## Thursday, August 2, 2012

### CBSE Class 7 Science - CH14 - Electric Current and its Effects

Electric Current and its Effects
 Why CFL is better than electric bulb?
NCERT Solutions and other Q & A

Q1: Draw in your notebook the symbols to represent the following components of electrical circuits: connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in the ‘ON’ position, and battery.

Q2: Draw the circuit diagram to represent the circuit shown in Fig. below.

Answer: The circuit diagram is showing switch in OFF position:

Q3: Fig.below shows four cells fixed on a board. Draw lines to indicate how you will connect their terminals with wires to make a battery of four cells.

Answer: To obtain maximum voltage, cells are connected in series i.e. negative(-) end of one cell is connected to positive (+) end of another cell and so on.

Q4: The bulb in the circuit shown in Fig.14.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.

Answer: In the circuit, cells are not connected properly i.e. (+ve) terminal is connected to +ve terminal of another cell. To make the connection closed, the negative terminal of first cell be connected to positive terminal of second cell, as shown below.

Q5: Name any two effects of electric current.

• Heating Effect of current: When an electric current passes through a wire, the wire gets heated. This is known as the heating effect of current. It is used in many applications like electric heater, electric ion, light bulb etc.
• Magnetic Effect of current: When an electric current flows through a wire, it produces magnetic effect around it. This effect is called magnetic effect of current. If the current carrying wire is wrapped around a piece of iron, the iron piece acts as an electromagnet. Magnetic Effect of electric current has many applications like power lift, electric bell, electric fan etc.
Q6: When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.

Answer: The current carrying wire produces magnetic effect around it. It causes deflection of magnetic needle. When the current is switched off, there is no magnetic field produced by the wire, magnetic needle does not deflect from its north-south position.

Q7: Will the compass needle show deflection when the switch in the circuit shown by Fig. (below) is closed?

Answer: No the compass needle will not show deflection even though the switch is closed, because there is no source of electric current. An electric cell or battery needs to be connected here. In the absence of electric current, there is no magnetic field produced by the wires. Hence there is no deflection.

Q8: Fill in the blanks:

(a) Longer line in the symbol for a cell represents its positive terminal.

(b) The combination of two or more cells is called a battery.

(c) When current is switched ‘on’ in a room heater, it becomes hot due to heating effect of electric current.

(d) The safety device based on the heating effect of electric current is called a fuse.

Q9: Mark ‘T’ if the statement is true and ‘F’ if it is false:

(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (F)✗

(b) When the electric current through the fuse exceeds a certain limit,  the fuse wire melts and breaks.(T)✓

(c) An electromagnet does not attract a piece of iron. (F)✗

(d) An electric bell has an electromagnet. (T)✓

Q10: Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain

Answer: An electromagnet acts like an ordinary magnet and it can attract iron pieces only. Plastic does not have any magnetic property. Electromagnet cannot be used to separate plastic bags from garbage heap.

Q11: An electrician is carrying out some repairs in your house. He wants to replace a fuse by a piece of wire. Would you agree? Give reasons for your response.

Answer: Fuses are devices that safeguard our electrical appliances, wiring. They are made up of special material which melt quickly and break when large electric currents are passed through them.
If the electrician does not use proper fuse wire, instead he uses any ordinary electric wire as fuse, it increases the risk of overheating of wires due to flow of excessive current. It may lead to short circuit in electric equipments and these appliances may catch fire. It is advised to use standard fuse wire or MCBs carrying ISI mark.

Q12: Zubeda made an electric circuit using a cell holder shown in Fig., a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda in identifying the  possible defects in the circuit.

Answer:  Following can be the possible reasons:
1. The bulb may be fused due to broken element.
2. Cells are not connected properly i.e. +ve terminal of first cell should connect to -ve terminal of the second one.
3. There may be loose connections. e.g. wire is not connected properly to switch or to the bulb.
4. The switch is not functioning well.
5. The cells are dried up. The power of the cell i.e. chemical inside the cell which produces electric current has been exhausted.

Q13:In the circuit shown below

(i) Would any of the bulb glow when the switch is in the ‘OFF’ position?
(ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?

(i) None of the bulb will glow when the witch is in the OFF position, since the electric circuit is not closed.
(ii) When the switch is moved to 'ON' position, circuit is complete and electric current will flow immediately. All of the bulbs will glow instantly.

Q14: Why CFL is considered better than filament based electric bulb?

Answer:  In comparison to electric bulb, CFL (Compact Fluorescent Lamp) produces very less amount of heat energy and thus reduces the energy wastage. That's why CFL are more environment friendly and are considered better alternative to electric bulbs.

Q15: What are MCBs?

Answer: MCBs or Miniature circuit breakers (MCBs) are electric safety devices being used in place of fuses. These are kind of switches which turns off (breaks electric circuit) automatically when excessive current flows due to short circuit.

Q16: Name the scientist who discovered magnetic effect of the current.

Answer: Hans Christian Oersted was the first person who noticed the deflection of compass needle every time, when the current was passed through the wire.

Q17: What are the reasons for excessive current flowing through a circuit?
• Short circuit: Due to faulty insulation or overheating, live wire comes in contact with neutral wire.
• Overloading: Many electric devices are connected beyond the capacity of electric circuit.

Q18: Name the material used as heating element in an electric iron.

Q19: Name the element used as filament in electric bulb.

Q20: What is a circuit diagram?

Answer: The representation of electric circuit by using electric symbols instead of electric components images is called circuit diagram.

Q21: What are the factors of conducting wire on which amount of heat produced depends?

Answer: Length, thickness and nature of material.
1. With increase in length of wire, increases the amount of heat produced.
2. With increase in thickness of wire, less amount of heat is produced.
3. Good conductors produce less amount of heat while bad conductors (e.g. Nichrome) produce more heat
Q22: What is the SI unit of electric current?

Q23: Name the instrument used to measure electric current.

Q24: How does electric cell produce current?

Answer: An electric cell stores some chemicals and their chemical energy gets converted to electrical energy.

Q25(MCQ): The unit to measure electric resistance is:

(a) Joule
(b) Ohm
(c) Ampere
(d) Volt

Q26(MCQ): The device used to make or break an electric circuit is :

(a) Resistor
(b) Battery
(c) Switch
(d) Ammeter

Q27: What is vampire power?

Answer: Many times the appliances like TV, air-conditioners or computers are switched off using remote sets instead of mechanical switches. Instruments like mobiles can be kept with plug connected even after complete charging. In these cases, appliances use the power and there is a waste of electric energy. That is known as vampire power or standby power.

1. They are very useful to me ,sir.
THANK YOU !

2. Awsomely Usefull!
THANKS...

3. i wonder to study it

4. really informative
good job

5. ihave having my exam tomorrow it will help me thank u

1. Have changes am

2. Have changes am

6. sorry, i am not getting my answer
which is,
An electrician wants to replace a fuse by a piece of nichrome wire. Is it safe? Why or why not?

1. it is not safe

2. it is not safe

3. not at all safe

4. it was good enough

5. it was good enough

6. Fuses are devices that safeguard our electrical appliances, wiring. They are made up of special material which melt quickly and break when large electric currents are passed through them.
If the electrician does not use proper fuse wire, instead he uses any ordinary electric wire as fuse, it increases the risk of overheating of wires due to flow of excessive current. It may lead to short circuit in electric equipments and these appliances may catch fire. It is advised to use standard fuse wire or MCBs carrying ISI mark.

7. The fuse wire is made up of lead and tin , i think so any 1 knows wat it is made up of ?

7. i like it ! excellent information

10. it was extremely useful...

11. it was extremely useful...

12. Very useful. Kids can study the entire chapter in one place

13. Very useful. Kids can study the entire chapter in one place

15. My teacher is like her concept is fully from textbook .. She makes 10questions from each line.. These questions are very useful for me .. Thanks A Lot..

16. water day is evrey year celebrated on 22 march

17. water day is evrey year celebrated on 22 march

18. Very nice questions

19. Very nice questions

20. it was very useful informative and nice. thank u

21. really knowledgeable very much thanks to them ...

22. nice but boring

23. nice but boring

1. Its good but I need more hard questions

2. Also by hard questions I mean like a whole page of them

24. Exellent work. It helps me very much. Because of this , i got 100 out of 100 in final exams. Thank u for helping me

1. R u serious ðŸ¤”ðŸ¤”ðŸ˜‘ðŸ˜‘

25. This comment has been removed by the author.

27. how does an electric bell works?
it would be better if i get answer till 27-2-2018

28. This comment has been removed by the author.

29. The most widely used form is the school bell, which produces a continuous sound when current is applied. The bell or gong (B), which is often in the shape of a cup or half-sphere, is struck by a spring-loaded arm (A) with a metal ball on the end called a hammer, actuated by an electromagnet (E). In its rest position the hammer is held away from the bell a short distance by its springy arm. When an electric current is enabled to pass through the winding of the electromagnet (via a closing of the switch (K) i.e. pressing the door bell) it creates a magnetic field that attracts the iron arm of the hammer, pulling it over to give the bell a tap. This opens a pair of electrical contacts (T) attached to the hammer arm, interrupting the current to the electromagnet. The magnetic field of the electromagnet collapses, and the hammer springs away from the bell. This closes the contacts again, allowing the current to flow to the electromagnet again, so the magnet pulls the hammerer over to strike the bell again. This cycle repeats rapidly, many times per second, resulting in a continuous ringing.

30. Construction and Working of an Electric Bell

Circuit

 An electric bell consists of an iron core, on which is wound a wire as a coil.
 One end of the coil is connected to one terminal of a battery, and the other end to a steel rod that acts like a spring for the hammer touching the screw contact.
 The other terminal of the battery is connected to the screw contact with a switch in the middle.

Working

 Electric current flows through the coil when the switch is ON, and the iron core acts as an electromagnet.
 The iron core attracts the hammer towards it.
 The hammer hits the bell and produces a sound.
 The circuit breaks at the screw contact when the hammer moves towards the iron core. At this point, the iron core ceases to be an electromagnet.
 The hammer is pulled back to its original position due to the spring action of the steel rod, and then touches the contact again to complete the circuit.
 The circuit is completed and current flows through the coil again, and the hammer strikes the bell again.
 The process repeats itself and you hear a ringing sound since the hammer keeps hitting the bell, until the switch is released.