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Current, charge and the direction problem questions
Three definitions carry the whole of electricity. Current is a rate of flow of charge. Potential difference is the energy each coulomb picks up or hands over. Divide one by the other and you have resistance. Alongside them runs a historical accident, because the arrows on every circuit diagram point the wrong way.
20 original questions · 58 marks · the current, charge and the direction problem notes · Electricity
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A charge of 30 C flows past a point in 10 s. Calculate the current.
Mark scheme
I = ΔQ/Δt = 30/10 (1)
I = 3.0 A (1)Define the coulomb.
Mark scheme
One coulomb is the charge that passes a point when a current of one ampere flows for one second (1); that is, 1 C = 1 A s (1).12 J of work is done moving a charge of 4.0 C between two points. Calculate the potential difference.
Mark scheme
V = W/Q = 12/4.0 (1)
V = 3.0 V (1)A camera flash produces a current of 0.75 A that lasts for 2.4 ms. Calculate the charge that flows through the flash lamp.
Mark scheme
Use of Q = IΔt with t = 2.4 × 10−3 s (1); Q = 0.75 × 2.4 × 10−3 = 1.8 × 10−3 C (1).A phone battery is rated at 3200 mA h, meaning it can supply a current of 3200 mA for one hour. Calculate the charge the battery stores.
Mark scheme
I = 3.2 A and t = 3600 s (1); Q = It = 3.2 × 3600 = 1.15 × 104 C (1).An electron moving between two electrodes in a vacuum tube gains 8.0 × 10−19 J of energy. Calculate the potential difference between the electrodes (e = 1.60 × 10−19 C).
Mark scheme
V = W/Q (1); V = (8.0 × 10−19)/(1.60 × 10−19) = 5.0 V (1).A current of 0.50 A flows for 2.0 minutes. Calculate (a) the charge that passes and (b) the number of electrons this represents (e = 1.60 × 10−19 C).
Mark scheme
(a) Q = It = 0.50 × 120 (1)
Q = 60 C (1)
(b) N = Q/e = 60/(1.60 × 10−19) = 3.75 × 1020 (1)A component has a potential difference of 6.0 V across it and carries a current of 0.25 A. Calculate its resistance.
Mark scheme
R = V/I = 6.0/0.25 (1)
R = 24 Ω (1)A charge of 8.0 C passes through a resistor with a potential difference of 12 V across it. Calculate the energy transferred in the resistor.
A steady current of 2.0 A flows in a lamp filament for 5.0 minutes. Calculate the charge that passes through the filament.
The flexible lead to a kettle contains copper wire of diameter 1.2 mm carrying a current of 13 A. Copper has 8.5 × 1028 free electrons per cubic metre (e = 1.60 × 10−19 C). Calculate (a) the cross-sectional area of the wire and (b) the drift velocity of the electrons.
A wire in a circuit tapers so that its cross-sectional area halves. State what happens to the drift velocity of the electrons where the wire narrows, and explain your answer using I = nAvq.
A cyclist's rear lamp is powered by a 3.0 V battery and draws a current of 0.20 A. Calculate the energy transferred by the lamp during a 10 minute ride.
A device draws a current of 3.0 A from a 230 V supply for 1.0 hour. Calculate (a) the charge that flows, (b) the energy transferred and (c) the number of electrons that pass (e = 1.60 × 10−19 C).
Explain what is meant by conventional current and how it relates to the actual direction electrons move in a metal wire.
Explain why the convention for current direction was not changed once electrons were discovered to be the charge carriers in metals.
An electric bicycle battery needs at least 2.0 × 105 C of charge to charge fully. It is connected for 8.0 hours overnight to a charger that supplies a steady current of 6.5 A. Deduce whether the battery charges fully.
A copper track (8.5 × 1028 free electrons per m3) and a semiconductor channel (5.0 × 1019 charge carriers per m3) in a sensor have the same cross-sectional area and carry the same current. Determine the ratio of the drift velocity in the semiconductor to that in the copper.
In a metal wire the electrons drift at less than a millimetre per second, yet a lamp lights the instant the switch is closed and the current can be several amperes. Explain fully how both observations are consistent with the drift of electrons.
A 9.0 V battery for a smoke alarm stores 1.9 × 104 C of charge. (a) Show that the energy stored is about 1.7 × 105 J. (b) In standby the alarm draws a steady current of 60 μA. The manufacturer claims a 10 year standby life. Determine whether the claim is justified (1 year = 3.15 × 107 s).
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