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Page No 217:

Question A:

Solve the following crossword puzzle:
Figure

Across
3. This device is based on direct
5. This device is used to close or open an electric circuit

Down
1. This substance allows the flow of current
2. A cylindrical coil of wire
4. It is a piece of wire twisted to form a circle

Answer:

Page No 217:

Question B.1:

Who discovered the magnetic effect of current?
(a) Joule
(b) Maxwell
(c) Oersted
(d) Newton

Answer:

(c) Oersted

Oersted was the first to discover the magnetic effect of electric current.

Page No 217:

Question B.2:

Electric bulb converts electrical energy into
(a) sound energy
(b) mechanical energy
(c) light energy
(d) magnetic energy

Answer:

(c) Light energy

An electric bulb converts electrical energy into light energy.

Page No 217:

Question B.3:

A solenoid carrying current behaves like a/an
(a) bar magnet
(b) electric bell
(c) generator
(d) insulator

Answer:

(a) Bar magnet

A solenoid carrying an electric current behaves like a bar magnet.

Page No 217:

Question B.4:

The device used to close or open an electric circuit is
(a) switch
(b) resistance
(c) ammeter
(d) voltmeter

Answer:

(a) Switch

A switch is a device used to close or open an electric circuit.

Page No 217:

Question B.5:

The safety device used for electrical appliances is
(a) fuse
(b) resistance
(c) connecting wire
(d) none of these

Answer:

(a) Fuse

Fuse is a safety device used in electrical appliances to avoid a short circuit.

Page No 217:

Question B.6:

Which of the following appliances is not based on the principle of heating effect of current?
(a) electric bell
(b) electric kettle
(c) electric bulb
(d) electric press

Answer:

(a) Electric bell

The electric bell is not based on the principle of the heating effect of current.

Page No 217:

Question B.7:

A coil refers to
(a) a fuse
(b) an electric wire
(c) a wire twisted in the form of a circle
(d) none of these

Answer:

(c) A wire twisted in the form of a circle

A coil refers to a wire twisted in the form of a circle.

Page No 217:

Question B.8:

In an electric bell, which of these gets attracted to the electromagnet?
(a) the hammer
(b) the soft iron strip
(c) the screw
(d) none of these

Answer:

(b) The soft iron strip

In an electric bell, the soft iron strip that is mounted on a springy metal gets attracted to the electromagnet.



Page No 218:

Question C:

Fill in the blanks:
1. An electric circuit is a closed path in which ............................. flows.
2. A solenoid carrying current behaves like a ............................. .
3. The space around a magnet where its influence can be experienced is called the ............................. .
4. The combination of two or more cells is called a ............................. .
5. When current is switched 'on' in a room heater, it gets ............................. .
6. The safely device based on the heating effect of electric current is called a ............................. .

Answer:

1. An electric circuit is a closed path in which current flows.
2. A solenoid carrying current behaves like a bar magnet.
3. The space around a magnet where its influence can be experienced is called the magnetic field.
4. The combination of two or more cells is called a battery.
5. When current is switched 'on' in a room heater, it gets heated.
6. The safety device based on the heating effect of electric current is called an electric fuse.

Page No 218:

Question D:

Match the items in Column A with the items in Column B:

Column A Column B
1. Rubber (a) Magnetic field
2. Solenoid carrying current (b) Electric circuit
3. Heating effect of current (c) Insulator
4. Closed path in which current flows (d) Fuse
5. Space around a magnet (e) Bar magnet

Answer:

 Column A  Column B
1. Rubber (c) Insulator
2. Solenoid carrying current (e) Bar magnet
3. Heating effect of current (d) Fuse
4. Closed path in which current flows (b) Electric circuit
5. Space around a magnet (a) Magnetic field

Page No 218:

Question E:

Write True (T) or False (F) against the following statements in the given brackets:
1. Current flows through a circuit when it is open. ( )
2. Electric bell is based on the heating effect of current. ( )
3. An electric bell has an electromagnet. ( )
4. To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. ( )
5. When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. ( )
6. The magnetic strength in a conductor decreases with the increase in current. ( )

Answer:

1. False
   Current flows through a closed circuit.

2. False
   The electric bell is based on the magnetic effect of current.

3. True

4. False
   To make a battery with two cells, the negative terminal of one cell is connected to the positive terminal of the other.

5. True

6. False
   The magnetic strength in a conductor increases with the increase in current.

Page No 218:

Question F:

Match the terms in Column I to the symbols in Column II and match further to the correct explanation in Column III:

Column I Column II Column III
1. Bulb (a) Figure (i) Has a coiled tungsten filament.
2. Switch (b) Figure (ii) Series of cells connected with correct polarity.
3. Fuse (c) Figure (iii) Source of electric current.
4. Cell (d) Figure (iv) Used to close or open an electric circuit.
5. Battery (e) Figure (v) Has a wire with how melting point.

Answer:

     Column I Column II Column III
1. Bulb (d) (i)  Has a coiled tungsten filament
2. Switch (a) (iv) Used to close or open an electric circuit
3. Fuse (e) (v)  Has a wire with a low melting point
4. Cell (b) (iii)  Source of electric current
5. Battery (c) (ii) Series of cells connected with correct polarity



Page No 219:

Question 1:

Air is not filled in electric bulbs. Why? Justify your answer.

Answer:

When an electric current passes through a bulb, the tungsten filament gets heated up to such an extent that it reacts easily with the surrounding elements. For an electric bulb filled with air, the tungsten filament reacts with the oxygen present in the air, which is a supporter of combustion, resulting in the burning of the bulb. Hence, the bulbs are filled with inert gases that are non-reactive.

Page No 219:

Question 2:

Rahul was carrying out repairs in his house. He wishes to replace a fuse by a piece of wire. Would you agree? Give reason for your response.

Answer:

No, replacing the fuse with a piece of wire is not advisable. The fuse consists of a thin wire of an alloy that is designed to easily heat up and melt down whenever the current flowing through a circuit exceeds the safety limit. This causes discontinuity in the circuit and stops the flow of current, thereby avoiding danger. Using a wire instead of the fuse causes overheating of the wire. This can cause fire in the circuit, resulting in damage to life and property.

Page No 219:

Question 3:

What kind of material can you place in figure, so that the circuit is complete and the bulb will glow?
Figure

Answer:

Switch: a switch completes the circuit shown in the figure, causing the bulb to glow.

Page No 219:

Question 4:

In which of the figures will the bulb glow and why?
Figure

Answer:

The bulb glows if it is connected according to the arrangement in figure (c). The circuit diagram in figure (c) shows a closed circuit. Current from the cell flows through the circuit causing the bulb to glow.

Page No 219:

Question 5:

In which of the following circuit, will the bulb glows the brightest? Given that all the cells used supply equal currents.
Figure

Answer:

Figure (a) consists of a closed circuit with three cells connected in series to form a battery. In this type of circuit, the bulb glows the brightest.

Figure (b) consists of a closed circuit with one cell connected in series. In this type of circuit, the bulb's brightness is lower than the one in figure (a).

Figure (c) consists of an open circuit with two cells connected in series. In this type of circuit, the bulb does not glow because the current will not flow.

Page No 219:

Question 6:

Which one of the following is a stronger electromagnet? Why?
Figure

Answer:

The electromagnet in figure (b) is stronger. The strength of a magnetic field increases with an increase in the number of turns of the coil of a solenoid. The solenoid in figure (b) has more number of turns than the one in figure (a). Hence, the solenoid in figure (b) is a stronger electromagnet.



Page No 220:

Question 7:

Will the compass needle show deflection when the switch the circuit shown in figure, is closed?
Figure

Answer:

No, the compass needle will not show deflection because the circuit given in the figure does not contain any source of electric current. A solenoid itself cannot produce any magnetic field. When electric current passes through it, a magnetic field is created around the conductor, which can deflect the needle of a compass when placed near to it.

Page No 220:

Question 8:

In the devices like electric heater, electric geyser, electric bulb-the heating effect of current can be put to good use. Can you think of the situations where this effect results in wastage of energy?

Answer:

The heating effect of electric current results in the wastage of energy in an electric bulb. In electric bulbs, the electrical energy is converted into light energy. A part of the electrical energy is converted into heat energy, which cannot be utilised. This results in the wastage of energy.



Page No 221:

Question A.1:

What is short-circuiting?

Answer:

When current in a circuit follows an unintended path,  it is called short-circuiting. For example, in a torch circuit, if the two terminals of the bulb touch each other, the bulb will not glow. Also, if the two terminals of the battery touch each other, the torch won't work. This condition is called short-circuiting

Page No 221:

Question A.2:

What are electromagnets?

Answer:

An electromagnet is a magnet consisting of a soft iron core with a coil of insulated wire wound round it. When current flows through the wire, the core becomes magnetised; when the current ceases to flow, the core loses its magnetisation.

Page No 221:

Question A.3:

What is meant by a fuse?

Answer:

An electric fuse is a safety device that works on the principle of the heating effect of electric current. It breaks the circuit when there is an excess flow of current.

Page No 221:

Question A.4:

Why does an overloaded electric circuit catch fire?

Answer:

When the current flowing through a circuit exceeds the safety limit, the wires carrying the current get overheated and catch fire.

Page No 221:

Question A.5:

What is nichrome? To what use is it put?

Answer:

Nichrome is a conductor with a high melting point. It can be heated to a very high temperature. Because of this property, it is used in electric iron boxes.

Page No 221:

Question B.1:

How can the strength of a magnetic field be increased?

Answer:

The strength of the magnetic field can be increased:
(i) by increasing the number of turns of the coil in the solenoid
(ii) by increasing the strength of the electric current passing through the solenoid
(iii) by winding the solenoid around a magnetic material

Page No 221:

Question B.2:

What is the lighting effect of current?

Answer:

When electric current passes through objects such as the filament of an electric bulb, the filament glows and produces light. Here, electrical energy is converted into light energy. This is the lighting effect of electric current.



Page No 222:

Question B.3:

Why are CFLs preferred to electric bulbs?

Answer:

CFLs are preferred over electric bulbs because they minimise the wastage of electrical energy. When electric current passes through the bulb, it not only gives light, but also gives out heat, which is not utilised. This results in the wastage of electrical energy. This can be minimised using CFLs.

Page No 222:

Question B.4:

How do short circuits occur?

Answer:

An electric short-circuit occurs when the path between two points (two ends of a wire), which are at a certain potential difference, becomes low in resistance in order to allow the flow of electricity.

Page No 222:

Question B.5:

Distinguish between conductors and insulators.

Answer:

 Conductors  Insulators
 These are materials that allow heat and electricity to pass through them. These are materials that do not allow heat and electricity to pass through them.
Examples include copper, aluminium, etc. Examples include plastic, rubber, etc.

Page No 222:

Question B.6:

Why does the fuse wire have low melting point?

Answer:

A fuse wire has a low melting point. Whenever the current flowing through an electric circuit exceeds the safety limit, the fuse wire heats up and melts down, resulting in a break in the path of current flow. This stops the flow of current in the circuit.

Page No 222:

Question C.1:

What are the reasons of melting of a fuse wire?

Answer:

The fuse wire melts for the following reasons:
(i) Frequent fluctuations in the flow of electric current in a power supply system
(ii) Worn out insulation covers on the connecting wires that may cause a short-circuit
(iii) The circuit may get overloaded due to a large withdrawal of current

Page No 222:

Question C.2:

What are MCBs? Write the full form of it.

Answer:

MCBs are safety switches used for the protection of appliances and human beings. If the current through a circuit exceeds the safety limit, it automatically turns off and protects the devices connected to the circuit. This automatic turn-off is accomplished by over-current sensors and temperature sensor circuitry along with a relay mechanism. MCBs are a replacement for safety fuses, which were used earlier.

MCB stands for Miniature Circuit Breaker.

Page No 222:

Question C.3:

Distinguish between:
(a) Cell and battery
(b) Open and closed circuit

Answer:

(a) Cell and battery

 Cell  Battery
 A cell is a single unit that is used as a source of electricity.  A battery consists of two or more cells connected to each other and used as a source of electricity.

(b) Open and closed circuit
 Open Circuit  Closed Circuit
Open circuit is an incomplete or broken circuit in which the flow of current is interrupted. Closed circuit is a complete circuit in which the flow of current is continuous.

Page No 222:

Question C.4:

Briefly explain the construction and working of an electric bell with the help of a labelled diagram.

Answer:




An electric bell works with the help of an electromagnet. It is made up of a coil of wire wound round an iron piece, resembling an electromagnet.When current passes through it, a magnetic field is produced.

When the circuit is complete, the hammer of the bell is attracted towards the electromagnet. As the hammer moves towards the electromagnet, the circuit breaks. This stops the electromagnet from attracting the hammer anymore. The hammer comes back to its original position due to the spring effect. While doing so, it touches the metal connector and the circuit is complete again. This cycle continues till the key is kept ON.

Page No 222:

Question C.5:

List fives uses of electromagnets.

Answer:

The uses of electromagnets can be listed as follows:
(i) Electric motors mainly use electromagnets.
(ii) They are used in electric bells.
(iii) Electromagnets are used to lift the loads of iron and steel.
(iv) They are used in telephones, telegraphs, generators, etc.
(v) Electromagnets are also used in cranes.

Page No 222:

Question C.6:

What is meant by magnetic effect of current? With the help of a labelled diagram, demonstrate the magnetic effect of current. Briefly explain Oersted's experiment.

Answer:



When an electric current passes through a conductor, a magnetic field is created around it. This is the magnetic effect of current. This was first observed by Hans Christian Oersted during an experiment. He observed that the needles of a compass placed near a current-carrying conductor were deflected every time a current passed through the conductor. When the direction of the current was reversed, the needles of the compass were deflected in the opposite direction. By this experiment, Oersted concluded that whenever current passes through a conductor, a magnetic field is created around it, causing it to behave like a magnet. The diagram below shows the magnetic effect of electric current.

http://images.tutorvista.com/contentimages/science/CBSEXScience/Ch534/images/img16.jpeg



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