Download CE-PHY I

HKCEE 1990 – 1998
PHYSIC – Long Question
Electromagnetism
(1990-CE-PHY I - 7)
7.
Figure 9
Figure 9 shows a 3-pin electrical plug and a transformer. The three wires X, Y, Z of the
transformer are to be connected to the plug. The plug will be connected to a 200 V a.c. supply.
The output voltage of the transformer will be 110 V. Assume the transformer is 100% efficient.
(a)
(i)
To which of the terminals A, B and C of the plug should each of the wires X, Y
and Z be connected ?
(ii)
Explain briefly why the fuse should be connected to the terminal B.
(iii)
Suggest one reason why it is necessary to have the X-wire connection.
(iv)
Find the turns ratio (primary coil to secondary coil) of the transformer.
(8 marks)
(b)
An iron of rated values '110 V, 1100 W' is connected to the output of the transformer and
switched on for half an hour.
(i)
Calculate the current drawn from the transformer by the iron.
(ii)
Calculate the cost of electricity if one kilowatt-hour of electrical energy costs 80.
(iii)
If fuses marked 1A, 3A and 7A are available, which one is most appropriate to be used in
the plug in Figure 9 ? Explain your choice.
(7 marks)
(1990-CE-PHY I - 8)
8.
Figure 10
Figure 10 shows the design of a water heating system. When the water is below a certain
temperature and when the contacts are covered with water, the heater is switched on by the
electromagnetic relay.
(a)
Construct a truth table relating the inputs A and B and the output Q of the 'AND' gate.
(Take 1 for 'high' and 0 for 'low' voltage.) What is the state of Q when the heater is on ?
(3 marks)
(b)
The state of input A is controlled by devices X and Y.
(i)
Name the devices X and Y.
(ii)
How will the resistance of device X change when the temperature of the water
increases ? State under what condition the input A will be 'high'.
(iii) What is the function of device Y?
(7 marks)
(c)
The state of input B is controlled by the contacts and the resistor R.
(i)
What will be the state of input B when the contacts are wholly immersed in
water ?
(ii)
Explain briefly why the contacts should be placed near the top of the tank.
(3 marks)
(d)
Suggest one safety device which could be added to the water heating
system.
(2 marks)
(1991-CE-PHY I - 6)
6.
(a)
A thermistor is connected to a resistor R and a 6 V power supply as shown in Figure 8.
The resistance of R is 470 . A voltmeter of high resistance is connected across R.
Figure 9 shows the variation of resistance of the thermistor with temperature.
(i)
(ii)
The reading of the voltmeter is 4.7 V. At this instant, find
(1)
the current flowing through R,
(2)
the resistance of the thermistor,
(3)
the temperature of the thermistor.
How would the voltmeter reading change if the temperature of the thermistor
increases ? Explain briefly.
(8 marks)
(b)
A student uses the thermistor in (a) and the components shown in Figure 10 to design an
alarm system. When the switch is on and the temperature of the thermistor rises to a
certain value, the buzzer will sound. Figure 11 shows an incomplete circuit of the system.
Figure 10
Figure 11
(i)
Construct a truth table of a NOR gate. (Take 1 for high voltage and 0 for low
voltage.)
(ii)
Copy Figure 11 into your answer book and complete the circuit.
(iii)
What is the function of the variable resistor in the circuit ?
(7 marks)
(1992-CE-PHY I - 5)
5.
A student uses the following components to measure the resistance of a light bulb :
A battery, an ammeter, a voltmeter, a switch,
a variable resistor and the light bulb.
Figure 7 shows an incomplete circuit for the experiment.
Figure 7
(a)
Copy Figure 7 into your answer book and use suitable circuit symbols to complete the
circuit. Indicate on your diagram the positive terminals of the ammeter and voltmeter
with " + " signs.
(5 marks)
(b)
What is the function of the variable resistor in the circuit ?
(2 marks)
(c)
Figure 8 shows the result obtained in the experiment.
Figure 8
(i)
(ii)
What is
(1)
the voltmeter reading,
(2)
the ammeter reading ?
Calculate the resistance of the light bulb.
(4 marks)
(d)
The rating of the light bulb is '200 V, 100 W'.
(i)
Calculate the resistance of the bulb when it is working at its rated value.
(ii)
Explain why the resistance found in (d) (i) is much greater than that found in (c)
(4 marks)
(1992-CE-PHY I - 6)
6.
(a)
Figure 9
Figure 9 shows the circuit of a burglar alarm system S1. The buzzer sounds when the
device X is illuminated by a burglar's torch.
(i)
Name the device X.
(ii)
Construct a truth table for a NOT gate.
(iii)
Explain briefly why the buzzer sounds when X is illuminated.
(iv)
A student says that the variable resistor is used to adjust the loudness of the
buzzer.
State whether the statement is true or false and explain briefly.
(8 marks)
(b)
Figure 10
Figure 10 shows the circuit of another burglar alarm system S 2. The device Y is made up
of two NAND gates.
(i)
Name the device Y.
(ii)
Device X
RESET switch
Buzzer
Step l
Illuminated
Open
ON
Step 2
Not illuminated Open
Step 3
Not illuminated Closed
Step 4
Not illuminated Open
The steps shown in the table above are performed to investigate how S 2 works. In
step 1, X is illuminated with the RESET switch open. As a result the buzzer
sounds (ON). Write down if the buzzer is ON or OFF in steps 2 to 4.
(iii)
State the advantage of using the alarm system S2 over S1.
(iv)
If X in Figure 10 is replaced by a thermistor, suggest one application of the
circuit.
(7 marks)
(1993-CE-PHY I - 5)
5.
(a)
Figure 6
Figure 6 shows a solenoid passing through a piece of horizontal cardboard. A direct
current passes through the solenoid from A to B to produce a magnetic field.
(i)
Describe a method to find the magnetic field pattern on the cardboard using iron
filings.
(ii)
Draw a diagram to show the pattern and direction of the magnetic field on the
cardboard.
(5 marks)
(b)
Figure 7
A student designs a simple door bell as shown in Figure 7. When switch S is pressed and
then released, two notes 'ding-ding' are heard.
(i)
Explain how the two notes are produced.
(ii)
Suggest one way to modify the bell so that two notes of different frequencies
'ding-ding' are produced.
(iii)
Comment on the following two statements :
Statement 1 : The bell does not work if the spring is made of copper.
Statement 2 : The bell does not work if the polarities of the battery are reversed.
(10 marks)
(1993-CE-PHY I - 6)
6.
Figure 8
Figure 8 shows the circuit of a lighting system. When switch S is closed
and the environment is dark, the four lamps will be turned on.
(a)
Name the devices X and Z.
(2 marks)
(b)
Construct a truth table for a NOR gate.
(2 marks)
(c)
Explain why the lamps are turned on when switch S is closed and the environment is
dark.
(5 marks)
(d)
The four lamps are connected to a 200 V mains supply and the rating of each lamp is
"200 V, 500 W".
(i)
Find the current drawn from the mains supply.
(ii)
Calculate the cost of electricity if the lamps are turned on for 8 hours and one
kilowatt-hour of electricity costs 80.
(4 marks)
(e)
Explain why the device Z is used in the circuit.
(2 marks)
(1994-CE-PHY I - 7)
7.
A moving coil galvanometer has a full scale deflection current of 5 mA and the resistance of the
coil in the galvanometer is 200 .
(a)
Find the full scale deflection voltage.
(2 marks)
(b)
With the help of a resistor, the galvanometer can be adapted to measure a voltage up to
10 V.
(i)
Should the resistor and galvanometer be connected in series or in parallel ?
(ii)
Find the resistance of the resistor.
(3 marks)
(c)
Figure 13
Figure 13 shows the structure of the galvanometer.
(i)
A current flows through the coil along the direction ABCD. Explain briefly how
the galvanometer measures the current. Your answer should include a diagram
showing the forces acting on the coil due to the current.
(ii)
Suggest TWO methods to increase the sensitivity of the galvanometer.
(iii)
Can the galvanometer measure 50 Hz alternating currents ? Explain briefly.
(10 marks)
(1995-CE-PHY I - 5)
5.
Figure 10
Two long wires AB and CD of total resistance 4  are used to connect a d.c. power supply to a
lamp. The lamp is working at its rated value 'l2 V, 24 W'.
(a)
Find
(i)
the resistance of the lamp,
(2 marks)
(ii)
the current flowing through the lamp,
(2 marks)
(iii)
the power loss in the wires,
(2 marks)
(iv)
the efficiency of the circuit supplying power to the lamp.
(2 marks)
(b)
To reduce the power loss in the wires, an a.c. power supply and two transformers are
used as shown in Figure 11.
Figure 11
(i)
In Figure 12, draw wires to connect the terminals of the components according to
Figure 11.
(3 marks)
*(ii)
Explain how the arrangement can reduce the power loss in the wires.
(4 marks)
If you attempt question 5, fill in the details in the first three boxes above and tie this sheet into
your answer book.
Figure 12
(1995-CE-PHY I - 6)
6.
Figure 13
Figure 13 shows a water heating system. The tank contains 15 kg of water. The heater is
switched on when the contacts are covered by water and the temperature of the water is below
45C.
(a)
The heater takes 5 minutes to raise the temperature of the water from 20C to 45C. The
specific heat capacity of water is 4200 J kg-1 K-1. Find
(i)
the energy used to heat up the water,
(2 marks)
(ii)
the output power of the heater.
(2 marks)
(b)
Figure 14 shows the circuit which controls the heating system.
Figure 14
(i)
Name the devices X and Y.
(2 marks)
(ii)
How would the resistance of X change with temperature ?
(1 mark)
*(iii) Explain why the heater is switched on when the contacts are covered by water and
the temperature of the water is below 45C.
(6 marks)
(iv)
Suggest one adjustment to a device in the circuit that would allow the water to be
heated to a temperature higher than 45C.
(2 marks)
(1996-CE-PHY I - 5)
5.
A student sets up a circuit as shown in Figure 9.
Figure 9
(a)
Name the devices C and D.
(2 marks)
(b)
Step
Switch A
Switch B
Device D
1
Closed
Closed
OFF
2
Closed
Open
3
Open
Closed
4
Open
Open
ON
The steps shown above are performed to investigate how the circuit works. When both
switches A and B are closed, device D does not emit light (OFF). When both switches are
open, D emits light (ON).
*(i)
Explain why device D is OFF when both switches are closed.
(5 marks)
(ii)
State whether D is ON or OFF in steps 2 and 3.
(2 marks)
(iii)
What kind of logic gate does the circuit represent if switches A and B are taken as
the inputs and device D as the output ? (Note : A closed switch denotes high input
and device D ON denotes high output.)
(1 mark)
(iv)
What is the function of the resistor R in the circuit ?
(1 mark)
(c)
In Figure 10, draw wires to connect the terminals of the devices according to the circuit
in Figure 9.
(4 marks)
If you attempt Question 5, fill in the details in the first three boxes above and tie this sheet into your
answer book.
Figure 10
(1996-CE-PHY I - 7)
7.
*(a)
Figure 12
A bar magnet is pushed with constant speed from left to right through a solenoid as
shown in Figure 12. Describe the change in the direction of the current passing through
the galvanometer during the motion of the magnet.
(4 marks)
(b)
Figure
13
Figure 14
Figure 13 shows the structure of a simple a.c. generator. An e.m.f. is induced when the
coil is set into rotation. The output of the generator is displayed on a CRO as shown in
Figure 14. The time base of the CRO is set at 20 ms cm-1 and Y-gain at 50 mV cm-1.
(i)
Which points (P , Q , R and S) in Figure 14 correspond to instants at which the
plane of the coil is parallel to the magnetic field ?
(2 marks)
(ii)
Find the peak voltage and frequency of the output of the generator.
(3 marks)
(iii)
Describe what happens to the peak voltage and frequency of the output of the
generator in each of the following cases :
(1)
Increasing the speed of rotation of the coil.
(2)
Winding the coil on a soft-iron core.
(4 marks)
(iv)
Steam is commonly used to drive generators in power stations. Suggest two other
practical means of driving generators.
(2 marks)
(1997-CE-PHY I - 5)
5.
Figure 6
A student sets up a circuit as shown in Figure 6 to control the operation of an air-conditioner.
(a)
Name the devices X, Y and Z.
How does the resistance of X change when its temperature is increased ?
(4 marks)
(b)
Construct a truth table for an AND gate.
(2 marks)
(c)
Assume that the input B is high when the temperature is higher than T0 and becomes low
when the temperature is lower than T0. State whether the air-conditioner is on or off in
each of the following cases :
Case
Switch S
Temperature
1
Closed
> T0
2
Closed
< T0
3
Open
>T0
4
Open
<T0
Air-conditioner
Table 1
(2 marks)
(d)
What is the function of the device Y in the circuit ?
(2 marks)
(e)
What is the advantage of using device Z to control the operation of the air-conditioner ?
(2 marks)
(f)
Figure 7
If a NOR gate is used to replace the AND gate in Figure 6, the student has to modify the
circuit so that the operation of the air-conditioner remains unchanged as in Table 1.
Figure 7 shows part of the modified circuit. Copy Figure 7 into your answer book and
complete the circuit using the devices
R, S, X and Y.
(3 marks)
(1997-CE-PHY I - 7)
7.
Two students suggest using a 24 V d.c. supply and a 24 V a.c. supply separately to operate a
lamp X of rating '6 V, 12 W'.
(a)
Figure 9
A student connects X in series with a 24 V d.c. supply and a resistor R (see Figure 9). If X
works at its rated value,
(i)
find the current flowing through X,
(ii)
find the voltage drop across R,
(iii)
find the resistance of R,
(iv)
what percentage of the electric power provided by the d.c. supply is dissipated in
R?
(8 marks)
(b)
The other student suggests that X can also be made to work by using a 24 V a.c. supply
together with a transformer.
(i)
Draw a circuit diagram to show how X, the a.c. supply and the transformer are
connected.
(2 marks)
(ii)
What is the advantage of using this method over the one shown in Figure 9?
(1 mark)
(iii)
Find the turns ratio (primary to secondary) of the transformer for X to work at its
rated value, and calculate the primary current if the transformer is 100% efficient.
(4 marks)
(1998-CE-PHY I - 4)
4.
Figure 5
Figure 5 shows a 3-pin plug and a kettle.
(a)
To which of the A, B and C of the plug should each of the wires X, Y and Z of the
kettle be connected ?
(2 marks)
(b)
(i)
Explain why it is safer to have pin A of the plug longer
than the other two pins.
(2 marks)
(ii)
Explain why switch S of the kettle is connected in wire X instead of wire
Y.
(2 marks)
(c)
The rating of the kettle is ‘220V, 2000 W’.
(i)
If the kettle is switched on for half an hour, calculate the cost of
electricity. (Given : One kilowatt-hour of electricity costs $0.9.)
(2 marks)
(ii)
Figure 6
(c)
(ii)
(continued)
A housewife plugs the kettle and an oven of rating
‘220 V, 2500 W’ into a 15 A socket. (See Figure 6.) Explain why this
connection is dangerous. Show your calculations.
(3 marks)
*(d)
A student makes the following note in his book :
In case either wire X or Y touches the metal case of the kettle accidentally, the kettle
will stop working.
Explain whether the student’s note is correct.
(4 marks)
(1998-CE-PHY I - 5)
5.
Figure 7 shows
a type of motor.
PQ and RS are
solenoids. The
solenoids
and
the coil ABCD
are
connected
in parallel to a
battery.
(a)
State
(i)
the polarity at end Q of the solenoid PQ,
(ii)
the direction of rotation of the coil as seen by the observer.
(2 marks)
(b)
Name the component E and explain its function.
(3 marks)
(c)
Suggest two ways of increasing the rotating speed of the coil.
(2 marks)
*(d)
A student says “If the battery in Figure 7 is replaced by a 50 Hz a,c, supply, the coil
will only oscillate to and fro. Hence the motor will not function properly.”
Explain why the student is incorrect.
(5 marks)
(e)
Describe, with the help of a diagram, how the motor in Figure 7 can be converted to a
direct current generator.
(3 marks)