digital circuits important and old question papers for gate
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Name _________________________ _____________________________________ ____________ 1.
CIRCUITS REVISION
Jenny made some electrical circuits using identical lamps and batteries.
(a)
A
B
C
D
Which circuit has only two lamps in series? .................................................................................................................................... (1)
(b)
Which circuit has the smallest current? .................................................................................................................................... (1)
(c)
Which circuit has the brightest lamps? .................................................................................................................................... (1)
(d)
In circuit B, Jenny removed one lamp from its holder. Suggest what happened to the other two lamps. .................................................................................................................................... Give a reason for your answer. .................................................................................................................................... (1)
(e)
The resistance of each lamp in circuit A is 4 ohms.
Using
R = R 1 + R 2
calculate the total resistance of the lamps in circuit A. ………………………………………………………………….…………… ohms.
(1) (Total 5 marks)
2.
In a motor vehicle, a relay is used to switch the 12 V, 60 W headlights on and off. The diagram shows the circuit. pivot armature coil iron core
The resistance of the relay coil is 150 . Calculate the current in the relay coil when it is connected to a 12 V supply. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. (Total 3 marks)
3.
The diagram shows a way of connecting 40 Christmas tree light bulbs to the mains electricity supply.
40 light bulbs
series circuit
(a)
(i)
When in use, the voltage across each light bulb is 6 V. What must be the voltage of the mains supply? ............................................................................................................................. (1)
(ii)
The resistance of the filament in each light bulb is 10 ohms. Calculate the current in a filament when it has a voltage of 6 V across it. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. (3)
(iii)
What is the current in the circuit when the 40 light bulbs are lit? ............................................................................................................................. (1)
(b)
(i)
Describe the energy changes that take place in the light bulbs when connected to the mains supply. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. (2)
(ii)
The Christmas tree lights are left on for 12 hours a day for the 12 days of Christmas. Calculate the total energy used by the Christmas tree lights during this period if they consume a total power of 144 W.
Calculate the cost of having the Christmas tree lights on for this period if the cost of 3.6 MJ of energy is 9 p. ............................................................................................................................. ............................................................................................................................. (2) (Total 12 marks)
4.
The diagram shows the circuit used by the torch.
(a)
How many cells are fitted to the torch? .................................................................................................................................... (1)
(b)
Suggest why the cells are connected in series rather than in parallel. .................................................................................................................................... (1)
(c)
A voltmeter is used to check the voltage across the battery. Draw the symbol for a voltmeter in the correct position on the diagram. (2)
(d)
If one cell was removed from the battery what would happen to the brightness of the torch bulb? .................................................................................................................................... (1) (Total 5 marks)
5.
(a)
The diagram shows a simplified model of the inside of a metallic electrical conductor.
The end A of the conductor is connected to the positive terminal of a battery and the end B to the negative terminal. (i)
What name is given to the negatively charged carriers in a metallic conductor? ............................................................................................................................ (1)
(ii)
Draw an arrow on the diagram to show the direction in which the negatively charged carriers move when the metallic conductor is connected to the battery. Explain why they move in the direction you have shown. ............................................................................................................................ ............................................................................................................................ (2)
(iii)
What name is given to the flow of the negatively charged carriers through the conductor? ............................................................................................................................ (1)
(b)
In an experiment using a conductor, the current in it is 0.5 A when a 12 V supply is connected across it. (i)
The supply is connected for 5 minutes. Calculate the number of coulombs of charge that pass through the conductor in this time. ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ (3)
(ii)
How many joules of energy are given to each coulomb passing through the conductor? ............................................................................................................................ ............................................................................................................................ (1)
(iii)
Calculate the total energy transferred in the conductor during the time the supply is connected. ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ......................................................................................................................... J (2) (Total 10 marks)
6.
A technician wishes to find the resistance of a lamp for an overhead projector. He measures the current and voltage in an electrical circuit which includes the lamp. The electrical symbol for the lamp is
(a)
Draw a circuit diagram to show how he finds the resistance of the lamp. Label any devices used in your circuit.
(2)
(b)
The lamp in an overhead projector is protected by a thermistor.
The graph shows how the resistance of the thermistor changes with temperature.
2000 Resistance in ohms 1000
0 0
20
40
60
80
Temperature in ºC
The thermistor and the lamp are connected in series. (i)
Describe how the thermistor protects the lamp when the projector is first switched on. ....................................…….................................................................………. ....................................…….................................................................………. ....................................…….................................................................………. ....................................…….................................................................……….
(2)
(ii)
What happens as the projector lamp warms up? .......................... ..........…….................................................................………. ....................................…….................................................................………. ....................................…….................................................................………. ....................................…….................................................................……….
(2)
(iii)
What is the resistance of the thermistor at 20 °C?
The resistance of the lamp is 300 In a series circuit:
at
20 °C.
Total Resistance R = R1 + R2 Calculate the current in the circuit at 20 °C when the projector is operated at 240 volts. ....................................…….................................................................………. ....................................…….................................................................………. ....................................…….................................................................………. ....................................…….................................................................………. ....................................…….................................................................………. ....................................…….................................................................……….
(3) (Total 10 marks)
7.
Sally made an electrical circuit. It contained two lamps, a variable resistor and a battery joined in series. (a)
(i)
Draw the circuit diagram.
(3)
(ii)
Use a word from the box to complete the following sentence. kinetic
potential
thermal
In the circuit some electrical energy is changed to ............................ energy. (1)
(b)
An electric fan can blow out hot or cold air.
The circuit diagram for the heater is shown below, A
B
live 230 V
neutral
heater fan
Suggest what happens when: (i)
Switch A is closed and switch B is open ........................................................................................................................... (1)
(ii)
Switch A is open and switch B is closed ........................................................................................................................... (1)
(iii)
Switch A and switch B are both closed ........................................................................................................................... (1) (Total 7 marks)
8.
Kate makes a simple alarm circuit using a light dependent resistor (LDR) and a buzzer.
6V
(a)
(i)
Explain why the buzzer sounds when light shines on the LDR. ......................................................................................................................... .........................................................................................................................
In bright light, the total resistance of the buzzer and the LDR is 400 ohms. What is the current in the circuit? ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... (2)
(b)
Kate makes another circuit which includes a variable resistor.
A
V
(I)
(i)
Describe how the brightness of the lamp changes as the resistance of the variable resistor increases. ......................................................................................................................... ......................................................................................................................... (1)
(ii)
She draws a graph for different values of the voltage and the corresponding current. voltage in V
current in A
Explain the shape of the graph. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... (2) (Total 7 marks)
9.
(a)
An aircraft in flight becomes charged. The aircraft tyres are made of rubber that conducts electricity. Suggest what happens to the charge on the aircraft when it lands. .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... (2)
(b)
– 4
The charge on an aircraft is 2.0 × 10 C. It passes through the tyres in 0.5 s. Calculate the current in the tyres when the charge is passing through them. .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... (3) (Total 5 marks)
10.
The graph shows how the current in a thermistor depends on its temperature.
0.10 current in A 0.08
0.06
0.04
0.02
0
20
30
40
50
60
70
temperature in °C
(a)
What is the current in the thermistor at a temperature of 37 °C? .............................. (1)
(b)
The thermistor is connected to a 2.5 V d.c. supply. Calculate the resistance of the thermistor at 37 °C. .....................................................................................................................………….
Use the graph to explain how the resistance of the thermistor changes when its temperature increases. .....................................................................................................................………… .....................................................................................................................…………
A thermistor is used to keep the temperature of an incubator at 37 °C. If a large current passes in the thermistor, it switches the incubator heater off at a temperature below 37 °C. Suggest how this happens. .....................................................................................................................…………. .....................................................................................................................…………. .....................................................................................................................………….
(1) (Total 8 marks)
11.
(a)
The diagram shows a diode connected in series with a resistor and an a.c. supply.
(i)
The graph on the left shows how voltage across the diode changes with time.
voltage
current
0
0 time
time
Draw a graph that shows how the current in the diode changes with time. Use the grid on the right. (2)
(ii)
The connections to the diode are now reversed. Draw a graph that shows how the current in the diode changes with time. Use the grid below.
