9. Electricity
A section of Physics, 9702
Listing 10 of 370 questions
State what is meant by an electric current. A metal wire has length L and cross-sectional area A, as shown in . L A I I is the current in the wire, n is the number of free electrons per unit volume in the wire, v is the average drift speed of a free electron and e is the charge on an electron. State, in terms of A, e, L and n, an expression for the total charge of the free electrons in the wire. Use your answer in to show that the current I is given by the equation I = nAve. A metal wire in a circuit is damaged. The resistivity of the metal is unchanged but the cross- sectional area of the wire is reduced over a length of 3.0 mm, as shown in . d 0.69 d damaged length cross-section Y cross-section X current 0.50 A 3.0 mm The wire has diameter d at cross-section X and diameter 0.69 d at cross-section Y. The current in the wire is 0.50 A. Determine the ratio average drift speed of free electrons at cross-section Y average drift speed of free electrons at cross-section X . ratio = The main part of the wire with cross-section X has a resistance per unit length of 1.7 × 10–2 Ω m–1. For the damaged length of the wire, calculate 1. the resistance per unit length, resistance per unit length = Ω m–1 2. the power dissipated. power = W The diameter of the damaged length of the wire is further decreased. Assume that the current in the wire remains constant. State and explain qualitatively the change, if any, to the power dissipated in the damaged length of the wire.
9702_w17_qp_22
THEORY
2017
Paper 2, Variant 2
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Define resistance. A cylindrical metal wire of length 2.4 m and cross-sectional area 8.0 × 10–6 m2 has a resistance of 0.33 Ω. There is a current in the wire of 4.7 A. Determine the resistivity of the metal from which the wire is made. resistivity = Ω m Calculate the charge that passes through the wire in a time of 5.0 minutes. charge = C The free electrons (charge carriers) in the wire have an average drift speed of 0.16 mm s–1. Determine the number density of charge carriers in the metal. number density = m–3 The wire in may be considered to be a fixed resistor. It is connected in series with a thermistor to a battery that has negligible internal resistance. Use circuit symbols to complete to show the circuit diagram of this arrangement. Explain, without calculation, how the power dissipated in the wire changes as the temperature of the thermistor is increased.
9702_w24_qp_23
THEORY
2024
Paper 2, Variant 3
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Define the potential difference across a component. The variation with potential difference V of the current I in a semiconductor diode is shown in . 0.5 1.0 V / V I Use to describe qualitatively: the resistance of the diode in the range V = 0 to V = 0.25 V the variation, if any, in the resistance of the diode as V changes from V = 0.75 V to V = 1.0 V. A battery of electromotive force (e.m.f.) 12 V and negligible internal resistance is connected to a uniform resistance wire XY, a fixed resistor and a variable resistor, as shown in . 1.6 m 5.0 Ω 2.0 m 2.7 A 1.5 A W 12 V Z X Y resistance wire (not to scale) The fixed resistor has a resistance of 5.0 Ω. The current in the battery is 2.7 A and the current in the fixed resistor is 1.5 A. Calculate the current in the resistance wire. current = A Determine the resistance of the variable resistor. resistance = Ω Wire XY has a length of 2.0 m. Point Z on the wire is a distance of 1.6 m from point X. The fixed resistor is connected to the variable resistor at point W. Determine the potential difference between points W and Z. potential difference = V The resistance of the variable resistor is now increased. By considering the currents in every part of the circuit, state and explain whether the total power produced by the battery decreases, increases or stays the same.
9702_m23_qp_22
THEORY
2023
Paper 2, Variant 2
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Questions Discovered
370