(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 around a straight current-carrying wire.
(b) When is an ammeter said to be (i) Sensitive (ii) accurate?
(c)(i) Explain, using a labelled diagram, how a delicate magnetic material could be protected, from the Earth's magnetic field.
(ii) A charge of 1.6 x 10\(^{-19}\) C enters a magnetic field of flux density 2.0 T with a velocity of 2.5 x 10\(^7\) ms\(^{-1}\) at an angle of 30° with the field. Calculate the magnitude of the force exerted on the charge by the field.
(d) State the laws of electro-magnetic induction.
(a)(i) Neutral point
A neutral point in a magnetic field is a point where the resultant magnetic flux density is zero, because the field due to a magnet and the earth's (or another) field are equal in magnitude and opposite in direction, so they cancel out. No magnetic field line passes through such a point.
(a)(ii) Field around a straight current-carrying wire
The magnetic field lines form concentric circles in a plane at right angles to the wire, centred on the wire. Using the right-hand grip rule (thumb pointing in the direction of conventional current, fingers curling round), the lines are anticlockwise when the current flows out of the page and clockwise when it flows into the page. The lines are closest together near the wire and spread out farther from it.
(b) Ammeter
- (i) Sensitive: when it gives a large deflection for a small current passing through it.
- (ii) Accurate: when its reading corresponds very closely to the true value of the current being measured (small error).
(c)(i) Magnetic shielding
A delicate magnetic material is protected from the earth's field by magnetic screening: it is enclosed inside a hollow box or ring made of soft iron (a soft-iron shield). The soft iron has very high permeability, so the external field lines are concentrated into and channelled through the iron walls, leaving the space inside almost field-free. The delicate material placed in the cavity is therefore shielded.
(c)(ii) Force on the moving charge
\[ F = qvB\sin\theta \] \[ F = (1.6\times10^{-19})(2.5\times10^{7})(2.0)\sin30^{\circ} \] \[ F = (1.6\times10^{-19})(2.5\times10^{7})(2.0)(0.5) = 4.0\times10^{-12}\ \text{N} \]
(d) Laws of electromagnetic induction
- Faraday's law: the magnitude of the induced e.m.f. in a circuit is directly proportional to the rate of change of magnetic flux linkage through the circuit.
- Lenz's law: the induced current always flows in such a direction as to oppose the change (in flux) producing it.
(a)(i) Neutral point
A neutral point in a magnetic field is a point where the resultant magnetic flux density is zero, because the field due to a magnet and the earth's (or another) field are equal in magnitude and opposite in direction, so they cancel out. No magnetic field line passes through such a point.
(a)(ii) Field around a straight current-carrying wire
The magnetic field lines form concentric circles in a plane at right angles to the wire, centred on the wire. Using the right-hand grip rule (thumb pointing in the direction of conventional current, fingers curling round), the lines are anticlockwise when the current flows out of the page and clockwise when it flows into the page. The lines are closest together near the wire and spread out farther from it.
(b) Ammeter
- (i) Sensitive: when it gives a large deflection for a small current passing through it.
- (ii) Accurate: when its reading corresponds very closely to the true value of the current being measured (small error).
(c)(i) Magnetic shielding
A delicate magnetic material is protected from the earth's field by magnetic screening: it is enclosed inside a hollow box or ring made of soft iron (a soft-iron shield). The soft iron has very high permeability, so the external field lines are concentrated into and channelled through the iron walls, leaving the space inside almost field-free. The delicate material placed in the cavity is therefore shielded.
(c)(ii) Force on the moving charge
\[ F = qvB\sin\theta \] \[ F = (1.6\times10^{-19})(2.5\times10^{7})(2.0)\sin30^{\circ} \] \[ F = (1.6\times10^{-19})(2.5\times10^{7})(2.0)(0.5) = 4.0\times10^{-12}\ \text{N} \]
(d) Laws of electromagnetic induction
- Faraday's law: the magnitude of the induced e.m.f. in a circuit is directly proportional to the rate of change of magnetic flux linkage through the circuit.
- Lenz's law: the induced current always flows in such a direction as to oppose the change (in flux) producing it.
