Answer: The region around the magnet, where force of attraction or repulsion can be felt by magnetic materials, is called as magnetic field.
2. What will be the frequency of an alternating current, if its direction changes after every 0.05 s?
Answer: The time period (T) of one cycle would be = 2 x (0.05 s) = 0.1 s.
frequency, f = 1/T. Hence, f = (1 / 0.1) = 10 Hz.
3.What is the principle on which working of electric generator is based? What are its important parts?
Answer: The working of electric generator is based on the principle of electromagnetic induction.
Important parts of electric generator are armature, slip rings, brushes and field magnets.
4. Why two magnetic lines of force don’t intersect each other?
Answer: If so then at the point of intersection there will be two different directions of magnetic field which is not possible.
5. Give two methods by which we can increase the strength of magnetic field produced by a circular coil carrying current?
Answer: The two methods by which we can increase the strength of magnetic field are given below:-
1. By increasing the number of turns of wire in the coil.
2. By increasing the current flowing through the coil.
6. What are the patterns of magnetic field lines inside and outside of a solenoid? What do they indicate?
Answer: (i)The field lines inside the solenoid are parallel straight lines. This indicates that the magnetic field is uniform and is therefore, same at all points inside it.
(ii)The field lines outside the solenoid are curved lines. This indicates that the magnetic field is non-uniform.
7. What are the factors which govern the force experienced by a current carrying conductor placed in a uniform magnetic field depends?
Answer: The factors which govern the force experienced by conductor which is placed in a uniform magnetic field are:-
1. strength of the magnetic field in which conductor is placed.
2. strength of current flowing through the conductor.
3. length of conductor.
8. Distinguish between an electric motor and generator?
Answer: The major differences between an electric motor and generator are stated below:
1. It converts electrical energy into mechanical energy.
2. It needs electrical energy for its working.
3. They are used as water pumps, marble grinders e.t.c
1. It converts mechanical energy into electrical energy
2. It needs mechanical energy for its working.
3. They are used as water pumps, marble grinders e.t.c
3. Diesel generator, hydro-electric generator are the examples of it.
9. Why mostly all electrical home appliances like refrigerator, toaster etc. are provided with a wire having green insulation?
Answer: Earth wire is the insulated green coloured copper wire which is connected to a metal plate deep inside the earth near the house. It is used as a safety measure.
It is kept at zero potential by connecting it to the ground, thus, providing a low-resistance conductive path for the current.
When by electric fault, if current flows through the metallic body of electric appliance and we touch it, then current finds the lower resistance path of earth wire than our body. So, current flows through earth wire only and we remain safe.
10. Describe domestic household circuit.
Answer: From an electric pole or underground cables, we receive electric supply in our homes. Domestic electric circuit consists of three main wires.The wire with red insulation is called live wire.
The wire with black insulation is called neutral wire. Potential difference between the neutral and the live wire is 220 V.
The wire with green insulation is called earth wire. It is connected to a metallic body deep inside the earth.
According to new International Convention, insulation of live wire should be of brown colour whereas neutral and earth wires should be of light blue and green (or yellow) insulation cover.
In our homes, we receive AC electric power of 220 V with a frequency of 50 Hz.
Live wire and neutral wire maintains the p.d. of 220 V.
These wires pass through the fuse board. Fuse is specially connected with the live wire. Current rating of this fuse depends on house load.
From the fuse board, these wires pass through the electric metre. From the metre, the earth wire is locally inserted inside the ground of the house.
After the electric metre, these wires pass through the main switches and fuses in different rooms.
Depending on the power of electrical appliances, two types of fuse (5A or 15 A) are used. From the switch and fuse board, the power lines are distributed to different electrical appliances.
11. Describe about short circuiting and overloading in detail.
Answer: Short Circuiting: Short-circuiting occurs in a circuit when the live wire comes in direct contact with the neutral wire. This causes much damage to the electric appliances connected with them.
Reason of short-circuiting: When the live wire touches the neutral wire, the contact portion of the two wires behave like load and due to very low resistance of copper wire, it draws very large amount of current. In the presence of this high current, the copper wire catches fire, due to joules heating effect.
Overloading: Overloading occurs in a circuit when the current in the circuit increases abruptly. It causes overheating of the wire and might lead to the fire. It can also occur due to accidental increase in the supply voltage or on connecting too many appliances to a single socket.
12. What is an electromagnet? On what factors its magnetic field depends?
Answer: Electromagnet is a magnet formed by the magnetization of a piece of a magnetic material (such as soft iron) by inserting it into a solenoid.
The magnetic field (strength) of an electromagnet depends upon:
(i) Amount of current passing through the coil
(ii) Number of turns of the coil
(iii) Nature of core material present inside coil
13. Mention the differences between an electromagnet and a permanent magnet.
Answer: Differences between electromagnet and permanent magnet are:
1.It is temporary magnet and can be demagnetized.
2.Magnetic strength can be changed. .
3.Its polarity can also be changed.
4.It is prepared from soft iron.
1.It is a permanent magnet and cannot be demagnetized easily.
2. Its strength is fixed
3.Its polarity is fixed.
4.It is prepared from hard steel.
14. What do you understand by magnetic effect of electric current?
Answer: The phenomenon due to which, a wire behaves like a magnet when electric current is passed through it, is called as the magnetic effect of electric current.
15. Explain Maxwell's right hand thumb rule?
Answer: According to Maxwell's right hand thumb rule,
“If we hold the current-carrying conductor in our right hand in such a way that the thumb is stretched along the direction of the current, then the curled fingers give the direction of the magnetic field produced by the current”,
16. What are the properties of magnetic field due to a current through a straight wire?
Answer: The properties of magnetic field due to a current through a straight wire are:
1. The magnitude of the magnetic field produced at a given point increases as the current through the wire increases.
2. The magnetic field produced by a given current in the conductor decreases as the distance from it increases.
3. The concentric circles representing the magnetic field around a current-carrying straight wire become larger and larger as we move away from it.
4. If the direction of the current is reversed in the wire, the lines will still be circular, but the directions of the lines will be reversed, which can be verified using the compass needle.
17. Which rule is used find the direction of the force on a conductor in a magnetic field. Explain?
Answer: To find the direction of the force on a conductor in a magnetic field, a simple rule known as Fleming's left-hand rule is used.
According to Fleming's left-hand rule, if you stretch the thumb, forefinger and middle finger of your left hand such that they are mutually at right angles, If the First finger points in the direction of the field. The second finger represents the direction of the current (in the classical direction, from positive to negative), then the thumb will point in the direction of the force acting on the conductor or in the direction of the resultant motion. This rule is used to know the direction of the induced current
18. Give characteristics of magnetic field lines.
Ans: (i) They represent the magnetic field.
(ii) They are directed from north to South Pole outside a magnet and from south to North Pole inside a magnet.
(iii) The field lines are closed curves.
(iv) The strength of magnetic field in a region is determined by closeness of the field lines in that region.
(v) The closer the field lines are, greater will be the field strength and vice–versa.
(vi) No two field lines ever cross each other as it would mean two different directions of field at point of intersection, which is not possible.
(viii) The parallel lines represent the uniform magnetic field whereas converging lines or diverging lines represents the non uniform magnetic field.
19. What do you mean by electromagnetic induction?
Answer: "The process, due to which a changing magnetic field in a conductor induces a current in another conductor, is called electromagnetic induction".
20. A current through a horizontal power line flows in north to south direction.What is the direction of magnetic field (i)at a point directly below it and (ii)at a point directly above it?
Answer: (i) West to East (ii) East to West
21. A straight wire carrying electric current is moving out of plane of paper and is perpendicular to it. What is the direction and type of induced magnetic field?
Ans . Induced magnetic field will be in the form of concentric circles in the plane of paper.
22.. How can it be shown that magnetic field exist around a wire carrying current?
Ans:By using magnetic compass which, shows deflection.
23 . How can a solenoid be used to magnetise a steel bar.
Answer: By inserting the steel bar inside the solenoid and switching on electric current.
24. Can a 5 A fuse be used in wire carrying 15 A current? Why?
Answer: No, because both of them would then be ineffective in controlling the amount of current flowing.
25. Give the factors that affect strength of magnetic field at a point due to a straight conductor carrying current.
Answer: Magnitude of electric current, perpendicular distance between that point and conductor.
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Q. Where do we connect a fuse: with live wire or with neutral wire?
Ans: It is always connected with live wire.
Q. Give two uses of electromagnets.
Ans: (i) It is used in cranes for lifting heavy loads.
(ii) used in electric bells.
Q. Name any two devices which use permanent magnets.
Ans: Loudspeakers, Galvanometer, voltmeter.
Q.A current-carrying straight conductor is placed in the east-west direction.
What will be the direction of the force experienced by this conductor due to earth’s magnetic field? How will this force get affected on?
(a) reversing the direction of floe of current (b) doubling the magnitude of current.
The direction of earth’s magnetic field is from G-south to G-north. Let current
is from west to east. Therefore force is vertically upwards.
(a) By reversing the direction of current, the direction of will be reversed i.e.
(b) The magnitude of the force is doubled.
Q. An electron enters a magnetic field at right angles to it as shown in fig. The direction of the force acting on the electron will be:
(a) to the right (b) to the left (c) out of the page (d) into the page
Answer: When a conductor carrying current is placed perpendicular to the direction of magnetic field, the acting on it is given by Fleming’s left hand rule.
Since the direction of current is the same as that of the motion of a positive charge, the
direction of force acting on it when moving perpendicular to the direction of magnetic field is the same as that acting on a current-carrying conductor placed
perpendicular to the direction of magnetic field. Obviously, the force acting on an electron is opposite to that. Therefore in this case it is into the page.
Q. A coil of insulated copper wire is connected to a galvanometer. What would happen if a bar magnet is (i) Pushed into the coil? (ii) Withdrawn from inside the coil? (iii) Held stationary inside the coil?
Answer: (i) Due to change in magnetic flux linked with coil, the galvanometer shows deflection (say towards right).
(ii) Due to change in magnetic flux linked with coil, the galvanometer shows deflection (say towards left opposite to that in case one).
( ii i) As it is stationary no change in magnetic flux linked with coil, so galvanometer shows no deflection.
Q.A magnetic compass needle is placed in the plane of paper near point A as shown in Figure 13.6.
In which plane should a straight current carrying conductor be placed so that it passes through A and there is no change in the deflection of the compass? Under what condition is the deflection maximum and why?
Answer: In the plane of the paper itself. The axis of the compass is vertical and the field due to the conductor is also vertical. It could result in a dip of compass needle which is not possible in this case (dips result only if axis of compass is horizontal). The deflection is maximum when the conductor through A is perpendicular to the plane of paper and the field due to it is maximum in the plane of the paper
Q. Under what conditions permanent electromagnet is obtained if a current carrying solenoid is used?
(i) The current through the solenoid should be direct current.
(ii) The rod inside is made of a magnetic material such as steel
Q. It is established that an electric current through a metallic conductor produces a magnetic field around it. Is there a similar magnetic field produced around a thin beam of moving (i) alpha particles, (ii) neutrons? Justify your answer.
(i) Yes, Alpha particles being positively charged constitutes a current in the direction of motion.
(ii) No. The neutrons being electrically neutral constitute no current.
Q. Meena draws magnetic field lines of field close to the axis of a current carrying circular loop. As she moves away from the centre of the circular loop she observes that the lines keep on diverging. How will you explain her observation.
Answer: Strength of the magnetic field falls as distance increases. This is indicated by the decrease in degree of closeness of the lines of field.
Q. What does the divergence of magnetic field lines near the ends of a current carrying straight solenoid indicate?
Ans: The divergence, that is, the falling degree of closeness of magnetic field lines indicates the fall in strength of magnetic field near and beyond the ends of the solenoid.
Q. What is the role of the two conducting stationary brushes in a simple electric motor?
Ans: The brushes are connected to the battery and touch the outer side of two halves of the split ring whose inner sides are insulated and attached to the axle.
Q. What is the difference between a direct current and an alternating current? How many times does AC used in India change direction in one second?
Ans: Direct current always flows in one direction but the alternating current reverses its direction periodically. The frequency of AC in India is 50 Hz and in each cycle it alters direction twice. Therefore AC changes direction 2 × 50 = 100 times in one second.
Q.What is the role of fuse, used in series with any electrical appliance? Why should a fuse with defined rating not be replaced by one with a larger rating?
Ans: Fuse is used for protecting appliances due to short-circuiting or overloading. The fuse is rated for a certain maximum current and blows off when a current more than the rated value flows through it. If a fuse is replaced by one with larger ratings, the appliances may get damaged while the protecting fuse does not burn off. This practice of using fuse of improper rating should always be avoided