(a) Explain what is meant by a magnetic field (b)(i) Describe an experiment to show that a magnetic field exists around a straight wire carrying current (ii...

(a) Magnetic field. A magnetic field is the region of space around a magnet or a current-carrying conductor within which a magnetic force is experienced (for example by a magnetic material, a moving charge, or another magnet). It is a vector quantity, represented by lines of magnetic flux whose direction at any point is the direction in which a free north pole would move.

(b)(i) Experiment to show a field around a straight wire (Oersted's experiment). A thick straight copper wire is passed vertically through a small hole in the middle of a horizontal piece of stiff cardboard. Several small plotting compasses are placed on the card around the wire, and the wire is connected through a switch to a battery. When a large current is switched on and the card is tapped gently, the compass needles swing round and settle so that they all point along circles centred on the wire. Iron filings sprinkled on the card set into the same pattern of concentric circles. This shows that a magnetic field exists around the wire. When the current is reversed, every compass needle reverses its direction, showing that the field direction depends on the current direction.

(b)(ii) Pattern and direction of the field. The lines of force are concentric circles centred on the wire and lying in the plane at right angles to it. Their direction is given by the right-hand grip rule: gripping the wire with the right hand so that the thumb points along the conventional current, the curled fingers give the field direction. With the current coming out of the page (shown by the central dot), the field lines run anticlockwise, as drawn below.

(c) Two parallel wires carrying current in the same direction. Each wire produces its own set of concentric circular field lines. At the midpoint of the line joining the two wires the two fields are equal in magnitude but opposite in direction, so they cancel exactly, giving a neutral point N. (Outside the pair the fields reinforce, which is why two such wires attract each other.)

(d) Protecting a delicate magnetic material (magnetic shielding). The instrument is enclosed in a thick ring or box of soft iron, which has a very high magnetic permeability. Because the flux prefers the easy path through the iron, the earth's field lines are drawn into the shield and channelled round its walls instead of crossing the enclosed space. The cavity inside is therefore left almost field-free, screening the delicate material from the earth's magnetic field.