, compass needles (because the arrow is always a magnetic north pole) always point towards a magnetic south pole. This means that Earth’s magnet actually has its south pole at the geographic North Pole.

The orientation of Earth’s magnetic changes slightly over time, and the geographic North Pole is no longer in the same place as the magnetic south pole. Therefore, when using a compass for navigational purposes, the magnetic deviation (angle between magnetic south and geographic north) must be taken into account)

Question 14

How to produce a magnetic field that can be readily changed?

Answer 14

The magnetic field of a magnet cannot be readily changed either in strength of direction, and so its uses are limited. One way to produce a magnetic field that can be readily changed - both in strength and direction - is to pass an electric current along a conducting wire.

Question 15

What is the shape of the magnetic field if the conducting wire is straight?

Answer 15

The shape can be represented by magnetic field lines that go in circles around the straight wire.

Question 16

How to draw magnetic field 3-D straight wire

Answer 16

When drawing, if both the wire and the field lines are to be shown, the drawing must be 3-dimensional.

To overcome this problem, a convention has been established that allows an arrow that is travelling into or out of the page to be drawn:
If the arrow is going into the page, all that can be seen is the tail of the arrow x
If the arrow is coming out of the page, all that can be seen is the tip of the arrow •

Question 17

How to find the circular direction of the magnetic field around a current-carrying wire?

Answer 17

Right-hand grip rule.

Question 18

Relationship between the strength of the magnetic field due to a current in a straight wire, size of the current and the distance the place is from the wire

Answer 18

The strength of the magnetic field at a particular place around a current-carrying wire depends on the size of the current in the wire and the distance the place is from the wire.

As the distance from the current-carrying wire increases, the spacing of the magnetic field lines become more spread out, indicating that the strength of the magnetic field is decreasing

Question 19

Why do the spacing of the magnetic magnetic field lines increase further away from the current-carrying wire?

Answer 19

Because the strength of the field is decreasing

Question 20

Equation for strength of a magnetic field at a particular point in a magnetic field

Answer 20

B = kI/d

B = strength of magnetic field, in tesla, T
k = value of constant, 2x10^-7
I = current in the straight wire
d = distance from the wire in m

Question 21

What is the magnetic field like due to a current in a coiled wire?

Answer 21

The magnetic field generated around each turn in the coil interacts with the magnetic fields around adjacent turns. The magnetic field along the centre of the coil is straight.

Question 22

Right hand grip rule for coiled wire

Answer 22

If the wire is straight, the right-hand grip rules relates the direction of a straight current to the direction of a curved field. If the wire is coiled, the right-hand grip rules relates the direction of a curved current to the direction of a straight field. When the hand is held in the grip position, the thumb is straight and the fingers are curved. For both magnetic field situations, if the thumb is pointed in the direction of whatever is straight, the fingers will indicate the circular direction of whatever is curved.

Question 23

Why is the shape of the magnetic field outside and around a coil similar to the shape of the magnetic field outside and around a bar magnet?

Answer 23

The coil has magnetic poles such that magnetic field lines leave the north pole and enter the south pole.

Because the shape of the magnetic field around a current-carrying coil of wire is so similar to the shape of the magnetic field around a magnet, a current-carrying coil of wire has the same magnetic behaviour as a bar magnet.

Question 24

What is a solenoid? and how will a magnet react if it was brought near?

Answer 24

A length of wire wound densely around a tubular core. If a current is passed through the coiled wire of a solenoid, the magnetic field along its core is straight and the magnetic field around the solenoid makes it into a magnet.

If a pole of a bar magnet is brought close to the end of the core of a current-carrying solenoid, the pole will react in exactly the same way as if it had been brought close to the pole of another magnet - either pushed away from the end of the solenoid or pulled towards.

Question 25

Describe the attraction of iron to a magnet

Answer 25

One of the most important properties of a magnet is that it will pull anything that is made of iron towards it. It is the special magnetic property of iron atoms that makes this attraction happen.

If the pole of a magnet is placed close to an object that is made of iron, it will make the object itself into a weak magnet.

Question 26

Eg of paperclip attaching to a magnet

Answer 26

If the S pole of a bar magnet is brought close to a paper-clip, the N poles of the atom magnets of the paper-clip are attracted to the bar magnet’s S pole, and they move a little bit in that direction, causing a very slight line up of the atoms, making the paper-clip into a very weak magnet. This weak magnet will have its north pole closest to the south pole of the bar magnet so will be attracted towards it.

Question 27

What is an electromagnet?

Answer 27

A current-carrying coil of wire and its magnetised iron core.

The strength of the magnetic effect of solenoid with current flowing through it can be vastly increased by filling the solenoid’s core with iron. The iron becomes magnetised by the magnetic field that runs along the core of the solenoid, and the strength of this magnet can be extremely great.

Question 28

Advantages of an electromagnet over an ordinary magnet?

Answer 28

• the magnetism can be switched on and off by switching the current on and off
• the direction of the magnetism can be changed by changing the current in the coil
• the strength of the magnetism that can be achieved in an iron rod is far greater if the rod is part of an electromagnet than if it has been made into a permanent magnet.

Because the extremely strong magnetic effect, it can be used to pick up and move heavy objects that are made of iron.

Question 29

Advantage of constructing electromagnet using a U-shaped instead of straight iron.

Answer 29

Both poles will be pulling on whatever the electromagnet is trying to lift. The total pull will be twice what it would have been if only one pole was pulling. So the electromagnetic is much stronger.