Frank Solutions for Chapter 8C Electricity and Magnetism Class 9 Physics ICSE
1. What are two kinds of charges?
Answer
Positive and negative are the two kind of charges.
2. What is the net charge on an atom?
Answer
Net charge on an atom is zero.
3. How many electrons are present in Na+? The atomic number of Na is 11.
Answer
10 is the number of electrons present in Na+.
4. Distinguish between an atom and an ion.
Answer
An atom does not have any net charge while ion is either a positive charge or a negative charged.
5. What causes electrification of two bodies when they are rubbed together?
Answer
The friction between two bodies when they are rubbed against each other is the reason of electrification.
6. What is the function of a lighting conductor?
Answer
A lightening conductor is a device used to protect the large buildings against lightening during a thunderstorm.
7. The lighting and the thunder are produced simultaneously, but the sound of the thunder is heard a few seconds after the lighting is seen. Why?
Answer
The speed of light is very fast as compared to the speed of sound in air. So, the thunder is seen fast and then we hear the sound of thunderstorm.
8. What is the purpose of an electroscope?
Answer
Electroscope is used to determine the nature of the charge and to detect the presence of charge on the body.
9. A positively charged rod is brought near the cap of a positively charged gold leaf electroscope. State your observation and give a reason.
Answer
When a positively charged rod is brought near the positively charged cap then , the positive charge on the cap will spread over because of repulsion and some of them will reach to the leaves . Now, the leaves will have the same positive charge, they will repel each other and hence, leaves will diverge.
10. What is direct current?
Answer
The flow of electrons in a particular direction is called direct current.
11. Write two differences between the primary and secondary cells.
Answer
The two differences between primary and secondary cell are :
- Primary cells cannot be charged again while secondary cells can be charged again and again.
- In primary cells, chemical reaction is irreversible while in secondary cells , chemical reaction is reversible.
12. What is the SI unit of current?
Answer
S.1. unit of current is ampere.
13. What is the difference between a cell and a battery?
Answer
A battery is the made up of multiple cells.
14. Is current a scalar or a vector quantity?
Answer
Current is a scalar quantity.
15. Name the physical quantity which defines the rate of flow of charge.
Answer
Current is defined as the rate of flow of charge.
16. Which is bigger: a milliampere or a microampere?
Answer
A milliampere is bigger than microampere.
17. What is the other name for a variable resistor?
Answer
Rheostat is other name of variable resistor.
18. Name two sources of current.
Answer
A cell and battery are the two sources of current.
19. Is voltmeter same as voltmeter?
Answer
No, voltmeter is not the same as voltmeter.
20. Name the instrument used to measure the magnitude of current flowing in the circuit.
Answer
Ammeter is used to measure the magnitude of current in the circuit.
21. What is responsible for the flow of current through a metallic conductor?
Answer
Electrons flow inside the metallic conductor and a battery and cell helps in the movement of electrons in it.
22. Which has a greater resistance: an ammeter or a voltmeter?
Answer
Voltmeter has high resistance than the ammeter.
23. What is an artificial magnet?
Answer
An artificial magnet is the magnetized piece of iron or steel.
24. How is a voltmeter connected in a circuit?
Answer
Voltmeter is always connected in parallel to the circuit.
25. How is an ammeter connected in a circuit?
Answer
Ammeter is always connected in series in the circuit.
26. Write the SI unit of resistance.
Answer
S.I. unit of resistance is ohm.
27. Define one ohm resistance.
Answer
The resistance of conductor is 1 ohm when a current of 1 ampere flows through it when the potential difference across it is 1 volt.
28. 0.8 C charge passes through a cross-section of a cross-section of a conductor in 5 s. Calculate the current.
Answer
I = Q/t = 0.8/5 = 0.16 Ampere.
29. Name the instrument used to control current in an electric circuit.
Answer
Rheostat is an instrument to control the current in the circuit.
30. What is a horse shoe magnet?
Answer
Horse-shoe magnet is the shape of the magnet. This shape helps in bringing the poles near to each other and make the strong magnet.
31. What do you mean by an induced magnetism?
Answer
The magnetism acquired by a magnetic material when it is kept near a magnet is called induced magnetism.
32. What are magnetic poles? Where are they situated?
Answer
The points of the magnet where attraction appears the maximum are called poles of magnet and they are situated at ends of the magnet.
33. Distinguish between the primary and secondary cells.
Answer
The differences between primary and secondary cell are :
- Primary cells cannot be charged again while secondary cells can be charged again and again.
- In primary cells, chemical reaction is irreversible while in secondary cells , chemical reaction is reversible.
34. What is a cell ? Name the important constituents of a cell.
Answer
A cell is a source of energy which converts the chemical energy into electrical energy. The cell consists of two electrodes in the form of conducting rod immersed in the solution called electrolyte.
35. Find the number of electrons that would flow per second through the cross-section of a wire when 1 A current flows in it.
Answer
I = Q/t = ne/t
So, n = I. t/e = 1.1/1.6 ×10-19 = 6.25× 1018
So, 6.25×1018 will be the number of electrons.
36. A current of 4.5 A flows through a conductor for 2.5 s. What amount of charge passes through the conductor?
Answer
Q = I.t = 4.5×2.5 = 11.25 C.
37. What is the function of a key in an electric current?
Answer
A key is used to put the current on and off in the circuit.
38. Define electrical resistance of a metal. Write its SI unit.
Answer
The effect to rate of flow of current by the wire is called its electrical resistance. Its S.I unit is ohm.
39. State the factors on which the resistance of a wire depends.
Answer
The resistance of the wire depends on the following factors :
- Length of the wire
- Cross-section of the wire
- Temperature of the wire.
- Material of the wire.
40. What is the effect of temperature on the resistance of a wire?
Answer
With increase in temperature of the wire, the resistance of wire increases.
41. A wire of resistance 1 Q is stretched to double its length. What will happen to the resistance of the stretched wire?
Answer
The resistance of wire will become 4 ohm.
42. What is a variable resistance? Explain its function using a labeled diagram.
Answer
Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available.
A variable resistance is a device used to change the resistance and hence the current flowing through a circuit; without changing the voltage.
43. How is a galvanometer different from an ammeter?
Answer
Galvanometer is used to detect the direction of current as well as to detect very weak current. Ammeter is used to measure the magnitude of an electric current in the circuit.
44. How is a voltmeter different from a voltmeter?
Answer
Voltmeter is used to measure the potential difference between two points in the circuit. Voltmeter is an instrument for measuring the voltaic electricity passing through it, by its effect in decomposing water or some other chemical compound acting as an electrolyte.
45. What is a natural magnet ? State three important properties of a magnet.
Answer
The magnets (pieces of lodestone) found in nature is called natural magnet.
The properties of magnet are:
- Attractive property
- Directive property
- like poles repel while unlike poles attract.
46. Show by diagrams two usual shapes of the artificial magnets.
Answer
The two usual shapes of magnet are bar magnet and horse shoe magnet. The diagrams for bar magnet and horse shoe magnet are as follows:
47. What do you understand by magnetic induction? What role does it play in the attraction of a piece of iron by a magnet?
Answer
The mechanism in which magnetism acquired by the magnetic material when kept near a magnet is called magnetic induction. When the magnetic material get magnetized, the pole developed on its side will be opposite to the magnet's pole taken near to it. So, opposite poles attract each other. So, induction causes attraction.
48. Explain the method of plotting the magnet field lines by using a small compass needle.
Answer
We will start drawing the magnetic field of lines from the north pole of magnet. When a small compass is placed at a point near the north pole of the magnet, the needle turns round on its pivot and settle in the fixed direction. At other points away from the pole the needle settles tangent to the drawn field line. By plotting the directions indicated by the needle, we can draw magnetic field of lines.
49. A small magnet is suspended by a silk thread from a rigid support such that the magnet can freely swing. How will it rest? Explain by drawing a diagram.
Answer
It will always rest in north-south direction. The diagram is as follows. The compass is telling that the magnet is pointing in north-south direction.
50. What do you mean by the term lines of magnetic field?
Answer
The magnetic field of lines are the lines which get formed by joining the poles of the magnet and they are the continuous curves in which tangent drawn to it will give the direction of magnetic field.
51. A magnet is carefully broken into two halves along its long axis as shown in fig. 1. How would the magnetic strength of each piece compare with that of the original magnet ?
By breaking the magnet into two halves, magnetic strength will always get half. So, the ratio of magnetic strength will be 1:2.
53. A conductor B is charged by bringing another charged body A in contact (Fig.3).
(ii) Explain the process of charging B.
Answer
(i) The charge gained by the B will be positive charge
(ii) The B will be charged by the charging by contact. The charge on A will spread on B.
54.What is an electroscope? Describe the different stages of charging a gold leaf electroscope negatively with the help of a diagram.
Answer
Electroscope is the device used to detect the presence of charge and the nature of charge on the body. The four stages of the charging a gold leaf electroscope by induction are :
- Bring a positively charged rod near the cap of an electroscope. The cap will acquire the negative charge on it by induction. The leaves will diverge
- Earth the electroscope by touching the cap will finger.
- Remove the finger and keep the positively charged rod as such.
- Now, take away the rod. The leaves will diverge because of charges induced by induction..
(i) Magnetization of Iron Bar by Single Touch Method: Place the iron piece on the table and stroke it with bar magnet from A to B with one end of the magnet. When the magnet reaches B it is lifted and brought back to A to repeat the stroke. After few strokes, we will notice that A has acquired the same polarity as the pole rubbed against it.
(ii) Magnetization of steel needle using electric current: Wind a length of copper wire around a steel needle and connect it to battery and switch. After the current is passed through the copper wire for some time, we will notice that steel needle got magnetized.
55. What is meant by electrostatic induction? You have been given two metal containers A and B and a positively charged body X. With the help of a diagram, explain the different stages of charging
(i) A positively, and
(ii) B negatively (Fig.4).
Electrostatic induction is the process of charging a body by bringing another charged body near to it. When the X will come near to body B, the opposite charge will be acquired by body B. So, negative charge will be acquired. Similarly, by electrostatic induction, A will acquire the opposite charge of B so A will acquire the positive charge.
56. (a) Write a short note on lighting conductors.
(b) Explain the following:
(i) Resistance of a conductor
(ii) Potential difference
(iii) Open circuit
(iv) Closed circuit
Answer
(a) Lightening conductor is a device used to safeguard the large buildings against lightening of thunderstorm. Normally, they are fitted on the top of tall buildings as a safety device. It consist of few metal spikes fitted to the highest part of building and connected to thick copper strip which is buried inside the earth .
(b)
- Resistance of the conductor: It is the property of the conductor by virtue of which it obstructs the flow of current in the circuit.
- Potential Difference: It is defined as the amount of work done in moving a charge from one point to another .
- Open Circuit: In the circuit, if the switch is off or the wire is cut then flow of current in circuit stops and we say that its an open circuit.
- Closed Circuit: The circuit in which current flows without any obstruction then it's a closed circuit.
57. Compare the brightness of two 60 watt bulbs
(i) When they are connected in series.
(ii) When they are connected in parallel.
Explain giving scientific reasons.
Answer
(i) When two bulbs will connect in series then resistance of the circuit will increase so the brightness of the bulbs will decrease.
(ii) When the two bulbs will be connected in parallel then resistance of circuit will decrease so the bulbs brightness will increase.
58. Differentiate between:
(i) Conductors and insulators.
(ii) Flow of electron and flow of conventional current.
(iii) Primary and secondary cell.
Answer
(i) Differences between Conductor and insulators:
- Conductors allow passing of electric current through it. Insulator does not allow passing of current through it.
- Silver, Gold are good examples of conductor of electricity. Rubber, Cotton are good examples of insulators.
(ii) Difference between Flow of electron and flow of conventional current:
Flow of electrons is the flow of negative charges inside the conductor while flow of conventional current is the flow of positive charges inside a conductor.
(iii) Difference between Primary and secondary cell:
Primary cells are the cells in which chemical reaction is irreversible so they can't be charged again and again. While, secondary cells are the cells in which chemical reaction is reversible and can be charged again and again.
59. (a) What are the properties of a magnet?
(b) State the laws of magnetism.
(c) Distinguish between
(i) Magnetic and non-magnetic materials.
(ii) Natural and artificial magnets.
(iii) Magnetic properties of iron and steel.
Answer
(a) Following are the properties of magnet
- Like poles of magnet repel each other while unlike poles attract.
- Magnet have an attractive property means it attract the magnetic material towards it.
- Magnet have the directive property means when it is held freely, it always point towards north-south direction.
(b) The Laws of Magnetism: Like poles repel each other and unlike poles attract each other.
(c) (i) Magnetic materials get attracted towards the magnet but non-magnetic materials does not get attracted towards the magnet.
(ii) Natural magnets are the pieces of lodestone which are found in nature while artificial magnet is the magnetized piece of iron or steel.
(iii) Iron has high susceptibility and low retentivity means it acquires the magnetism faster and removes it soon as well. While , steel has low susceptibility and high retentivity.
60. With the help of a diagram explain the following:
(i) Magnetisation of an iron bar by single touch method.
(ii) Magnetisation of steel niddle using electric current such that the pointed end should have north polarity.
Answer
(i) Magnetization of Iron Bar by Single Touch Method: Place the iron piece on the table and stroke it with bar magnet from A to B with one end of the magnet. When the magnet reaches B it is lifted and brought back to A to repeat the stroke. After few strokes, we will notice that A has acquired the same polarity as the pole rubbed against it.
(ii) Magnetization of steel needle using electric current: Wind a length of copper wire around a steel needle and connect it to battery and switch. After the current is passed through the copper wire for some time, we will notice that steel needle got magnetized.
61. (a) Explain, what is meant by magnetic induction.
(b) How would you test whether a piece of steel was magnetised
(i) If you had a magnet.
(ii) If there was no magnet available.
(c) Define the following terms:
(i) Magnetic meridian
(ii) Geographic meridian
(iii) Angle of declination
(iv) Angle of dip
Answer
(a) Magnetic induction is the process in which magnetism is acquired by the magnetic material when a magnet is brought near it.
(b) (i) If we had a magnet then we will bring the magnet near to steel , if there is force of repulsion or attraction is experienced then steel was magnetized otherwise not.
(ii) We will held the steel freely, if it points to north-south direction only then it is magnetized otherwise not.
(c) (i) Magnetic Meridian: It is the vertical plane containing the magnetic axis of a freely suspended magnet at rest under the earth's field.
(ii) Geographic Meridian: It is the vertical plane containing the geographic north and south poles of the earth and passing through the given place in the surface of earth.
(iii) Angle of declination: The angle between geographic and magnetic meridian is called angle of declination.
(iv) Angle of Dip: The angle between the horizontal and earth's magnetic field is known as angle of dip.
62. Give scientific reasons:
(i) A ‘cracking’ noise may be heard on removing a nylon shirt, blouse or stockings.
(ii) A charge can be obtained on a rubbed ebonite rod held in the hand but not on a rubbed metal rod held in the hand.
(iii) Soft iron is used to make electromagnets.
(iv) Two steel needles hanging from the lower end of a vertical bar magnet do not hang vertically (Fig. 5).
Answer
(i) When a nylon shirt , blouse or stocking s are removed then they get rubbed with the hairs and due to friction between hairs and nylon , charges get induced on them and attractive force get induced between them.
(ii) The rubbed ebonite rod contains more electrons so it acquires the negative charge so a charged is acquired by it. But, in the rubbed metal rod, electrons are transferred but through hands and body they pass to the earth so no charges will be there while metal rod is held in hand.
(iii) Soft iron is used to make electromagnets because it has high susceptibility and low retentivity, it get magnetized very faster and loses magnetism as soon as the inducing magnet is removed.
(iv) Two steel needles hanging from the lower end of the vertical bar magnet do not hang vertical but will diverge because they got the like poles on it and like poles repel each other so they diverge.
(v) If magnetic material is brought near to the magnet then it get attracted although its not magnetized but the magnetic material will be repelled only in case it is magnetised. So repulsion is the sure test of magnetism.
63. (a) What is a neutral point?
(b) Draw the magnetic field lines around a bar magnet when a bar magnet is placed such that the N pole points North.
Answer
(a) Neutral point is the point where magnetic field of the magnet is equal to the magnitude of earth’s horizontal magnetic field, but in opposite direction.
(b)
64. State two disadvantages of setting up large dams for hydro-electricity power station.
Answer
Disadvantages of setting up large dams for hydro-electricity power station::
- It leads to large catchment area being submerged
- Displacement of people from their places.
65. State two particles in the use of energy those cut down energy consumption.
Answer
Improved practices in the use of energy :
- Use the improved cook stove which will save the bio-mass energy
- Use the improved solar panels and solar furnaces.
66. What are the functions of Bureau of energy efficiency set up by the government?
Answer
The functions of bureau of energy efficiency setup are :
- control and keep an eye on consumption pattern
- develop appliances suitable for the energy change
- organize posters and slogans for developing awareness in society.
67. Suggest best particles in use of energy (a) at school (b) at home that save energy.
Answer
(a) At School : Use of solar cells to light up the electricity in schools and do rain water harvesting, keep the electrical appliances off after the needed period of time.
(b) At Home : Use of bio energy for cooking and lightening, use CFLs instead of filament bulbs, keep the electrical appliances off after the needed period of time.
68. Complete the following diagrams of magnets to show the lines of force in each case. [Fig. 6 (a, b, c, d)].
(i)
(ii)
