Selina Concise Class 7 Physics Solution Chapter No. 7- ‘Electricity and Magnetism’ For ICSE Board Students.
Selina Concise Physics Solutions for Class 7th Chapter 7 Electricity and Magnetism with easy language, expert answers and doubt clearing sections. The book is especially designed keeping in mind the syllabus of ICSE board and the difficulty level faced by the students of ICSE boards class 7.
Selina Class 7 Physics Chapter 7 Solution:
1.) Write true or false for each statement :
a) A current carrying coil when suspended freely can rest in any direction.
Answer: False,
A current carrying conductor act as a bar magnet and it always stable in North-South direction only.
b) A coil carrying current behaves like a magnet.
Answer: True.
The concept of magnetism is associated with electric current.
c) In an electromagnet. the core is made up of
Answer: False.
In an electromagnet. the core is made up of soft iron.
d) An electric bell uses an electromagnet.
Answer: True.
Electrical bell is based on the principle of electromagnet.
e) An electromagnet with soft iron core is temporary magnet.
Answer: True.
Electromagnet works when the current is passing through it. If the current is stopped it does not works as an electromagnet.
f) We use cell as the source of electricity to run an electric immersion rod.
Answer: False.
The power provided by cell is so weak to run electric immersion rod.
g) A torch bulb glows if the terminals of the bulb are connected to the terminals of a cell by the metallic wire.
Answer: True.
The circuit is connected by metallicwires and we know that metals are good conductor of electricity.
h) Wool is a conductor of electricity.
Answer: False.
Wool is an insulator of electricity.
i) Silver is an insulator of electricity.
Answer: False.
Silver is a conductor of electricity.
j) Our body is a conductor of electricity.
Answer: True.
Our body conducts electric current.
k) For a circuit to be complete, every part of it must be made up of conductors.
Answer: True.
If the circuit is disconnected at any point then the current does not flow through it.
l) All metals are conductors of electricity.
Answer: True.
Metals has free electron in it’s outer orbit to flow electric current.
m) The switch should not be touched with wet hands.
Answer: True.
If we touch the switch then electricity may pass through our body and we get a shock.
n) A switch is an on-off device in an electric circuit.
Answer: True.
We can on-off circuit with the help of switch.
2.) Fill in the blanks :
Answer:
a) A magnet has two
b) Like poles repel each other and unlike poles attracts .
c) An electromagnet is used to separate large mass of iron
d) The strength of magnetic field of an electromagnet is increased by inserting a core of soft iron.
e) In a torch, we use dry cell as the source of electricity.
f) To light a table lamp and to run a refrigerator, we use mains as the source of electricity.
g) A group of two or more cells is called a battery.
h) Conductors pass electricity through them.
i) Insulators not pass electricity through them.
3.) Match the following:
Answer:
Column A Column B
(a) Human body (iii) conductor
(b) Silk thread (v) insulator
(c) Charge in motion (ii) current
(d) Soft iron (iv) electromagnet
(e) Electromagnet (i) electric bell
4.) Select the correct alternative :
a) A freely suspended magnet rests in :
(i) east-west direction
(ii) north-south direction
(iii) north-east direction
(iv) north-west direction.
Answer: (ii)north-south direction.
Magnet has directional property.
b) Electromagnets are made up of :
(i) steel
(ii) copper
(iii) brass
(iv) soft iron.
Answer: (iv) soft iron.
Soft iron is a magnetic substance therefore it increases the strength of electromagnet.
c) An electromagnet is used in :
(i) electric oven
(ii) ammeter
(iii) electric bell
(iv) radio set.
Answer: (iii) electric bell
Electric bell works on the principle of electromagnet.
d) The purpose of armature in an electric bell is :
(i) to make and break the circuit
(ii) to produce sound
(iii) to produce magnetic field
(iv) to provide spring action.
Answer: (i) to make and break the circuit
Armature coil is made up from magnetic material. So it attracts towards U- tube magnet or repel due to spring effect.
e) In a torch, the source of electricity is :
(i) the bulb
(ii) the switch
(iii) the cell
(iv) the mains.
Answer: (iii) the cell.
The source of electricity in battery is a cell. We can use multiple dry cells in battery.
f) Electricity can flow through:
(i) wood
(ii) rubber
(iii) plastic
(iv) copper wire.
Answer: (iv) copper wire.
The copper wire is an example of good conductor.
g) Electricity does not flow through:
(i) human body
(ii) animal’s body
(iii) rubber
(iv) silver
Answer: (iii) rubber
Rubber is a bad conductor of electricity.
h) We should not touch the switch with wet hands, otherwise :
(i) electricity may pass through our body
(ii) electricity may not pass through the appliance
(iii) circuit may break
(iv) the switch may get off.
Answer: (i) electricity may pass through our body.
Human body conducts electricity.
B.) Short/Long Answer Questions :
1) State two properties of a bar magnet.
Answer:
Properties of bar magnet-
- Monopole of magnet does not exists. We can not separate two poles of magnet.
- When bar magnet suspend in space freely by thread then it always stable in north-south direction.
2) How will you test whether a given is a magnet or not ?
Answer: Test for identify magnet.
Put another magnet near a given substance. The substance attracts or repel by the magnet. Then put other pole of magnet near that substance and see the change.
If the substance attracts or repel in both the cases then that substance will be magnet otherwise not.
3) How will you test whether a given rod is made of iron or not ?
Answer: Test for iron rod-
Put a magnet near the rod. If the rod attracts towards magnet then it will be iron rod otherwise not.
4) You are given two similar bars. One is a magnet and the other is of soft iron. How will you distinguish and identify them ?
Answer:
We know that magnet has directional property. It always stable at north-south direction. On the basis of this property we identify the substance is magnet or not.
- Hang both the bars by a string to the stand.
- Keep both bars directing towards east-west and release it.
- The one who moves and stable at north-south direction will be a magnet.
- The one who remains east-west direction will be iron bar.
5) You are given a magnet. How will you use it to find north-south direction at a place ?
Answer:
We know the directional property of magnet.
- Tia a magnet to a stand by tread.
- Rotate the magnet.
- Wait while it will retain at state of rest.
- Magnet always stable at north south direction. Therefore the north pole of magnet indicates north direction and south pole of magnet indicates south direction.
- In this way we find direction with the help of magnet.
6) Describe a simple experiment to illustrate that like poles of two magnets repel each other while unlike poles attract.
Answer:
Unlike poles of magnet attracts each other and like poles repel each other. We illustrate this by a simple experiment.
- Tia a bar magnet by string to the stand.
- Put north pole of another magnet directing toward the south pole ofa bar magnet which tied. Both the magnets attracts each other. From this we conclude that unlike poles attracts each other.
- Now put north pole of secondmagnet near the north pole magnet which is tied. Both magnets repel each other at this time. From this we conclude that like poles repel each other
7) “Poles exist in pairs”. Comment on this statement.
Answer: We can not separate two poles of magnet. If we break bar magnet into two parts then both poles arises into that two small magnets. The electron revolving around the nucleus produces magnetism in it. Therefore we can not separate two poles of magnet.
8) What is a magnetic compass ? State its use.
Answer:
Magnetic needle is a device which indicates the direction. Magnetic needle is based on the principle of directional property.
Magnetic needle can be used for identifying direction in ship.
9) Explain the meaning of the term, ‘magnetic field’.
Answer:
Magnet produces it’s effect near to it. In this area magnetic force exist. This area is called as magnetic field.
Magnetic Field- The area around the magnet in which magnetic effect exists is called magnetic field.
10) What is an electromagnet ?
Answer: A device which is made up from soft iron core wounding insulated copper wire around it is called electromagnet.
When a current passing through the conductor then it produces magnetic field around it. Current carrying conductor produces magnetic field around it.
Electromagnet is based on above principle.
11) Name the material of an electromagnet.
Answer:
The material used for making electromagnet is soft iron bar.
12) Draw a labelled diagram to make a soft iron bar as an electromagnet. Describe in steps the procedure.
Answer:
Electromagnet-Electromagnet is a type of magnet which is made up from soft iron core with the help of current passing through it.
Diagram for making electromagnet-
Construction and working-
- Wound insulated copper wire on the soft iron core as shown in figure.
- Connect a battery in series with this copper wire by key.
- The arrangement is as shown in figure above.
- Now on the key and allowcurrent to pass through the wire.
- The iron core shows a property ike magnet.
- Remember that, when current is passing through the wire, That time the bar magnet works as a magnet. When you switch off the button then it will retain as a iron bar.
In this way we can make electromagnet
13) You are given a U shaped soft iron piece, insulated copper wire and a battery. Draw a circuit diagram to make a horse shoe electromagnet.
Answer: When we pass a current through iron coil then it shows magnetic properties.
Following diagram illustrated that the soft iron core becomes magnet.
14) Name two factors on which the strength of magnetic field of an electromagnet depends.
Answer: The strength of magnetic field depends on-
- Strength of magnetic field depends on current flowing in the coil. We can increase the strength of field by increasing current in coil.
- Strength of magnetic field also depends on number of turns. If the number of turns increases then filed strength also increases.
15) State two ways by which the strength of magnetic field of an electromagnet can be increased.
Answer: We can increase the strength of magnetic field as-
- Insert a soft iron core in electromagnet.
- Strength of magnetic field also depends on current flowing in coil. We can increase the strength of field by increasing current in coil.
16) State two common uses of electromagnets.
Answer:
Uses of electromagnet-
- Electromagnet is used in DC motor.
- Electromagnet is also used in speakers.
- Electromagnet is used in mixer grinder.
17) Name a domestic device in which an electromagnet is used.
Answer: Electromagnet is used in electric bell which has domestic use.
18) Draw a neat and labelled diagram of an electric bell and describe its working.
Answer: Electric bell is based on the principle of electromagnet.
Electric bell is made up from horse shoe electromagnet, a soft iron, battery, switch, gong, screw, hammer, metallic spring etc
The circuit diagram of electromagnet is as shown below.
Working of electric bell-
- When we on the key then current will pass though the circuit and magnetism arises in coil.
- Armature coil attracts towards the electromagnet. Due this movement hammer strikes on a gong and bell ring.
- At same time the armature coil goes far from screw and circuit break. Therefore magnetism vanishes in electromagnet.
- The armature goes back towards screw because of spring effect.
- When the screw touch the armature , circuit becomes on and bell will ring.
- In this way electricbell works.
19) The incomplete diagram of an electric bell is given in fig. 7.39. Complete the diagram and label its different parts.
Answer:
The diagram in question is of electric bell.
The complete diagram of electric bell is as,
20) What is declination? Draw a diagram to show the angle between the declination and true direction of geographic north.
Answer:
We know that magnet has a directional property. It always stable towards north south direction. But the north pole of magnet does not coincide with earth’s north pole. This small horizontal angle is called magnetic declination.
The horizontal angle between earth’s north pole and north pole of magnet is called declination.
21) Define the term ‘current’.
Answer: The rate of flow of charge is called as electrical current. Electron are responsible for flow of current in solid. Electron and protons both are responsible for flow of current in liquid and gases.
22) Name four appliances which work electricity.
Answer:
The appliances works on electricity are-
- Electric fan
- Electric bulb
- Mixer
- Refrigerator
23) Name two sources of electricity.
Answer: Sources of electricity
- Thermal power plant.
- Hydro power plant.
- Dry cell
- Battery
24) What is a battery ?
Answer: Combination of two or more cells are called battery. Battery is a source of electrical energy in circuit.
25) What is an electric circuit ?
Answer:
The rate of flow of charges is called electric current.
Electric current has direction but it is a scalar quantity.
26) Describe an experiment to show that electricity flows only if the circuit is complete and it does not flow if the circuit is incomplete.
Answer:
We know that if the circuit is completed properly then the bulb will glow otherwise not.
To understand this we perform an experiment.
Bulb will glow.
- Connect a bulb in series with battery through key k.
- Make sure that all connection are proper.
- Now on the key.
- Bulb will glow.
Bulb does not glow.
- Connect one end of bulb to a battery and another oneto a key.
- Disconnect one end of key with battery.
- The connections are not proper. Therefore bulb will not glow.
27) You are provided with a torch bulb, a cell two plastic coated metal wires. Draw a diagram to show a complete circuit to light the bulb
Answer: The circuit diagram to glow a bulb is as,
28) In which of the following cases the bulb glow:
(i) Only one terminal of a cell is joined with a metal wire to one terminal of the bulb.
(ii) Both terminals of the bulb are joined with two metal wires to one terminal of the ceil
(iii) One terminal of the cell is joined to one terminal of the bulb and other terminal of the cell to the other terminal of the bulb.
Answer:
The case for the bulb glow is the terminal of the cell is joined to one terminal of the bulb and other terminal of the cell to the other terminal of the bulb.
29) Distinguish between conductors and insulators of electricity. Give two examples of each.
Answer:
Conductors-
- It allows flow of electric current through it.
- All metals are insulators.
- Free electron for conduction of current is easily available.
- Conduction band and valence band are overlap on each other.
- Examples- aluminum, iron, copper, gold, silver, etc.
Insulators-
- The substance which restrict the flow of charge through it is called insulator.
- All non metals are insulators.
- Free electron does not available for conduction.
- Forbidden gap between valence band and conduction band is greater than 5 ev.
- Examples- wood, glass, paper, pure water, rubber, plastic,
30) Select conductors and insulators from the following:
Glass, silver, copper, woodpaperpure water, impure water, aluminium, iron, leather, plastic, steel, human body and ebonite.
Answer: The substance which allows you flow electric current is called conductor. The substance which restrict the flow of electric current is called insulator.
The example of conductors are-
silver, copper, impure water, aluminum, iron, steel, steel, human body.
The examples of insulators are-
Glass, wood, paper, pure water, leather, plastic, ebonite rod.
31) The following diagram (fig. 7.40) shows four circuits A. B, C and D. Each circuit has a cell and a torch bulb. Name the circuits in which bulb will glow? Give reason to your answer.
Answer:
Current will flow in circuit A. Silk and cotton are bad conductors therefore they does not flow current in circuit.
In circuit ‘A’ copper and aluminum are used. Both are good conductor of electricity. Therefore the bulb will glow.
32) The diagram given below (fig. 7. 41) shows a bulb connected with a cell having terminals A and B. Mark the direction of current in the bulb.
Answer: Current always flow from terminal of high potential to terminal of low potential. The direction of flow of current in is as-
33) State the function of each of the following in an electric circuit and draw its symbolic representation : (i) Switch and (ii) Cell.
Answer:
Switch: The device which can make circuit on and off is called switch. The main function of switch in circuit is to make circuit on and off.
Cell- The device which provides energy to run a circuit is know as cell. The diagram of cell is as,
34) Draw a circuit diagram for a bulb connected to a cell with a switch. Mark arrows in the diagram to indicate the direction of flow of current.
Answer:
The diagram below indicates that a bulb connected to a cell with a switch.
35) In which arrangement are the appliances connected in the electric circuit of our homes, Series or Parallel? Give one reason for your answer.
Answer:
All the appliances are Connected in parallel in our homes. Voltage does no divides in parallel connection. Maximum equipment in our homes requires approximately 230 V. Therefore we have to connect appliances in parallel connection.
36) State two precautions that you must take when switching on an electric circuit.
Answer:
Human body can flows current through it. Therefore we have to take care while operating electrical devices.
Precautions that you must take when switching on an electric circuit.
- Conform that all wires are connected properly.
- Always wear safety kit while operating circuit.
- Do not touch circuit by wet hand.