13. MAGNETIC EFFECTS OF ELECTRIC CURRENT
Questions:
1. The magnetic is effect of current was discovered by:
(a) Faraday (b) Henry (c) Oersted (d) Maxwell
Ans: (c) Oersted
2. Match the column below:
Ans: 1. (b), 2. (c), 3. (d), 4. (a).
3. Why does a compass needle show deflection when brought near a current carrying conductor?
Ans: Due to production of magnetic field around the current carrying conductor.
4. State the observation made by Oersted on the basis of his experiment with current carrying conductors.
Ans: Every current carrying conductor has a magnetic field around it.
5. How will the magnetic field intensity at the centre of a circular coil carrying current change, if the current through the coil is doubled and the radius of the coil is halved?
Ans: Magnetic field at centre of a coil, B ∝ RI , when current I is doubled and radius R is halved; the magnetic field becomes four times the original field.
6. Name and state the rule which determine the direction of magnetic field around a straight current carrying conductor.
Ans: Right Hand Thumb Rule: Imagine holding the current carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then the fingers of right hand wrap around the conductor in the direction of field lines of the magnetic field.
7. Draw magnetic field lines produced around a current carrying straight conductor passing through cardboard. How will the strength of the magnetic field change, when the point where magnetic field is to be determined, is moved away from the straight wire carrying constant current? Justify your answer.
Ans: Using compass needle. When we move away from the straight wire, the deflection of the needle decreases which implies the decreasing strength of the magnetic field.
8. In the experiment to show that a current carrying conductor when placed in the uniform magnetic field experiences a force. What happens when:
(i) You reverse the terminals of the battery?
(ii) The direction of current is perpendicular to the direction of magnetic field? State your observation.
Ans: (i) On reversing the terminals of the battery, the deflection of rod will be in opposite direction.
(ii) When the direction of current is perpendicular to the direction of magnetic field, then the rod will experience maximum force.
9. (i) What is meant by a magnetic field? Mention two parameters that are necessary to describe it completely. (ii) If field lines of a magnetic field are crossed at a point, what does it indicate?
Ans: (i) The space around the magnet or current carrying conductor within which its influence can be felt by the magnetic substance is known as magnetic field. Magnitude and direction.
(ii) It would mean that at the point of intersection, compass needle would point to two directions which is impossible.
10. Name, state and explain with an example the rule used to determine the direction of force experienced by a current carrying conductor placed in a uniform magnetic field.
Ans: Fleming’s Left Hand Rule: The direction of force which acts on the current carrying conductor placed in a magnetic field is given by Fleming’s left hand rule. It states that if the forefinger, thumb and middle finger of left hand are stretched mutually perpendicular and the forefinger point along the direction of external magnetic field, middle finger indicates the direction of current, then thumb points along the direction of force acting on the conductor. Example: When an electron enters a magnetic field at right angles, the direction of force on electron is perpendicular to the direction of magnetic field and current according to this rule.
11. Draw a diagram to show the magnetic field lines around a bar magnet. List any two properties of magnetic field lines.
Ans: (i) Two magnetic field lines never interest each other. (ii) Outside the magnet, the magnetic field lines are directed from North pole of magnet towards South pole.
12. Explain whether an alpha particle will experience any force in a magnetic field if: (i) it is placed in the field at rest. (ii) it moves in the magnetic field parallel to field lines. (iii) it moves in the magnetic field perpendicular to field lines.
Ans: (i) No, because, a charged particle at rest does not interact with magnetic field.
(ii) No, because, the force is zero if current and field are in the same direction.
(iii) Yes, because, the force is maximum when current and magnetic field are maximum.
13. (a) Magnetic field lines of two bar magnets A and B are as shown below. Name the poles of the magnets facing each other.
(b) Two magnetic field lines never intersect each other. Why? (c) How does the strength of the magnetic field at the centre of a current carrying circular coil depend on the (i) radius of the coil, (ii) number of turns in the coil, and (iii) strength of the current flowing in the coil?
Ans: (a) North poles.
(b) Intersection of magnetic field lines at a point means that the compass needle would point towards two directions at that point, which is not possible.
(c) (i) Inversely proportional; more radius less strong magnetic field.
(ii) Directly proportional; more turns more strong magnetic field.
(iii)Directly proportional; more strength of current more strong magnetic field.
14. Two coils of insulated copper wire are wound over a non conducting cylinder as shown. Coil I has
larger number of turns.
(i) Write your observations when, (a) key K is closed; (b) key K is opened;
(ii) Give reason for your observations.
(iii) Mention the name of the phenomenon involved and define it.
(iv) Name of two coils used in this experiment.
(v) State the rule which gives the direction of induced current.
Ans: (i) (a) A momentary deflection is shown by the galvanometer. (b) A momentary deflection is shown by the galvanometer but in the opposite direction.
(ii) When key is closed or opened, the current in the coil I changes, therefore the magnetic field linked with coil II changes and hence a current is induced in it.
(iii) Electromagnetic induction: The process by which a changing magnetic field in a conductor induces current in another conductor.
(iv) Primary coil coil I Secondary coil : coil II
(v) Fleming’s Right Hand Rule: Stretch the first three fingers of the right hand mutually perpendicular to each other such that the forefinger gives the direction of magnetic field and the thumb points in the direction of the motion of a conductor then, the middle finger will give the direction of the induced current.
15. Describe briefly an activity to: (i) demonstrate the pattern of magnetic field lines around a straight current carrying conductor and,
(ii) find the direction of magnetic field produced for a given direction of current in the conductor. Name and state the rule to find the direction of magnetic field around a straight current carrying conductor. Draw a diagram to explain the same activity.
Ans: (i) (a) Take a battery (12V), a variable resistance, an ammeter (0 - 5A), a plug key and a long straight thick copper wire.
(b) Insert the thick wire through the centre, normal to the plane of a rectangular cardboard.
(c) Connect the copper wire vertically between X and Y as shown in series with battery, a plug key and a rheostat.
(d) Sprinkle some iron fillings uniformly on the cardboard and keep the variable of the rheostat at a fixed position and note the current through the ammeter.
(e) Close the key so that a current flows through the wire. Gently tap the cardboard a few times. Observe the pattern of the iron filings.
(ii) To find the direction of magnetic field lines, place a compass at a point P over a circle
and observe the direction of the needle. The direction of North Pole of the compass would give the direction of magnetic field lines. Maxwell’s Right Hand Thumb Rule is used to find the direction of magnetic field lines. It states that “Imagine you are holding a current carrying conductor in your right hand such that the thumb points in the direction of current. Then your fingers will wrap around the conductor in the direction of the magnetic field lines.”
16. A domestic electrical appliance requires alternating current of 15 V If 220 V of
alternating current is supplied to the house, then the device that helps in the functioning of that electrical appliance is: [SSLC April, 2016]
(a) induction coil (b) step-up transformer (c) AC dynamo (d) step-down transformer
Ans: (a) Induction coil
17. In Fleming’s right hand rule, middle finger indicates the direction of: [SSLC April, 2016]
(a) magnetic field (b) induced electric current
(c) mechanical energy (d) motion of the conductor
Ans: (b) induced electric current
18. Name the device used to prevent damage to the electrical appliances and the domestic circuit due to overloading.
Ans: Electric fuse.
19. Draw the diagram of an electric motor and label the following parts. (i) Split rings (b) Brushes [SSLC July, 2018-19]
Ans:
20. Mention the provision of two different current ratings in our domestic circuits. Explain with reason, the advantage of such a provision.
Ans: The two different current ratings provided in domestic circuits are 5A and 15A. This is because different electrical appliances have different power ratings, so they draw different currents when connected in the mains. Some appliances need smaller currents, while some other need heavy currents.
21. State Faraday’s laws of electromagnetic induction. [SSLC June, 2016]
Ans: I Law: Whenever a magnetic field linked with a conductor changes, an induced e.m.f. of is generated in the conductor. II Law: The magnitude of induced e.m.f of is directly proportional to the rate of change of magnetic field.
22. Distinguish between alternating current and direct current. Explain why alternating current is preferred over direct current for transmission over long distances.
Ans: Alternating Current: If the current changes direction after equal intervals of time, it is called alternating current. The positive and negative polarities of AC are not fixed. Direct Current: If the current always flows in the same direction, it is called direct current. It can be obtained from a cell or a battery. The positive and negative polarities of DC are fixed for long distance transmission. AC is preferred as it causes minimum loss of energy during transmission.
23. 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.
Ans: When a force conductor carrying current is placed perpendicular to the direction of magnetic field, the magnetic field 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.
24. (i) Define electromagnetic induction. [SSLC July, 2018-19]
(ii) Two coils P and S are wound over the same iron core. Coil P is connected to battery and key and the coil S is connected to galvanometer. Write your observations when:
(a) Current in the coil P is started by closing the key.
(b) Current continues to flow in coil P.
(c) Current in coil P is stopped by removing the key. Explain the reason for such observation.
Ans: (i) Electromagnetic Induction: The process by which a change in magnetic field in a conductor induces a current in another conductor.
(ii) (a) There is deflection in galvanometer connected with coils, due to induced current. (b) Deflection becomes zero. (c)Deflection in galvanometer is in opposite direction. Reason:
(i) E.M. induction takes place because field is changing.
(ii)Since current becomes stationary, no change in field takes place. So no EM induction.
(iii) E.M. induction takes place but induced current is in opposite direction.
25. (i) Explain what is the difference between a direct current and an alternating current. Write one important advantage of using alternating current. (ii) An air conditioner of 2kW is used in an electric circuit having a fuse of 10A rating. If the potential difference of the supply is 220 V, will the fuse be able to withstand, when the air conditioner is switched on? Justify your answer.
Ans: (i) The current whose direction gets reversed after every half cycle is called an alternating current or AC. There is no change in the direction of DC. The most important advantage of using AC over DC is that in the AC mode electric power can be transmitted over long distances with less loss of power. (ii) Here P = 2kW = 2000W, V = 220 Volt P 2000 P = VI, the current I = V = 220 = 9.09 A As the current is 9.09 A, below the rating of fuse, the fuse will withstand i.e., it will not blow off when AC is on.
26. (i) The given figure shows a domestic electric circuit. Study this circuit carefully. List any two errors in the circuit and justify your answer.
(ii) Give one difference between the wires used in the element of an electric heater and in a fuse.
(iii) List two advantages of parallel connection over series connection.
Ans: (i) Two errors are: (a) Fuse is incorrectly connected to the neutral wire (N), it must be connected to the live wire (L).
(b) Bulb B2 is not connected to the neutral, wire.
(c) Two switches S1 anl S2 in both B1 circuit and no switch in bulb B2 circuit.
(ii) Element of electric heater - high melting point Element of fuse wire - low melting point
(iii) (a) Each appliance has equal potential difference. (b) Each appliance has separate switch to ON/OFF the flow of current through it.
27. Why don’t two magnetic lines of force intersect each other?
Ans: Magnetic field lines do not intersect each other because there can’t be two directions of magnetic field at any one point.
28. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.
Ans: Current in the coil is clockwise. Using right hand thumb rule at every point of the wire, the direction of magnetic field inside the loop is found to be into the plane of table and outside the loop as out from the plane of table.
29. An electric oven of 2 KW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.
Ans: P = 2KW = 2000W, V 220V P = VI P 2000 I = V = 220 = 9.091 A The circuit will break and the fuse will blow off.
30. What precaution should be taken to avoid the overloading of domestic electric circuits?
Ans: Provide fuses with proper rating and do not connect appliances exceeding the total load capacity of the circuit.
Textbook Exercises:
1. Which of the following correctly describes the magnetic field near a long straight wire? (a) The field consists of straight lines perpendicular to the wire. (b) The field consists of straight lines parallel to the wire. (c) The field consists of radial lines originating from the wire. (d) The field consists of concentric circles centred on the wire.
Ans: (d) The field consists of concentric circles centred on the wire.
2. The phenomenon of electromagnetic induction is (a) The process of charging a body. (b) The process of generating magnetic field due to a current passing through a coil. (c) Producing induced current in a coil due to relative motion between a magnet and the coil. (d) The process of rotating a coil of an electric motor.
Ans: (c) Producing induced current in a coil due to relative motion between a magnet and the coil.
3. The device used for producing electric current is called a (a) generator (b) galvanometer (c) ammeter (d) motor.
Ans: (c) ammeter
4. The essential difference between AC generator and a DC generator is that (a) AC generator has an electromagnet while a DC generator has permanent magnet (b) DC generator will generator will generate a higher voltage (c) AC generator will generate a higher voltage (d) AC generator has slip rings while the D.C generator has a commutator.
Ans: (d) AC generator has slip rings while the DC generator has a commutator.
5. At the time of short-circuit, the current in the circuit (a) reduces substantially (b) does not change (c) increases heavily (d) varies continuously
Ans: (c) increases heavily.
6. State whether the following statements are true of false. (a) An electric motor converts mechanical energy into electrical energy: (b) An electric generator works on the principle of electromagnetic induction. (c) The field at the centre of a long circular coil carrying current will be parallel straight lines. (d) A wire with a green insulation is usually the live wire.
Ans: (a) False: An electric motor converts electric energy into mechanical energy. (b) True (c) True (d) False: A wire with a green insulation is usually the earth wire.
7. List two methods of producing magnetic fields.
Ans: Magnetic field can be produced by (i) a natural magnet (ii) a current carrying conductors
8. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current solenoid with the help of a bar magnet? Explain.
Ans: Current carrying solenoid behaves as a magnet. The end in which the current is anti clockwise acts as north pole and other end in which current is clockwise acts as south pole.
Suspend a current carrying solenoid. Bring North Pole of the bar magnet towards one end of the solenoid. If the solenoid is attracted towards its north pole then this end of solenoid is south pole and vice-versa.
9. When is the force experienced by a current – carrying conductor, placed in a magnetic field largest?
Ans: When the conductor is placed perpendicular to the direction of magnetic field.
10. Imagine that you are siffing in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Ans: Vertically downwards.
11. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?
Ans: Principle of Electric Motor: 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 then force acting on the conductor will be perpendicular to both and will be given by Fleming’s left-hand rule. Due to this force the conductor begins to move.
Working of Electric Motor: Current in the coil ABCD enters form the source battery through conducting brush X and flows back to the battery through brush Y. The current in arm AB of the coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction of current through arm AB. We find that the force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards. Thus the coil and the axle 0, mounted free turn about an axis, rotate anticlockwise. At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current in the coil gets reversed and flows along the path DCBA. The reversal of current also reverses the direction of force acting on the two arms AB and CD. Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up is now pushed down. Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.
Role of split ring: To change the direction of current flowing through the coil after each half rotation.
12. Name some devices in which electric motors are used.
Ans: Electric fans, water-pumps, coolers, refrigerators, mixers, blenders, washing machines.
13. A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?
Ans: (i) When a bar magnet is pushed into the coil of insulated copper wire connected to a galvanometer, an induced current is set-up in the coil due to change of magnetic field through it. As a result, galvanometer gives a deflection (say towards left). (ii) When the bar magnet is withdrawn from inside the coil, again an induced current is set up in the coil due to change of magnetic field through it. As a result galvanometer gives a deflection in the reverse direction. (say towards right). (iii) If the bar magnet is held stationary inside the coil, then there is no induced current in the coil, because there is no change in magnetic field through it. As a result, galvanometer does not show any deflection.
14. 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 the coil B? Give reason.
Ans: Due to change in magnetic field, an electric current is induced in coil B.
15. State the rule to determine the direction of a (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 and (iii) current induced in a coil due to its rotation in a magnetic field.
Ans: (i) Right hand thumb rule (ii) Fleming’s left hand rule (iii) Fleming’s right hand rule
16. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
Ans: An electric generator works on the principle of electromagnetic induction. According to it
, whenever a coil is rotated between the poles of a magnet, an induced current is set up in the coil, whose direction is given by Fleming’s right-hand rule.
Working: When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet. The coil ABCD is rotated clockwise in the arrangement. By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD. Thus an induced current flows in the direction ABCD. If there are larger numbers of turns in the coil, the current generated in each turn adds up to give a large current through the coil. This means that the current in the external circuit flows from B2 to B1. After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction DCBA. The current in the external circuit now flows from B1 to B2. Thus after every half rotation the polarity of the current in the respective arms changes. Such a current, which changes direction after equal intervals of time, is called an alternating current (abbreviated as AC). This device is called an AC generator. Function of brushes: It helps in transferring current from the coil ABCD to the external circuit.
17. When does an electric short circuit occur?
Ans: When live wire and neutral wire may come in direct contact or insulation of wires used in an electrical circuit is damaged or there is a fault in the appliance, then the current in the circuit abruptly rises and short-circuiting occurs.
18. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Ans: The metallic body of electric appliances is connected to the earth by means of earth wire so that any leakage of electric current is transferred to the ground. This prevents any electric shock to the user.
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