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Page No 198:

Question 1:

Which statement about electromagnets is incorrect?
(a) They have a core of steel.
(b) They remain magnets when electricity is switched off.
(c) They can function only when electricity is switched on.
(d) They can be used to lift heavy iron loads.

Answer:

Statements (a) and (b) are incorrect.

The core of electromagnets must be of soft iron.
Electromagnets behave like magnets only when current passes through it.

Page No 198:

Question 2:

Which option has the correct answers  for the following match the columns?
 

Column I Column II
A. voltage of a single dry cell i. complete path for the flow of electricy
B. battery ii. 6 V
C. cell iii. conductor
D. circuit iv. generates electricity
E. four dry cells in series v. a collection of cells
F. conducts electricity vi. 1.5 V

(a) A-iv; B-ii; C-v; D-i; E-vi; F-iii
(b) A-ii; B-iv; C-iii; D-i; E-v; F-vi
(c) A-vi; B-v; C-iv; D-i; E-ii; F-iii
(d) A-iv; B-iii; C-vi; D-i; E-v; F-ii

Answer:

(c) A-vi; B-v; C-iv; D-i; E-ii; F-iii

 

Column I Column II
A. voltage of a single dry cell  vi. 1.5 V
B. battery  v. a collection of cells
C. cell  iv. generates electricity
D. circuit  i. complete path for the flow of electricity
E. four dry cells in series  ii. 6 V
F. conducts electricity  iii. conductor

Page No 198:

Question 3:

When the switch was closed in the circuit shown here, the bulbs did not light up.
May be one of the bulbs is fused. How can it be determined which bulb is fused?
figure

(a) Close the switch and notice which bulb does not light up.
(b) Close the switch after removing one bulb at a time until the others light up.
(c) Close the switch and replace the three bulbs with a new one.
(d) There is no way to find out.

Answer:

(b) Close the switch after removing one bulb at a time until the others light up.
In this way, we can identify the bulb that is fused.

Page No 198:

Question 4:

Which switch must be open in this circuit to ensure that no bulbs light up?
figure

Answer:

Switch A should be open to ensure that no current passes through the bulbs. In the absence of flow of current, no bulbs will light up. If current passes through it, bulbs will light up.

Page No 198:

Question 5:

Study the circuit given below.
figure

If a second cell is connected in series with the first cell, what would happen to bulbs 1 and 2?
(a) They will continue to glow as before.
(b) Bulb 1 will glow brighter as it is closer to the cell.
(c) Bulb 2 will become dimmer.
(d) Both will glow brighter than before.

Answer:

(d) Both will glow brighter than before.
If a second cell is connected in series with the first cell, both bulbs will glow more brightly due to greater voltage generated by the cells.



Page No 199:

Question 6:

If in the above circuit, a third bulb is connected next to bulb 2, what would happen to bulbs 1 and 2?
(a) their brightness would decrease.
(b) Their brightness would double.
(c) They will not glow.
(d) They will continue to glow as before

Answer:

(a) Their brightness would decrease.
If a third bulb is connected, the same voltage will now be shared by three bulbs. Therefore, the brightness of the bulbs would decrease.

Page No 199:

Question 7:

Dhruv makes a circuit with four bulbs. When he completes the circuit all four bulbs glow. later bulb 1 fuses. But the other three continue to glow. Which one of the following is the circuit made by Dhruv?
figure

Answer:

The correct option is b.
Here, bulb 1 is connected in parallel with other bulbs. Therefore, if bulb 1 gets fused, the other three bulbs will continue to glow.

Page No 199:

Question 8:

In the circuit below, bulbs X and Y are glowing.
figure

A third bulb is added to the circuit as shown here. With switch P closed and switch Q open, which bulbs will glow?
 figure
 

  Bulb X Bulb Y Bulb Z
a. glow glow not glow
b. glow not glow glow
c. not glow glow glow
d glow glow glow

Answer:

Option (a) is correct.
Bulbs X and Y will glow but bulb Z will not glow if switch Q is open. This is because no current passes through bulb Z in this case.



Page No 200:

Question 9:

Which of the following statements are true about the circuit drawn here?
figure
A. Bulbs, X, Y and Z are connected in parallel.
B. If bulb X fuses, bulbs Y and Z will not glow.
C. Bulbs X, Y and Z are connected in series.
D. The current has only one path to flow through this circuit.

(a) All are correct.
(b) A, B and D are correct.
(c) B, C and D are correct.
(d) A and B are correct.

Answer:

(c) B, C and D are correct.
All the bulbs are connected in series; therefore, if one bulb gets fused, other bulbs will also not glow. Also, a series circuit has only one path for the electricity to follow.

Page No 200:

Question 10:

Convert these drawings of circuits into electrical circuit diagrams with proper symbols.
figure

Answer:

Page No 200:

Question 11:

Draw a circuit diagram showing a collection of six cells connected with three bulbs which in turn are connected in a parallel circuit with each other. Each bulb has its own switch. Show the arrangement in such a way that one bulb glows and the other two do not.

Answer:


In the above figure, bulbs 1, 2 and 3 are connected in parallel and bulb 1 is glowing.

Page No 200:

Question 12:

Prakash made a model of a house lighting circuit.
(a) Which bulb will always be on?
(b) Which bulb will never glow?
(c) Can we have only bulb B glowing in this circuit?
figure

Answer:

(a) Bulb A is always 'ON' if the circuit is closed.

(b) Bulb C will never glow because the circuit is not complete in this case.

(c) No. Since, the switch for bulb A and B is common; when we close the switch, both bulbs A and B will start glowing.



Page No 201:

Question 13:

Ron and his father arranged a string of lights on their Christmas tree and switched them on. But they did not light up. His father checked all the connections at the plug. There did not seem to be any problem. He then started checking the bulbs one by one. What do you think may have happened?

Answer:

This problem may occur if any of the bulbs in the string of lights is fused. Here, all the bulbs are connected in series. If one bulb gets fused, the circuit will break and all bulbs will be turned off.

Page No 201:

Question 14:

(a) With the help of a circuit diagram, explain how you could make an iron nail into an electromagnet.
(b) State two ways in which an electromagnet can be made more powerful.

Answer:

(a) To make an electromagnet, an iron nail is placed inside a solenoid. A solenoid is a long coil containing close turns of insulated copper wire.When current is passed through it, magnetic field is produced and iron nail is magnetised by induction. This combination of a solenoid and iron core is called electromagnet.



(b) Two ways to make an electromagnet more powerful:

  • By increasing the number of turns in the coil
  • By increasing the current flowing through the coil

Page No 201:

Question 15:

Look carefully at these two circuit diagrams and answer the questions that follow.
figure

(a) How many cells are there in each diagram?
(b) Which of the circuits, i and ii, is a parallel circuit?
(c) In circuit i, if bulb A fuses, what will happen?
(d) In circuit ii, how could you get only bulb D to light?

Answer:

(a) There are four cells in each diagram.

(b) Circuit (ii) is a parallel circuit.

(c) The bulbs in circuit (i) are connected in series; therefore, if bulb A fuses, both the bulbs will not light up.

(d) In circuit (ii), if bulb D is switched ON, only bulb D will light up.

Page No 201:

Question 16:

Look at the circuit diagram and answer the questions.
figure

(a) What happens when B is closed and A is open?
(b) What happens when B is open and A is closed?
(c) Which bulbs are connected in series?
(d) Which bulbs are connected in parallel?

Answer:

(a) When switch B is closed and switch A is open, no bulb will glow because the circuit is not complete in this case.

(b) When switch B is open and switch A is closed, bulbs 1, 2 and 3 will glow.

(c) Bulbs 2 and 3 are connected in series.

(d) Bulbs 1 and 4 are connected in parallel.

Page No 201:

Question 17:

Look at the circuit diagram and answer the questions.
figure

(a) What happens when B is closed and A is open?
(b) What happens when B is open and A is closed?

Answer:

(a) When B is closed and A is open, only bulb 4 will glow.

(b) When B is open and A is closed, bulbs 1, 2 and 3 will glow.



Page No 202:

Question 18:

Give reasons to explain the following statements.
(a) Fuses are used in the electrical circuits at home and in some electrical appliances.
(b) In your house, if one bulb is fused, the rest of the bulbs continue to glow.
(c) In your home, all the lights and fans go off if the main switch is turned off.
(d) A torch with four cells is brighter than one with only two cells.

Answer:

(a) A fuse is a safety device with low melting point. It melts and breaks the circuit if current exceeds a safe value. Thus, fuses are used in the electrical circuits at homes and in some electrical appliances.

(b) In our house, all the bulbs are connected in parallel. If one of the bulbs gets fused, other bulbs remain unaffected and they keep on glowing.

(c) A circuit needs to be closed for the flow of current. When the main switch is turned off there will be no power supply in the house as switching off the main supply involves breaking the circuit. So, all the electrical appliances stop working.

(d) Four bulbs will give more voltage than two cells. So, a torch with four cells is brighter than one with only two cells.

Page No 202:

Question 19:

Explain the working of a simple electric bell with the help of a circuit diagram.

Answer:

Electric bell is an electronic device with mechanical parts that move when current is passed through it. The bell functions through an electromagnet. It produces repetitive buzzing or clanging sound. It consists of a hammer, a metal strip, a gong and a switch. 


It is made up of a coil of wire wound on an iron piece. The coil acts as an electromagnet when electric current is passed through it. It works on the principle of electromagnetism.

The hammer of the bell is attracted towards the electromagnet when the circuit is complete. Further, the circuit breaks after the hammer moves towards the electromagnet. When the circuit breaks, the electromagnet does not attract the hammer and the hammer returns to its original position because of spring effect. As the hammer goes back to its original position, it touches the metal connector and the circuit is complete again. This cycle continues till the time the switch is kept ON.

Page No 202:

Question 20:

A student took a test tube and wound 100 turns of copper wire around it. Next he filled water in the tube and made an iron nail float on it. he then connected the ends of the copper wire to a cell. the pictures here show his experiment. Can you say why the nail is at different positions in the two pictures shown here?
figure

Answer:

The second position of the iron nail in the picture is different from its first position because of the magnetic effect of electric current. When current is passed through the copper coil, a magnetic field is produced and the iron nail gets magnetised and sinks in water.



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