(a)(i) What is meant by neutral point in a magnetic field? (ii) Draw and label a diagram to show the pattern and direction of the magnetic field produced ar...

(a)(i) Neutral point

A neutral point is a point at which the resultant magnetic flux density is zero. At this point, the magnetic fields acting there are equal in magnitude and opposite in direction.

(a)(ii) Magnetic field around a straight current-carrying wire

The field consists of concentric circles centred on the wire. For a current out of the page, the direction of the field is anticlockwise, as shown.

(b) Ammeter

(i) Sensitive: An ammeter is sensitive when it produces an appreciable or large deflection for a very small current.

(ii) Accurate: An ammeter is accurate when the current it measures is very close to the true value of the current flowing in the circuit.

(c)(i) Protection from the Earth's magnetic field

The delicate magnetic material is enclosed in a thick soft-iron or other high-permeability magnetic shield. The shield provides an easier path for the Earth's magnetic field lines, so that very little magnetic field exists in the cavity containing the material.

(c)(ii) Force on the charge

The magnetic force is given by:

\[F=qvB\sin\theta\]

\[F=(1.6\times10^{-19})(2.5\times10^{7})(2.0)\sin30^\circ\]

\[F=(1.6\times10^{-19})(2.5\times10^{7})(2.0)(0.5)\]

\[\boxed{F=4.0\times10^{-12}\ \text{N}}\]

(d) Laws of electromagnetic induction

1. Whenever the magnetic flux linkage through a circuit changes, an e.m.f. is induced in the circuit. The magnitude of the induced e.m.f. is proportional to the rate of change of magnetic flux linkage.
2. The induced current flows in such a direction as to oppose the change in magnetic flux that produces it.