Q 1. Define magnetic effect of electric current.
Ans. The production of magnetic field around a conductor when electric current is passed through it.
Q 2. Give one important advantage of AC over DC
Ans. A.C can be stepped up and stepped down which means that the voltage can be increased or decreased. Hence it can be transmitted to long distances without much loss of energy. So A.C is preferred over D.C.
Q 3. Give a note on Magnetism in Human beings.
Ans. Whenever there is an electric current, there is a magnetic field. Even the extremely weak ion currents that travel along the nerve cells in our body produce magnetic fields. When we try to touch something, our nerves carry an electric impulse to the muscles we need to use. This impulse creates a temporary magnetic field. These fields are about one billionth as weak as the Earth’s field. Two main organs in the human body where the magnetic field produced is significant are heart and brain.
Q 4. What is fuse?
Ans. Fuse is the most important safety device, used for protecting the circuits due to short-circuiting or overloading of the circuits.
Q 5. What is the pattern of magnetic field around a current carrying conductor?
Ans. The magnetic field around a current carrying conductors forms a pattern of concentric circles.
Q 6. Draw an appropriate schematic diagram showing common domestic circuits and discuss the importance of fuse.
Ans.
Q 7. What is electric motor? Write down its principle of working.
Ans. Electric motor: An electric motor is a rotating device that converts electrical energy to mechanical energy. Principle: A current-carrying conductor when placed in a magnetic field experiences a force.
Q 8. What is galvanometer?
Ans. A galvanometer is an instrument that can detect the presence of a current in a circuit. The pointer remains at zero (the centre of the scale) for zero current flowing through it. It can deflect either to the left or to the right of the zero mark depending on the direction of current.
Q 9. What will happen if a current carrying conductor is placed in magnetic field? How can the direction of magnetic field be found out?
Ans. A current-carrying conductor when placed in a magnetic field experiences a force. If the direction of the field and that of the current are mutually perpendicular to each other, then the force acting on the conductor will be perpendicular to both as given by Fleming’s left-hand rule.
Q 10. What does the direction of thumb indicate in the right-hand thumb rule? In what ways this rule is different from Fleming’s left-hand rule?
Ans. The thumb indicates the direction of current in the straight conductor held by curled fingers, whereas Fleming’s left-hand rule gives the direction of force experienced by current carrying conductor placed in an external magnetic field.
Q 11. 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 12. 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 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.
Q 13. On which factors does the magnetic field produced by a current carrying conductor at a given point depend?
Ans. The magnetic field produced by a given current decreases as the distance from it increases. The magnitude of the magnetic field produced at a given point increases as the current through the wire increases.
Q 14. What is electric generator? Write down its principle of working.
Ans. Electric Generator: A generator converts mechanical energy into electrical energy.
Principle: It works on the basis of electromagnetic induction. AC generator produces AC current and DC generator produces DC current.
Q 15. Define Electromotive force.
Ans. The motion of a magnet, with respect to the coil, produces an induced potential difference. This induced potential difference is called electromotive force which sets up an induced electric current in the circuit. The motion of a magnet, with respect to the coil, produces an induced potential difference.
Q 16. What is difference between AC and DC? Write down advantage of AC over DC?
Ans. The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.
In India, the AC changes direction after every 1/100 second, that is, the frequency of AC is 50 Hz. An important advantage of AC over DC is that electric power can be transmitted over long distances without much loss of energy.
Q 17. With the help of a labelled circuit diagram illustrate the pattern of field lines of the magnetic field around a current carrying straight long conducting wire. How is the right hand thumb rule useful to find direction of magnetic field associated with a current carrying conductor?
Ans. Right hand thumb rule states that if a current carrying straight conductor is held in the right hand with the thumb pointing towards the direction of current, then the fingers will wrap around the conductor in the direction of the field lines of the magnetic field.
Q 18. Describe the activity that shows that a current-carrying conductor experiences a force perpendicular to its length and the external magnetic field. How does Fleming’s left-hand rule help us to find the direction of the force acting on the current carrying conductor?
Ans.
- Take a small aluminium rod AB (of about 5 cm). Using two connecting wires, suspend it horizontally from a stand.
- Place a strong horse-shoe magnet in such a way that the rod lies between the two poles with the magnetic field directed upwards. For this put the north pole of the magnet vertically below and south pole vertically above the aluminium rod.
- Connect the aluminium rod in series with a battery, a key and a rheostat.
- Now pass a current through the aluminium rod from end B to end A.
- It is observed that the rod is displaced towards the left. You will notice that the rod gets displaced.
- Reverse the direction of current flowing through the rod and observe the direction of its displacement. It is now towards the right.
The displacement of the rod in the above activity suggests that a force is exerted on the current-carrying aluminium rod when it is placed in a magnetic field. According to Fleming’s left hand rule stretch the thumb, forefinger and central finger of your left hand such that they are mutually perpendicular.
If the forefinger points in the direction of magnetic field and the central in the direction of current, then the thumb will point in the direction of motion or force acting on the conductor.
Q 19. Write down a short note on magnetic field produced by a solenoid.
Ans. Magnetic field due to a solenoid
Solenoid: A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
- The pattern of the magnetic field around a current carrying solenoid is same as that of a bar magnet. One end of the solenoid behaves as a magnetic North pole, while the other behaves as a South pole.
- The field lines inside the solenoid are in the form of parallel straight lines.
- The field is uniform inside the solenoid.
- A strong magnetic field produced inside a solenoid can be used to magnetise a piece of magnetic material, like soft iron, when placed inside the coil. The magnet so formed is called an electromagnet.
Field lines of the magnetic field through and around a current carrying solenoid
A current-carrying solenoid coil is used to magnetise steel rod inside it-an electromagnet
Q 20. Describe the magnetic field produced due to current-carrying circular loop.
Magnetic field due to current-carrying circular loop
Ans.
- At the centre of the current carrying loop the magnetic field appears to be a straight line.
- The magnetic field produced by a current carrying wire at a given point depends directly on current passing through it.
Magnetic field lines of the field produced by a current-carrying circular loop
Q 21. Define electromagnetic induction. Explain the ways by which magnetic field linked through a coil can be changed.
Ans. Electromagnetic Induction: The phenomenon of electromagnetic induction is the production of induced current in a coil placed in a region where the magnetic field changes with time.
1. The magnetic field may change due to relative motion between the coil and the magnet.
Moving a magnet towards a coil sets up a current in the coil circuit, as indicated by deflection in the galvanometer needle.
current is included in coil – 2 when current in coil – 1 changed
Q 22. Define magnetic field and magnetic field lines. Write down the properties of magnetic field lines.
Ans. Magnetic field:
The region surrounding a magnet, in which the force of the magnet can be detected, is said to have a magnetic field.
Magnetic field lines:
Magnetic field lines are the imaginary lines drawn in a magnetic field along which a north magnetic pole would move.
- Properties of magnetic field line: Magnetic field lines are closed curves.
- The relative strength of the magnetic field is shown by degree of closeness of the field lines.
- No two field lines can cross each other as at the point of intersection the compass needle would point towards two directions, which is not possible.
Q 23. AB is a current carrying conductor in the plane of the paper as shown in the Figure. What are the directions of magnetic fields produced by it at points P and Q? Given r1 > r2, where will the strength of the magnetic field be larger?
Ans. Into the plane of paper at P and out of it at Q. The strength of the magnetic field is larger at the point located closer i.e., at Q.
Q 24. Under what conditions permanent electromagnet is obtained if a current carrying solenoid is used? Support your answer with the help of a labelled circuit diagram.
Ans.
- The current through the solenoid should be direct current.
- The rod inside is made of a magnetic material such as steel.
Q 25. 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.
Ans. The deflection increases. The strength of magnetic field is directly proportional to the magnitude of current passing through the straight conductor.
Q 26. 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.
Ans. (i) Yes, Alpha particles being positively charged constitute a current in the direction of motion.
(ii) No, the neutrons being electrically neutral does not constitute any current.
Q 27. Why does a magnetic compass needle pointing North and South in the absence of a nearby magnet get deflected when a bar magnet or a current carrying loop is brought near it. Describe some salient features of magnetic lines of field concept.
Ans. Current carrying loops behave like bar magnets and both have their associated lines of field. This modifies the already existing earth’s magnetic field and a deflection results. Magnetic field has both direction and magnitude. Magnetic field lines emerge from N – pole and enter S – pole. The magnetic field strength is represented diagrammatically by the degree of closeness of the field lines
Q 28. Field lines cannot cross each other as two values of net field at a single point cannot exist. Only one value, a unique net value, can exist. If in a given region, lines of field are shown to be parallel and equispaced, the field is understood to be uniform.
Q 29. Ravi took his grandma for MRI test. His younger sister asked some questions and Ravi answered?
(i) What is basic concept behind MRI?
(ii) Write down values shown by Ravi.
Ans. (i) MRI or Magnetic Resonance Imaging is based on magnetic effect of electric current.
(ii) Ravi is a knowledgeble and caring too.
Q 30. Give the circuit symbol for a fuse. Explain its importance in a circuit.
Ans.
A fuse is a very important device used for protecting electric circuits. It is a wire made out of a metal like tin or tin alloy having a very low melting point. When a high current flows through a circuit, the fuse wire gets heated or melts due to short circuiting or overloading. Hence the circuit is broken and the current stops flowing. This saves all the appliances of the circuit.
Fuse wires are of various capacities. A fuse with 5 ampere capacity will be thinner than a fuse with 15 ampere capacity. A fuse of 5 amps is used in circuits where lights and fans are connected whereas a fuse of 15 amps is used in power circuits where appliances like electric heater, geyser, electric iron and air conditioner are connected.
Q 31. Define a solenoid. Compare the magnetic field produced by a solenoid with that of a bar magnet?
Ans. A coil of many circular turns of wire wrapped in the shape of a cylinder, is called a solenoid. The magnetic field lines in a solenoid, through which current is passed, is very similar to that of a bar magnet. One end of the coil acts like a magnetic north pole, while the other acts like a south pole. The magnetic field produced by a long solenoid has all the properties of the field produced by a bar magnet.
Q 32. PQ is conductor × represents magnetic is ⊥ to the paper field and into the plane of the paper.
Ans. Fleming’s left hand rule gives the direction of force experienced by a current carrying conductor kept in a magnetic field. According to it, when the thumb, first finger and second finger of the left hand are kept perpendicular to each other such that the first finger points towards the direction of magnetic field, the central finger is along the direction of current, then the thumb shows the direction of the force acting on the conductor.
Q 33. State Fleming’s right Hand Rule. Give the principle, construction and working of the AC generator with a simple diagram. What modification will you suggest so that the output is DC
Ans. According to Fleming’s right hand rule, when the thumb and the central finger of right hand are kept perpendicular to each other, the thumb shows the direction of motion of the conductor, the first finger the direction of magnetic field when the current induced is in the direction of central finger.
AC generator
Principle: It works on the principle of electromagnetic induction. Induced current is produced, whenever current is produced.
Construction: A generator consists of mainly four parts, namely coil, magnets, slip rings and brushes just like an electric motor.
Coil: A large number of insulated copper wires wound on a rectangular frame.
Magnets: A large permanent magnet to provide a strong a magnetic field.
Slip rings: Two solid rings connected the two ends of the coil used to convey the current produced to outside circuit.
Brushes: Two carbon brushes remain in sliding contact with slip rings.
Working: The coil of the generator is rotated with the help of an axel. When coil rotates, it cuts through the magnetic filed of the magnet. So a current is induced in the coil by electromagnetic induction. The direction of this current is given by Fleming’s right hand rule.
As the coil turns clockwise, arm AB moves up and arm CD goes down. The direction of the current is from A to B and C to D. When after half rotation CD starts going up and AB starts coming down, the direction of the current in the coil also reverses. Now it is from D to C and from B to A. This alternating current with the help of slip rings which are in sliding contact with brushes B1 and B2 is given out to the circuit. Hence the current produced by the generator is alternating and such a generator is called AC generator.
To get direct current in place of slip rings, split rings are used so that one brush is always in contact with the arm that goes downward. Then the current given out to the outer circuit is in the same direction. This type of generator is called DC generator.
Q 34. Distinguish between a solenoid and a bar magnet. Draw the magnetic lines for both
Ans.
The solenoid is a long coil containing a large number of close turns of insulated copper wire. The magnetic field produced by the current carrying solenoid is similar to the magnetic field produced by a bar magnet. A solenoid is used for making electromagnets.
Differences between a bar magnet and solenoid:
Bar magnet
It is a permanent magnet.
The strength of a bar magnet cannot be changed.
The polarity (North – South) of a bar magnet cannot be changed.
Solenoid
It is a temporary magnet. It acts as a magnet only as long as the current passes through it.
The strength of a solenoid can be changed by changing the number of turns in its coil or by changing the current passing through it.
The polarity of a solenoid can be changed by changing the direction of current in its coil.
Q 35. What is electromagnetic induction? Explain how the movement of a magnet towards or away from a coil carrying a galvanometer produce current? Write the rule to find the direction of current in this above coil.
Ans. Whenever the magnetic field through a conductor changes, and induced current and e. m. f. is set up in the conductor. This is known as electromagnetic induction.
Q 36. Explain why, two magnetic lines of force do not intersect.
Ans. The magnetic lines of force do not intersect one another due to the fact that the resultant force on a north pole at any point can be only in one direction. But if the two magnetic lines of force intersect one another, then the resultant force on a north pole placed at the point of intersection will be along directions, which is not possible.
Q 37. State the right hand thumb rule.
Ans.
If you hold the thumb, the forefinger and the centre finger of your right hand at right angles to one another. Adjust you hand in such a way that forefinger points in the direction of magnetic field, and the thumb points in the direction of motion of conductor, then the direction in which centre finger points, gives the direction of induced current in the conductor.
Q 38. How will you find out the direction of the magnetic field produced by current-carrying conductor?
Ans. The direction of lines of force of the magnetic field produced by a straight wire carrying current is obtained by Maxwell’s right hand thumb rule. According to Maxwell’s right-hand thumb rule, “Imagine that the current carrying wire is in the right hand so that the thumb points in the direction of current, then the direction in which the fingers encircle the wire gives the direction of magnetic lines of force around the wire.
Imagine a current carrying wire AB in which the current is flows vertically upwards. To find out the direction of magnetic lines of force produced by this current, we imagine the wire AB to be held in the right hand, so that the thumb points in the direction of current towards A. Now, the direction in which the fingers are folded gives the direction of the lines of force. In this case the fingers are folded in the anti-clockwise direction, so the magnetic lines of force are also in the anti-clockwise direction.
Q 39. What type of core should be put inside a current-carrying solenoid to make an electromagnet?
Ans.
A soft iron core is placed inside a solenoid to make an electromagnet. When a soft iron core is placed inside a solenoid, then the strength of the magnetic field becomes very large because the iron core gets magnetized by induction. This combination of a solenoid and a soft iron core is called an electromagnet.
Q 40. Distinguish between a bar magnet and an electromagnet.
Ans. Bar Magnets
The bar magnet is a permanent magnet.
It produces a comparatively weak force of attraction.
The strength of a bar magnet cannot be changed.
The polarity of a bar magnet is fixed and cannot be changed.
Electromagnets
An electromagnet is a temporary magnet.
It produces a very strong magnetic force.
The strength of an electromagnet can be changed by changing the number of turns in its coil or by changing the current passing through it.
The polarity of an electromagnet can be changed by changing the direction of current in its coil.
Q 41. Derive the formula for the force acting on a charged particle moving in a magnetic field.
Ans.
The force acting on a current-carrying conductor placed in a magnetic field is,
F = B × I × L
The current I is the rate of flow of charge.
Now, if a charge Q flows in time t then the current I = Q/t. Hence substituting for I in
the above equation, we get,
F = (B × Q × L)/t
Suppose the particle carrying the charge Q travels a length L in time t, then the velocity
v = L/t. So substituting this value, we get
Force on moving charge F = B × Q × v.
Q 42. Explain the principle and working of an electric motor with the help of a diagram. What is the function of a split ring commutator?
Ans.
An electric motor converts electrical energy into mechanical energy. It works on the principle that – a current carrying conductor placed in a magnetic field experiences a force.
Following are the essential parts of an electric motor.
(i) Coil: It is a rectangular coil of insulated copper wire having large number of turns.
(ii) A large permanent magnet provides strong magnetic field between its pole pieces. The coil rotates between these pole pieces.
(iii) Split rings: The two ends of the coil are connected to two split rings, which are two halves of a slip rings.
(iv) Brushes: Two carbon brushes keep in sliding contact with split rings.
Working
When a current is passed through the coil, the direction of current in AB and CD is in opposite direction but both are perpendicular to magnetic field. Therefore, by Fleming’s left hand rule AB arm of the coil experiences an upward force and CD arm experiences a downward force. These two forces being equal and opposite to each other form a couple which rotates the coil. Arms BC and DA are parallel to the field and the force on them is zero. The forces, on AB and CD turn the coil in clockwise direction. After half revolution, the split rings change their position. Now S2 is in contact with brush B1 and S1 is in contact with B2. So the direction of current in the coil reverses. Therefore, AB now experiences downward force and CD upward force. The couple now acting on the coil again moves it in clockwise direction. Due to the function of split ring commutator and brushes, coil continues to turn in clockwise direction.
Split ring commutator changes direction after every half rotation, so that the direction of current going in the coil also reverses and the arm of the coil which goes up in the first half, goes down in second half. As a result, the coil continues to rotate in one direction. Anything connected to the axis of the coil also rotates. So, the electrical energy given to the coil changes into mechanical energy.
Q 43. With the help of a labelled diagram, explain the working of an A.C. generator.
Ans. “A. C. generator” means “Alternating Current generator”. That is, an A. C. generator produces alternating current, which alternates (changes) in polarity continuously. We will now describe the construction an working of the A. C. generator or A. C. dynamo.
Construction of an A. C. generator
A simple A. C. generator consists of a rectangular coil ABCD that can be rotated rapidly between the poles N and S of a strong horseshoe type magnet M. The coil is made of a large number of turns of insulated copper wire. The ends A and D of the rectangular coil are connected to two circular pieces of copper metal called slip rings R1 and R2. As the slip rings R1 and R2 rotate with the coil, the two pieces of carbon called brushes, B1 and B2, keep contact with them. So, the current produced in the rotating coil can be tapped out through slip rings into the carbon brushes. From the carbon brushes B1 and B2 we take the current into various electrical appliances like radio, T. V., electric iron, bulbs, etc. But in this figure, we have shown only a galvanometer G connected the two carbon brushes.
Working of an A. C. generator
Suppose that the generator coil ABCD is initially in the horizontal position. Again suppose that he coil ABCD is being rotated in the anticlockwise direction between the poles N and S of a horseshoe type magnet.
(i) As the coil rotates in the anticlockwise direction, the side AB of the coil moves down cutting the magnetic lines of force near the N-pole of the magnet, and side CD moves up, cutting the lines of force near the S-pole of the magnet. Due to this, induced current is produced in the sides AB and DC of the coil. On applying Fleming’s right hand rule to the side AB and DC of the coil, we find that the currents are in the direction B to A and D to C respectively. Thus, the induced currents in the two sides of the coil are in the same direction, and we get an effective induced current in the direction BADC.
(ii) After half revolution, the sides AB and DC of the coil will interchange their positions. The side AB will come on the right hand side and DC will come on the left side. So, after half a revolution, side AB starts moving up and side DC starts coming down. As a result of this, the direction of induced current in each side of the coil is reversed after half a revolution. Since the direction of induced current in the coil is reversed after half revolution so the polarity (positive and negative) of the two ends of the coil also changes after half revolution. The end of coil which was positive in the first half of rotation becomes negative in the second in the second half. And the end which was negative in the first half revolution becomes positive in the second half of revolution. Thus, in 1 revolution of the coil, the current changes its direction 2 times.
The alternating current (A. C.) produced in India has a frequency of 50 Hz. That is, the coil is rotated at the rate of 50 revolutions per second. Since in 1 revolution of coil, the current changes its direction 2 times, so in 50 revolutions of coil, the current changes its direction 2 × 50 = 100 times. Thus, the A. C. supply in India changes its direction 100 times in 1 second. Another way of saying this is that the alternating current produced in India changes its direction every 1/100 second. That is, each terminal of the coil is positive (+) for 1/100 of a second and negative (-) for the next 1/100 of a second. This process is repeated again and again with the result that there is actually no positive and negative in an A. C. generator. We will now describe why the direction of induced current in the coil of an A. C. generator changes after every half revolution of the coil.
After every half revolution, each side of the generator coil starts moving in the opposite direction in the magnetic field. The side of the coil which was initially moving downwards in a magnetic field, after half revolution, it starts moving in opposite direction – upwards. Similarly the side of coil which was initially moving upwards, after half revolution, it starts moving downwards. Due to the change in the direction of motion of the two sides of the coil in the magnetic field after every half revolution, the direction of current produced in them also changes after every half revolution.