(a) Define electromotive force. (b) State: (i) the principle of operation of a potentiometer, (ii) two advantages that a potentiometer has over a voltmeter ...
(i) the principle of operation of a potentiometer,
(ii) two advantages that a potentiometer has over a voltmeter in measuring potential difference.
(c)(i) Sketch and label a diagram of a gold-leaf electroscope.
(ii) Give one use of a gold-leaf electroscope.
(d)(i) Explain the action of a magnetic relay.
(ii) List two factors which determine the magnitude of an induced emf in a coil.
(iii) A current of 5 A passes through a straight wire in a uniform magnetic field of flux density 2.0 x10\(^{-3}\) T. Calculate the force per unit length exerted on the wire when it is inclined at 30° to the field.
(a) Electromotive force (emf) The electromotive force of a source is the total energy supplied by the source in driving one coulomb of charge round the complete circuit; that is, the work done per unit charge by the source (equal to the terminal potential difference of the source on open circuit). Its SI unit is the volt (V).
(b) Potentiometer (i) Principle of operation: A steady current is passed through a uniform resistance wire, so the potential difference across a portion of the wire is directly proportional to its length, giving a uniform potential gradient. An unknown emf or p.d. is balanced against the p.d. across a length of the wire; at balance no current flows through the galvanometer (null method), so \( V \propto L \), where \(V\) is the p.d. and \(L\) is the balance length.
(ii) Two advantages of a potentiometer over a voltmeter:
At balance it draws no current from the cell being measured, so it measures the true emf and eliminates error due to the internal resistance of the cell (a voltmeter draws current and reads less than the emf).
It is more sensitive and more accurate, since it uses a null method and needs no scale calibration that could introduce zero or scale errors.
(c)(i) Labelled sketch of a gold-leaf electroscope:
Labelled sketch of a gold-leaf electroscope: a metal cap and rod carry charge down to a metal plate and a gold leaf, which diverges when charged; the rod is held by an insulating plug in an earthed metal case with glass windows.
The metal cap collects charge and passes it down the metal rod to the metal plate and the gold leaf. Because the plate and leaf then carry like charges, the leaf is repelled and diverges from the plate. The rod is held by an insulating plug in the top of an earthed metal case that has glass windows for viewing.
(ii) Use of a gold-leaf electroscope: to detect the presence of electric charge on a body (it can also be used to test the sign of a charge and to compare the magnitudes of charges).
(d)(i) Action of a magnetic relay: A small current in the coil (the control circuit) magnetises a soft-iron core, turning it into an electromagnet. The electromagnet attracts a pivoted soft-iron armature, and the movement of the armature closes (or opens) a pair of contacts in a separate circuit. In this way a small current in the first circuit is used to switch on or off a much larger current in the second circuit.
(ii) Two factors which determine the magnitude of an induced emf in a coil:
The rate of change of magnetic flux linkage (the speed at which the flux through the coil changes).
The number of turns in the coil.
(iii) Force per unit length on the wire
Given: current \(I = 5\ \text{A}\), flux density \(B = 2.0\times10^{-3}\ \text{T}\), angle to the field \(\theta = 30^{\circ}\).
The force on a current-carrying conductor is \(F = BIL\sin\theta\), so the force per unit length is
(a) Electromotive force (emf) The electromotive force of a source is the total energy supplied by the source in driving one coulomb of charge round the complete circuit; that is, the work done per unit charge by the source (equal to the terminal potential difference of the source on open circuit). Its SI unit is the volt (V).
(b) Potentiometer (i) Principle of operation: A steady current is passed through a uniform resistance wire, so the potential difference across a portion of the wire is directly proportional to its length, giving a uniform potential gradient. An unknown emf or p.d. is balanced against the p.d. across a length of the wire; at balance no current flows through the galvanometer (null method), so \( V \propto L \), where \(V\) is the p.d. and \(L\) is the balance length.
(ii) Two advantages of a potentiometer over a voltmeter:
At balance it draws no current from the cell being measured, so it measures the true emf and eliminates error due to the internal resistance of the cell (a voltmeter draws current and reads less than the emf).
It is more sensitive and more accurate, since it uses a null method and needs no scale calibration that could introduce zero or scale errors.
(c)(i) Labelled sketch of a gold-leaf electroscope:
Labelled sketch of a gold-leaf electroscope: a metal cap and rod carry charge down to a metal plate and a gold leaf, which diverges when charged; the rod is held by an insulating plug in an earthed metal case with glass windows.
The metal cap collects charge and passes it down the metal rod to the metal plate and the gold leaf. Because the plate and leaf then carry like charges, the leaf is repelled and diverges from the plate. The rod is held by an insulating plug in the top of an earthed metal case that has glass windows for viewing.
(ii) Use of a gold-leaf electroscope: to detect the presence of electric charge on a body (it can also be used to test the sign of a charge and to compare the magnitudes of charges).
(d)(i) Action of a magnetic relay: A small current in the coil (the control circuit) magnetises a soft-iron core, turning it into an electromagnet. The electromagnet attracts a pivoted soft-iron armature, and the movement of the armature closes (or opens) a pair of contacts in a separate circuit. In this way a small current in the first circuit is used to switch on or off a much larger current in the second circuit.
(ii) Two factors which determine the magnitude of an induced emf in a coil:
The rate of change of magnetic flux linkage (the speed at which the flux through the coil changes).
The number of turns in the coil.
(iii) Force per unit length on the wire
Given: current \(I = 5\ \text{A}\), flux density \(B = 2.0\times10^{-3}\ \text{T}\), angle to the field \(\theta = 30^{\circ}\).
The force on a current-carrying conductor is \(F = BIL\sin\theta\), so the force per unit length is
