Try these "busters" to exercise your brain ... they should help you grasp the concepts underlying the properties of magnetic fields, etc. To gain the maximum effect you should attempt to answer them before looking at the answers!
[1] A steady current flows in the circuit shown below. If a compass were placed beneath the circuit at points A, B and C what would you expect to observe in the way of deflection?
[2] Here's a strange one! The north pole of a compass needle is attracted to the north pole of the Earth. Since like poles are supposed to repel, how can that be? What's the explanation?
[3] ... and another! One of your friends says that when a compass is taken across the equator from the northern hemisphere to the southern hemisphere, the needle turns round and points the other way. Another friend says that's nonsense! What's your opinion?
[4] You have two bars of iron; one is magnetized while the other is not. If there are no other items or materials nearby that you could use, how could you tell which one is the magnet?
[5] The picture shows an end-on view of three parallel wires that are perpendicular to the page. In two of the wires the current is directed into the page; in the third it is out of the page. The two outermost wires are fixed but the middle wire is free to move.
Will it move? Explain.
[6] A so-called electromagnet can be made by wrapping a coil around a "core", e.g., of iron. When a current flows through the coil it produces a magnetic field that is intensified by the "core"; the resulting field could be as much as 100's or 1000's of times greater than without the "core". In the arrangements below
explain whether the electromagnets will be attracted towards or repelled from the permanent magnets on the right.
[7] For each electromagnet at the left of the picture, work out whether it will be attracted to or repelled from the nearby electromagnet on the right.
[8] If the earth's magnetism is assumed to originate from a large circular loop of current within the Earth, how is the plane of this loop oriented relative to the magnetic axis of the Earth and what is the direction of the current?
[9] The diagram shows 4 wires viewed end-on; they are long, straight and parallel to each other and their axes lie at the corners of a square. Each wire carries a current of the same magnitude. What is the direction of the current in each wire so that if any single current is switched off, the total magnetic field at the point P, the center of the square, is directed towards one corner of the square.
Explain your answer. Hint: there are 4 possible solutions ... see if you can get them all.
[10] The diagram shows 3 wires viewed end-on; they are long, straight and parallel to each other and their axes lie at the corners of a square. The currents in #1 and #2 are equal and directed into the page. If you want the magnetic field at the vacant corner to be zero can you figure out the direction and magnitude of the current in #3?
Explain your reasoning.
[11] You are given a length of wire, L, and you must bend it into the shape that gives the largest magnetic field at the center when a current passes along the wire; the choices are (a) a square, (b) a circle and (c) an equilateral triangle. Which would you choose?