Question

1. Two bar magnets are arranged so that they are parallel to each other. In case 1, north poles face each other on the right side. In case2, south poles face each other on the right side. In what way is the resulting field pattern the same, and in what way is it different?

2-A bar magnet is placed on a flat surface so that the north pole is on the left side and the south pole on the right. At locations “x” and “y”, the magnetic field lines point away from the magnet. What happens in the region directly above, up and away (towards the reader) from the north pole and how is this different from what happens at the south pole?

3-Describe the action of a magnetic compass and a magnetic dipping needle at:

1-the equator

2-the south magnetic pole

Answer 1

Before we begin, knowing one thing will help us solve all of these questions. Magnetic Field Lines and Electric Field Lines NEVER intersect. Think of it as a fundamental rule when drawing them. They may diverge from a point or converge to a point but they never ever intersect. Knowing that, we can now begin to consider our two cases.

A bar magnet alone has the following field lines. Note two things. They don't intersect each other and they always seem to flow North to South outside the magnetic and from South to North inside.

Before we begin, knowing one thing will help us solve all of these questions. Magnetic Field Lines and Electric Field Lines NEVER intersect. Think of it as a fundamental rule when drawing them. They may diverge from a point or converge to a point but they never ever intersect. Knowing that, we can now begin to consider our two cases. A bar magnet alone has the following field lines. Note two things. They don't intersect each other and they always seem to flow North to South outside the magnetic and from South to North inside. b. Consider two magnets kept parallel to each other. Now, first we consider what will happen if the north faces north and south faces south. Using the rule of north to south flow and the property of non-intersection, we can draw the field lines of the resultant system. The field between the magnets will have squished lines, all of them pressed closer together

1.

b. Consider two magnets kept parallel to each other. Now, first we consider what will happen if the north faces north and south faces south. Using the rule of north to south flow and the property of non-intersection, we can draw the field lines of the resultant system. The field between the magnets will have squished lines, all of them pressed closer together, unable to overlap each other. This means the more we press them together, the more squished they get and hence there is repelling force between the magnets.

a. On the other hand, we flip one of the magnets so now North faces South and vice-versa. Now, field lines, while can't intersect, love flowing from North to South. So, the field lines emerging from North of one magnet immediately find the South of the other and terminate there. This creates a force of attraction between magnets.

So, the field between the magnets is different in both cases while the field on the opposite side of each magnet remains unchanged. This is because of a lack of change in field values on the other side.

2.

In this case, we take a bar magnet again and see that original diagram we used in the first question. The field lines outside the magnet flow from north to south. Now, this is a 2D image, the magnetic fields lines of the bar magnet towards the reader and into the plane of the paper behave the same way. They flow away from North and into the South. Hence, anywhere in 3D near the North will see field lines directed away from the magnet, while a similar symmetry exists at the South Pole where at any point near it, the fields will point towards the magnet.

3.

Earth can also be considered a giant bar magnet. The Geographical South Pole is actually the North Pole of the Earth Magnet, and vice-versa. If we take a compass, and stand at the equator, we will see that it aligns itself in the direction of the field line, such that its north pole aligns itself to the magnetic south pole which is the geographical north pole. This is of course true at any point on the Earth's Surface. So, at the equator the compass needle will point north (geographical).

While, things get peculiar at the south pole (magnetic). The field lines are flowing into the south pole, but due to the curve of the lines, the lines hit the compass needle from above. The compass is made to move only horizontally, and can't align itself in the vertical direction. So, it will freely move about its axis at the magnetic south pole.

Magnetic Dipping Needles are special compasses, hoisted on a horizontal support. This means, they can move in any direction except the horizontal. They are basically a compass rotated 90 degrees. They too are just small magnets. So, when they encounter a magnetic field, they try to align themselves in its direction. In this case, the needle can't point north horizontally, but it can point in the direction of the field line. The direction makes an angle with the horizontal surface of Earth. So Dipping needles tell you the precise field direction of the field lines and hence the angle of the point on surface makes with the horizontal at equator.

At equator then, the magnetic dipping needle will be unmoved. Since, the field line will be parallel to the surface. At the magnetic south pole, on the other hand, the field lines are moving downwards, towards the south pole, the north of the needle will align itself such that it too points towards the south pole, and hence rotating by a 90 degrees.