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Electrical Revision
MJC P2 Q4
A small electric torch is powered by a single cell
which supplies 1.6 J of energy per coulomb of
charge passing through the cell. When the torch is
switched on, the cell supplies a constant current of
0.50 A to the bulb X. The potential difference across
the bulb is 1.2 V.
(a) What is meant by “1.6 J of energy per coulomb
of charge passing through the cell”?
(b) Show that the internal resistance r of the cell is
0.80 Ω.
(c) The bulb X is replaced by another bulb Y which
is found to take a current of 0.30 A. Calculate
(i) the new potential difference across the
internal resistance of the cell,
(ii) the potential difference across bulb Y.
(d) Show quantitatively why bulb Y is more efficient.
Electrical Revision
RJC P2 Q3
The circuit in Fig 3.1 below has a thermistor
connected in series to a 200 Ω resistor and a 12 V
battery of negligible internal resistance. Fig 3.2
shows how the resistance Rth of the thermistor
varies with temperature.
(a) (i) Calculate the current in the circuit when the
temperature is 25 °C.
(ii) Calculate the potential difference across the
thermistor at 25 °C.
(b) Without further calculation, explain how you
would expect the potential difference across the
thermistor to change as temperature increases
from 25 °C.
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(c) The circuit in Fig 3.1 is modified by removing the
200 Ω resistance to give the circuit shown in Fig
3.3. The temperature of the thermistor is
increased at a steady rate from 25 °C to 45 °C in
10 min.
(i) Calculate the power dissipated in the
thermistor at 25 °C and 45 °C.
(ii) By taking the average of your answers in
(c)(i), calculate an approximate value for the
energy supplied by the battery during the
period in which the temperature of the
thermistor increases.
(iii) Explain why the energy determined in (c)(ii)
is only an estimate.
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RJC P3 Q4
A 4.0 V cell of negligible internal resistance is
connected in series with two resistors of resistances
600 Ω and 400 Ω respectively (See Fig 4.1).
(a) What is the potential difference across the 400 Ω
resistor?
(b) A voltmeter is connected across the 400 Ω
resistor as shown in Fig 4.2.
If the voltmeter reads 1.40 V, calculate the
resistance of the voltmeter.
(c) What resistance must the voltmeter have in
order that it measures the potential difference
across the 400 Ω resistor in Fig 4.2 to an
accuracy of 1%?
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JJC P3 Q8
(a)
Fig 8.1 shows a 15 V source of negligible
internal resistance.
(i) With switch K open, find the effective
resistance between point C and point D.
(ii) With switch K closed and given that the
potential at point A is – 3.0 V,
1. determine the potential at point C and,
2. calculate the current I.
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(b)
Fig 8.2 shows a constantan wire XY of length
75.0 cm of uniform cross-sectional area 1.2 ×
108 m2. The 2.0 V source is of negligible internal
resistance.
(i) Given that the resistivity of constantan is
49.0 × 10-8 Ω m, show that the resistance of
the wire is 30.6 Ω.
(ii) If the galvanometer registers a null
deflection when length XP is 50.0 cm,
1. show that the potential difference across
XY is 1.82 V and
2. determine
the
ammeter
reading
assuming it has negligible resistance.
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(c)
The 2.00 Ω resistor is now replaced with a 4.00
Ω resistor as shown in Fig 8.3. The current
shown on the ammeter is observed to be 0.382
A when the galvanometer registers a null
deflection.
(i) Determine the e.m.f. and internal resistance
of cell Q.
(ii) Describe and explain the qualitative change
(if any) in the balance length XP if the
ammeter has non-negligible resistance.
Electrical Revision
TJC P2 Q5
A light bulb, rated 80 W, is connected to an
alternating power supply whose voltage is given by
V = 110 sin (100πt)
(a) Calculate the value of the root-mean-square
current in the bulb.
(b) Calculate the maximum power used by the bulb
at any given instant of time.
(c) On Fig 5.1, sketch the variation of the power
dissipated in the bulb with respect to time over a
duration of 0.040 s. Mark appropriate numerical
values along the axes.
Electrical Revision
PJC P3 Q5
In a 1200 W hairdryer shown in Fig 5.1, alternating
current of 50 Hz passes mostly through the heating
coils which act as a pure resistor.
(a) (i) Suppose the hair dryer is connected to the
mains supply of 120 V r.m.s., calculate the
peak current through the coils.
(ii) Write an equation in terms of t where t is
time, for the current that passes through the
heating coil, given that power output is zero
at t = 0.
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(iii) The hairdryer is connected to a c.r.o. whose
Y-plate sensitivity and time-base are set at
100 V sm-1 and 5.0 ms cm-1 respectively.
On Fig 5.2, sketch what is seen on the c.r.o.
to illustrate the variation of voltage through
the coil with time.
(b) The primary coil of a transformer shown in Fig
5.3 is connected to a 2.4 kV r.m.s. supply. The
secondary coil is connected to the hair dryer and
the current flowing through the heating coil has
the same value as that calculated in (a)(i),
Given that the transformer is non-ideal and that
electrical energy is converted to thermal energy
in the windings of the transformer at a rate of
600 W, determine the primary current Ip.