1. A current through a horizontal power line flows in east to west direction. What is the direction of magnetic field at a point directly below it and at a point directly above it?

Solution

The electric current is flowing horizontally in east to west direction. Applying right hand thumb rule, we find that   i. The direction of magnetic field at a point directly below the wire is from north to south. ii. The direction of magnetic field at a point directly above the wire is from south to north.

2. What is a generator? Name the principle used in generator.

Solution

Generator is a device that converts mechanical energy to electrical energy. It works on the principle of electromagnetic induction.

3. When is the force on a current carrying wire maximum or minimum?

Solution

The force on a current carrying wire is maximum only when the wire is perpendicular to the direction of magnetic field. It is minimum, that is, no force acts on the conductor when it is parallel to the magnetic field.

4. What is an electromagnet? How is it different from a permanent magnet ? State two uses of electromagnet.

Solution

(a) When a piece of metal like soft iron is placed inside a solenoid and current is passed through it, it gets magnetised. The magnet so formed is called an electromagnet. (b) Electromagnet is a temporary magnet. Its polarity can be reversed by reversing the direction of current, whereas the polarity of a permanent magnets is fixed. Uses :- Electric crane, Electric bell.

5. Positively charged particle projected towards west is deflected towards north by a magnetic field. What is the direction of the magnetic field? State the rule applied to determine direction of magnetic field.

Solution

Vertically upwards Fleming’s Left Hand Rule – If we stretch the thumb, forefinger and middle finger of our left hand such that they are mutually perpendicular and if the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of force acting on the conductor.

6. Why does a current-carrying solenoid, when suspended freely rests along a particular direction? Explain.

Solution

A current-carrying solenoid behaves like a bar magnet. We know that a freely suspended bar magnet aligns itself in the north-south direction. So, a freely suspended current-carrying solenoid also aligns itself in the north-south direction.

7. Explain the role of fuse in series with any electrical appliance in an electric circuit? Why should a fuse with defined rating for an electric circuit not be replaced by one with a larger rating?

Solution

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.

8. Which gas is filled in the electric bulb and why?

Solution

Argon or Neon. These are inactive or inert gases to prolong life of the filament.

9. What are the two main circuits in domestic wiring and what is the usual capacity of fuse used in these circuits?

Solution

Usually two separate circuits are provided in a house 1) The lighting circuit with a 5 A fuse for running low power-rating devices such as electric bulbs, tube-lights, fans, radio, television, etc. which draw small current. 2) The Power circuit with a 15 A fuse for running high power- rating devices such as electric iron, room heater, geysers, refrigerators, etc. which draw heavy current. These two circuits are connected in parallel so that  short circuit in one does not effect the working of the other circuit.

10. What is an electric fuse? Briefly explain its function.

Solution

An electric fuse is a device which is used in series of an electric circuit as a safety device to prevent the damage caused by short-circuiting or overloading of the circuit. It is a small, thin wire of a material whose melting point is very low. If due to some fault, electric circuit gets short-circuited, then a strong current begins to flow. Due to such strong flow of current, the fuse wire is heated up and it melts. As a result, the electric circuit is broken and current flow stops. Thus, possible damage to the circuit and appliances is avoided.

11. What change in the deflection of the compass needle placed at a point near current carrying straight conductor shall be observed, if the (a) current through the conductor is increased, (b) direction of current in the conductor is reversed? (c) compass is moved away from the conductor?

Solution

(a) Deflection increases when current through conductor increases. (b) Direction of deflection is reversed when direction of current is reversed. (c) Deflection decreases when the compass is moved away.

12. Series arrangements are not used for domestic circuits. List any three reasons.

Solution

(i) In series arrangement same current will flow through all the appliances, which is not required. (ii) Total resistance of domestic circuit will be sum of the resistances of all appliances and current drawn by the circuits will be less. (iii) We cannot use independent On/Off switches with individual appliances. (iv) All appliances are to be used simultaneously even if we don’t need them. (Any three)

13. How will the magnetic field produced at a point due to a current carrying circular coil change if we: (i) increase the current flowing through the coil? (ii) reverse the direction of current through the coil? (iii) increase the number of turns in the coil?

Solution

(i) On increasing the current flowing through the coil, the field will increase. (ii) On reversing the direction of current through the coil, the direction of field will reverse. (iii) On increasing the number of turns in the coil, the field will increase.

14. How are the electric appliances in a room connected (in series/parallel) and why?

Solution

All the electrical appliances like bulbs, fans and sockets, etc. in a room are connected in parallel across the live wire and the neutral wire because of the following advantages: 1) In a parallel circuit, if one of the appliances is switched off, or gets fused, there would be no effect on the other appliances and they keep on operating. 2) In parallel circuit, same voltage of the mains is available for all the electrical appliances. Example: all bulbs connected in parallel would get same voltage and would glow brightly whereas, if connected in series, bulbs would get less voltage and would glow less brightly.

15. A stationary charge is placed in a magnetic field. Will it experience a force?

Solution

No, a magnetic field exerts a force only on a moving charge.

16. Under what condition does a current carrying conductor kept in a magnetic field experience maximum force?  On what other factors does the magnitude of this force depend?  Name and state the rule used for determination of direction of this force.

Solution

When the conductor is held perpendicular to the magnetic field, it experiences maximum force.  Magnitude of this force depends on strength of magnetic field, orientation of the conductor, length of the conductor.  Fleming’s left hand rule – If we stretch the thumb, the first finger and the central finger of the left hand so that they are mutually perpendicular to each other and if the first finger points in the direction of the magnetic field, the central finger points in the direction of current, then the thumb points in the direction of motion of the conductor.

17. A coil made of insulated copper wire is connected to a galvanometer. What will happen to the deflection of the galvanometer if this coil is moved towards a stationary bar magnet and then moved away from it? Give reason for your answer and name the phenomenon involved.

Solution

When coil is moved towards a stationary magnet, the magnetic field associated with the coil will change and so current will be induced in the coil. This causes galvanometer to show deflection in one direction. Now when coil is moved away, the magnetic field will decrease and so current induces in the opposite direction causing galvanometer to show opposite deflection. The phenomenon is electromagnetic induction.

18. What important property of the field is indicated by the field line pattern of a long straight solenoid? Name any two factors on which the magnitude of the magnetic field due to the solenoid depends.

Solution

Important Property: The magnetic field (of a long straight solenoid) is nearly uniform inside it. The magnitude of the field depends on strength of current and the number of loops of the solenoid.

19. What does the direction of thumb indicate in the right-hand thumb rule? In what way this rule is   different from Fleming’s left hand rule?

Solution

The thumb in right hand rule indicates the direction of current in the straight conductor held by curled fingers, whereas the thumb in Fleming’s left-hand rule gives the direction of force experienced by current carrying conductor placed in an external magnetic field.

20. A student while studying the force experienced by a current carrying conductor in a magnetic field records the following observations (a) The force experienced by the conductor increases as the current is increased (b) The force experienced by the conductor decreases as the strength of the magnetic field is increased. Which of the two observation is correct and why? State the rule which gives the direction of force acting on a current carrying conductor placed in a magnetic field.

Solution

Observation a is correct. Because force experienced by a current carrying conductor in a magnetic field is proportional to the strength of the current.   Flemings left hand rule gives the direction of force acting on a current carrying conductor placed in a magnetic field. Fleming’s left hand rule: Hold the forefinger, the centre finger and the thumb of your left hand at right angles to one another. Adjust your hand in such a way that the forefinger points in the direction of magnetic field and the centre finger points in the direction of current, then the direction in which thumb points, gives the direction of force acting on the conductor.

21. How will the magnetic field Produced in a current carrying a circular coil change if we (i) Increase the value of current. (ii) Increase the distance from the coil (iii) Increase the no. turns of the coil

Solution

(i) Magnetic field will increase (ii) Magnetic field will decrease (iii) Magnetic field will increase

22. (a) What is the direction of magnetic field lines inside a bar magnet and out side of it? (b) What does the degree of closeness of the field lines represents?

Solution

(a) The direction of magnetic field lines outside a bar-magnet is from North seeking pole to south seeking pole while inside the magnet it is from south to north seeking pole. (b) Degree of closeness of magnetic field line determines the strength of magnetic field. Stronger is the field at places where field lines are crowded.

23. What are permanent magnets and electromagnets? Give one use of each.

Solution

Permanent magnets have constant magnetic fields around it. When a piece of metal like soft iron is placed inside a solenoid and current is passed through it, it gets magnetised. The magnet so formed is called an electromagnet. Uses. Permanent magnet – Electric generator, loud speaker. Electromagnet – Cranes, electric bell.

24. Why does a current carrying conductor kept in a magnetic field experience force? On what factors does the direction of this force depend? Name and state the rule used for the determination of direction of this force.

Solution

A current carrying conductor produces a magnetic field around it which interacts with the magnetic field in which it is held. Thus, it experiences a force. Direction of this force depends upon the direction of current and the magnetic field. Fleming’s left hand rule – If we stretch the thumb, forefinger and middle finger of our left hand such that they are mutually perpendicular, and if the first finger points in the direction of magnetic field, the second finger in the direction of current, then the thumb will point in the direction of force acting on the conductor.

25. (a) State the rule to determine the direction of a current induced in a coil due to its rotation in a magnetic field. (b) Differentiate between AC and DC. Write one advantage of AC over DC

Solution

(a) Fleming’s Right hand rule – If the fore finger, second (central) finger and thumb of the right hand are stretched at right angles to each other, with the forefinger in the direction of the field, the thumb in the direction of the motion of the wire then the induced current in the wire is in the direction of the second or central finger.   (b) Direction of AC changes after equal intervals of time. Direction of DC does not change. Advantage of AC over DC – AC can be transmitted to long distances without much loss of energy.

26. (a) Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in coil B? Give reason. (b) State the rule to determine the direction of : (i) magnetic field produced around a straight conductor carrying current. (ii) force experienced by a current carrying straight conductor placed in a magnetic field, which is perpendicular to it.

Solution

(a) Yes, magnetic field changes in A. So magnetic flux linked with B changes and current is induced. (b) (i) Right hand thumb rule – If one holds a straight conductor carrying current in right hand in such a way that the thumb indicates the direction of current, then the folded fingers indicate the direction of magnetic field surrounding the conductor. (ii) Fleming’s left hand rule – If we stretch the first three fingers of the left hand mutually perpendicular to each other such that the fore finger points along the direction of magnetic field, the middle finger points along the direction of current then thumb indicates the direction of force experienced by the conductor.

27. Why does a current carrying conductor experiences a force when it is placed in a magnetic field? State Fleming’s left hand rule.

Solution

Because of interaction between the field due to current carrying conductor and the magnetic field in which the conductor is placed. Fleming’s left hand rule – If we stretch the thumb, the first finger and the middle finger of the left hand so that they are mutually perpendicular to each other and if the first finger points in the direction of the magnetic field, the middle finger points in the direction of current, then the thumb points in the direction of motion of the conductor.

28. How can you prepare an electromagnet?

Solution

By placing a soft iron core inside a current carrying solenoid coil.

29. If a magnet is brought near a current carrying conductor free at one end, would the wire move?

Solution

The wire would move on bringing magnet near it, as the magnet would exert a mechanical force on the current carrying wire, and this force is enough to produce motion in the wire. This result can also be obtained by the Newton’s third law of motion, that is, if a current carrying wire exerts a force on a magnet, then the magnet will exert an equal and opposite force on the wire.

30. Can a fuse made up of pure copper wire? How is a fuse connected in domestic wiring?

Solution

A pure copper wire can not be used as a fuse wire because it has a high melting point due to which it will not melt easily when a short circuit occurs. Usually a short length of a thin, tin plated copper wire having low melting point is used in a fuse. A fuse is connected in series in domestic electric circuit.

31. A magnetic compass shows a deflection when placed near a current carrying wire. How will the deflection of the compass get affected if the current in the wire is increased? Support your answer with a reason.

Solution

The deflection increases. The strength of magnetic field is directly proportional to the magnitude of current passing through the straight conductor.

32. Does the direction of induced current in the coil of an A.C. generator change after half rotation of the coil? Explain why?

Solution

After every half rotation, each side of the generator coil starts moving in the opposite direction in the magnetic field. The side of the coil which was earlier moving downwards in the magnetic field, after half rotation it starts moving upwards. Similarly the side of the coil which was initially moving upwards, after half rotation starts moving downwards. Thus, due to change in the direction of motion of the two sides of the coil in the magnetic field after every half rotation, the direction of induced current also changes.

33. Whenever there is a relative motion between a magnet and a coil, a current is induced in the coil. Name this phenomenon?

Solution

This phenomenon is called Electromagnetic Induction.

34. Explain briefly two methods of producing induced current.

Solution

(1) By varying current in one coil, current can be induced in the neighbouring coil. (2) By moving a magnet towards or away from a coil, current can be induced in it.

35. Explain why electrical appliances are connected in parallel in household circuits?

Solution

(i) In parallel, electric potential is same for all appliances. (ii) In parallel, equivalent resistance decreases. (iii) Different switches can be provided for different appliances.

36. Distinguish between electric motor and electric generator

Solution

Electric motor           Electric generator A battery is used in an electric motor to pass a current through the   armature coil. A battery is not used in an electric generator This works on the principle that a current carrying conductor placed in a magnetic field experiences a force. This works on the principle of electromagnetic induction. In this, electrical energy is converted into mechanical energy. In this, mechanical energy is converted into electrical energy. A split ring commutator is used in an electric motor. Rings are used in an electric AC generator. These motors are used in electric cars, trains, fans, hair dryers, lifts, etc. This is used in power stations.

37. (a) When do we state that an electrical appliance is earthed? Mention the function of earth wire in electric lines. Why is it necessary to earth the electric appliances having metallic body?

(b) Explain what is short circuiting and overloading in an electric supply.

Solution

(a) When the metal body of any electrical appliance is connected to a metal wire which is connected to a metal plate kept deep inside the earth it is said to be earthed. It is necessary to earth all electrical appliances to avoid electric shock.

(b) (i) An electric circuit is said to be short circuited if live wire and neutral wire come in contact with each other.

(ii) Overloading means to connect too many appliances with single socket and using faulty device in electrical circuit.

38. (a) Name the scientist who discovered that a moving magnet can be used to generate electric current. With which name is this phenomenon known ? (b) Two coils 1 and 2 of insulated copper wire having large but different number of turns are wound over a cardboard cylinder. Coil 1 is connected to battery and a plug key. Coil 2 is connected to a galvanometer. How will the galvanometer reading change when ? (i) key is plugged in, and (ii) key is taken out. Give reason for your answer. (c) Name and state the rule used for determination of direction of induced current due to a changing magnetic field and give one practical application of this phenomenon in everyday life.

Solution

(a) Michael Faraday ; Electromagnetic induction (b) (i) Galvanometer will show a momentary deflection as the number of magnetic field lines will change around coil 2 due to increase in current through coil 1 from zero to a maximum value. (ii) Galvanometer will show momentary deflection in the opposite direction as the number of magnetic field lines will change around the coil 2 as the current in coil 1 falls from maximum to zero. (c) Rule: Fleming’s right hand rule. It states that if we stretch the thumb, fore finger and middle finger of our right hand such that they are perpendicular to each other. If fore finger indicates direction of magnetic field and the thumb shows the direction of motion of conductor, then middle finger will show direction of induced current. Application: Electrical generator

39. Insulation cover of which colour is conventionally used for earth wire? Why is an earth wire connected to metallic parts of appliances?

Solution

Green coloured wire is used for earth wire. Any leakage of current to the metallic parts is safely conducted to the ground, thereby reducing chances of severe shock.

40. What is the direction of magnetic field lines (i) Outside magnet (ii) inside a magnet?

Solution

(i) From N-pole towards S-pole outside a magnet, and (ii) from S-pole towards N-pole inside a magnet.