 # Chapter 13 Magnetic effects of Electric Current

Electricity and magnetism were discovered independently but both are closely related. Magnets are commonly used nowadays in preparing floppies, credit cards, ATM, toys etc. In the field of medicine magnetic device i.e. Magnetic resonance imaging scanners are used to diagnose the patients. In this chapter we will study about magnets, its type, magnetic field, magnetic field line, electromagnet, electric motor, electric generator and many more.

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### Exercise 1 ( Page No. : 224 )

•  Q1 Why does a compass needle get deflected when brought near a bar magnet? Ans: Compass needle is a bar magnet when it is brought near a bar magnet, its magnetic field interacts with the magnetic field line of the bar magnet. Therefore deflection occurs in the compass needle.

### Exercise 2 ( Page No. : 228 )

•  Q1 Draw magnetic field lines around a bar magnet. Ans: The region surrounding a magnet in which the force of a magnet can be detected is said magnetic field. On putting magnet filings around magnet the arrange in pattern representing magnetic field line. Magnetic field lines of a bar magnet emerge from the north pole and merge at south pole. Inside the magnet the direction of the field line is from south pole towards north pole. Q2 List the properties of magnetic field lines. Ans: The properties of magnetic field lines are:  (a) Magnetic field lines emerge from the north pole. (b) They merge at the south pole. (c) The direction of field lines inside the magnet is from the south pole to the north pole. (d) Magnetic lines do not intersect with each other. Q3 Why don’t two magnetic field lines intersect each other? Ans: Two magnetic lines never intersect because if two field lines of a magnet intersect, then at the point of intersection, the compass needle points in two different directions which is not possible at same time a needle shows to different directions. Hence, they never interact.

### Exercise 3 ( Page No. : 230 )

•  Q1 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: Inside the loop = Pierce inside the table Outside the loop = Appear to emerge out from the table For the downward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop. Similarly, for the upward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop, as shown in the given figure. Q2 The magnetic field in a given region is uniform. Draw a diagram to represent it. Ans: The magnetic field lines inside a current-carrying long straight solenoid are uniform. Q3 Choose the correct option. The magnetic field inside a long straight solenoid-carrying current (a) is zero. (b) decreases as we move towards its end. (c) increases as we move towards its end. (d) is the same at all points. Ans: (d) The magnetic field inside a long, straight, current-carrying solenoid is uniform. Therefore, it is the same at all points inside the solenoid.

### Exercise 4 ( Page No. : 232 )

•  Q1 Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.)   (a) mass                  (b) speed   (c) velocity               (d) momentum Ans: (c) and (d) When a proton enters an area of magnetic field, it experiences a magnetic force. As a result of the force, the path of the proton becomes circular. Therefore, its velocity and momentum change. Q2 In Activity 13.7, how do we think the displacement of rod AB will be affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased? Ans: A current-carrying conductor placed in a magnetic field experiences a force. The magnitude of force increases with the amount of current, strength of the magnetic field, and the length of the conductor. Shuffle, the magnetic force exerted on rod AB and its deflection will increase if i. current in rod AB is increased i.) a stronger horse-shoe magnet is used iii. length of rod AB is increased Q3 A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is    (a) towards south                 (b) towards east    (c) downward                       (d) upward Ans: (d) The direction of the magnetic field can be determined by applying Fleming’s left hand rule. According this rule, if we arrange the thumb, the centre finger, and the forefinger of the left hand at right angles to each other, then the thumb points towards the direction of the magnetic force, the centre finger gives the direction of current, and the forefinger points in the direction of magnetic field. As the direction of a positively charged alpha particle is towards west, the direction of current will be the same i.e., towards the west. If the direction of magnetic force is towards the north. Hence, according to Fleming’s left hand rule, the direction of the magnetic field will be upwards.

### Exercise 5 ( Page No. : 233 )

•  Q1 State Fleming’s left-hand rule. Ans: Fleming’s left hand rule states that if we arrange the thumb, the centre finger, and the forefinger of the left hand at right angles to each other, then the thumb points towards the direction of the magnetic force, the centre finger gives the direction of current, and the forefinger points in the direction of magnetic field. Q2 What is the principle of an electric motor? Ans: The principle of an electric motor is based on the magnetic effect of current. A current-carrying loop experiences a force and rotates when placed in a magnetic field. The direction of rotation of the loop is determined by Fleming's left-hand rule. Q3 What is the role of the split ring in an electric motor? Ans: The split ring in the electric motor is a commutator. The commutator reverses the direction of current flowing through the coil after each half rotation of the coil. Due to this direction of current getting reverse, the coil continues to rotate in the same direction.

### Exercise 6 ( Page No. : 236 )

•  Q1 Explain different ways to induce current in a coil. Ans: The different ways to induce current in a coil are as following: a. If a magnet is moved relative to a coil, then an electric current is induced in the coil. b. If a coil is moved rapidly between the two poles of a horse-shoe magnet, then an electric current is induced in the coil.

### Exercise 7 ( Page No. : 237 )

•  Q1 State the principle of an electric generator. Ans: The principle of an electric generator is electromagnetic induction. It generates electricity by rotating a coil applied in a magnetic field. Q2 Name some sources of direct current. Ans: DC generator is a source of direct current. Q3 Which sources produce alternating current? Ans: AC generators, power plants are sources that produce alternating current. Q4 Choose the correct option. A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced current changes once in each        (a) two revolutions         (b) one revolution        (c) half revolution           (d) one-fourth revolution Ans: (c) When a rectangular coil of copper is rotated in a magnetic field, the direction of the induced current in the coil changes once in each half revolution. Therefore, the direction of current in the coil remains the same.

### Exercise 8 ( Page No. : 238 )

•  Q1 Name two safety measures commonly used in electric circuits and appliances. Ans: Two safety measures commonly used in electric circuits and appliances are as follows: i. Electric fuse. It prevents the flow of excessive current through the circuit. When the current passing through the wire exceeds the maximum limit of the fuse element, the fuse melts to stop the flow of current through that circuit, hence protecting the appliances connected to the circuit. ii. Earthing is a must to prevent electric shocks. Any leakage of current in an electric appliance is transferred to the ground and people using the appliance do not get the shock. Q2 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: We can calculate current drawn by electric oven is Given - Power = 2kW = 2000 W Potential difference of 220 W by applying the formula P = VI By putting the value = Q3 What precaution should be taken to avoid the overloading of domestic electric circuits? Ans: The precautions that should be taken to avoid the overloading of domestic circuits are as follows: a. Different appliances should not be used at the same time. b. Different appliances should not be connected to a single socket. c. Fuse should be connected in the circuit. d. Faulty appliances should not be connected in the circuit.

### Exercise 9 ( Page No. : 240 )

•  Q1 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 magnetic field lines, produced around a straight current-carrying conductor, are concentric circles. Their centres lie on the wire. Q2 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) When a straight coil and a magnet are moved relative to each other, a current is induced in the coil. This phenomenon is known as electromagnetic induction. Q3 The device used for producing electric current is called a (a) generator. (b) galvanometer. (c) ammeter. (d) motor. Ans: (a) Generator is the device used to produce electric current. Q4 The essential difference between an 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 generate a higher voltage. (c) AC generator will generate a higher voltage. (d) AC generator has slip rings while the DC generator has a commutator. Ans: (d) The main difference is an AC generator has two rings called slip rings. A DC generator has two half rings called commutator. Q5 At the time of short circuit, the current in the circuit (a) reduces substantially. (b) does not change. (c) increases heavily. (d) vary continuously. Ans: (c) At the time of short circuit the current increase heavily, when two naked wires of an electric touch each other the amount of current that is flowing in the curcuit increasse. Q6 State whether the following statements are true or 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 of an electric supply. Ans: (a) False As an electric motor converts electrical energy into mechanical energy. (b) True A generator is a device that generates electricity by rotating a coil in a magnetic field. It works on the principle of electromagnetic induction. (c) True A long circular coil is a long solenoid. The magnetic field lines inside the solenoid are parallel lines. (d) False Red insulation covers the live wire whereas green colour. Q7 List two methods of producing magnetic fields. Ans: Three sources of magnetic fields are: a. Electromagnets b. Current-carrying conductors c. Permanent magnets Q8 How does a solenoid behave like a magnet? Can you determine the north and south poles of a current–carrying solenoid with the help of a bar magnet? Explain. Ans: A solenoid is a long coil of circular loops of insulated copper wire. Magnetic field lines are produced around the solenoid when a current is allowed to flow through it. The field lines produced in a current-carrying solenoid is shown magnetic field line emerges from North pole towards South pole, whereas inside the solenoid magnetic field lines parallel. When the north pole of a bar magnet is brought near the end to the negative terminal of the battery, the solenoid repels the bar magnet as like poles repel each other, the end connected to the negative terminal of the battery behaves as the north pole of the solenoid and the other end behaves as a south pole. So, one end of the solenoid behaves as a north pole and the other end behaves as a south pole. Q9 When is the force experienced by a current–carrying conductor placed in a magnetic field largest? Ans: When the direction of current is perpendicular to the direction of magnetic field. The force experienced by the current carrying conductor is maximum. Q10 Imagine that you are sitting 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: The direction of the magnetic field is determined by Fleming’s left hand rule. Magnetic field inside the chamber will be perpendicular to the direction of current and direction of deflection either upward or downward. The direction of current is from the front wall to the back wall because negatively charged electrons are moving from the back wall to the front wall. The direction of magnetic force is rightward. By using Fleming’s left hand rule, it can be concluded that the direction of magnetic field inside the chamber is downward. Q11 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: An electric motor converts electrical energy into mechanical energy. Electric motor is based on the principle of the magnetic effect of current. A current-carrying coil rotates in a magnetic field. When a current is allowed to flow through the coil MNST by closing the switch, the coil starts rotating anti-clockwise. This happens because a downward force acts on length MN and at the same time, an upward force acts on length ST. As a result, the coil rotates anti-clockwise. Current in the length MN flows from M to N and the magnetic field acts from left to right, normal to length MN. Therefore, according to Fleming’s left hand rule, a downward force acts on the length MN. Similarly, current in the length ST flows from S to T and the magnetic field acts from left to right, normal to the flow of current. Therefore, an upward force acts on the length ST. These two forces cause the coil to rotate anti-clockwise. After half a rotation, the position of MN and ST interchange. The half-ring D comes in contact with brush A and half-ring C comes in contact with brush B. Hence, the direction of current in the coil MNST gets reversed. The current flows through the coil in the direction TSNM. The reversal of current through the coil MNST repeats after each half rotation. As a result, the coil rotates unidirectional. The split rings help to reverse the direction of current in the circuit. These are called the commutator. Q12 Name some devices in which electric motors are used. Ans: The devices in which electric motors are used are as follows: a. Electric mixers b. Water pumps c. Washing machines d. Electric fans Q13 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: A current induces in a solenoid if a bar magnet is moved relative to it. This is the principle of electromagnetic induction. (i) When a bar magnet is pushed into a coil of insulated copper wire, a current is induced in the coil. As a result of which the needle of the galvanometer deflects direction of current. (ii) When the bar magnet is withdrawn from inside the coil of the insulated copper wire, a current is again induced in the coil in the opposite direction. As a result, the needle of the galvanometer deflects in the opposite direction. (iii) When a bar magnet is held stationary inside the coil, no current will be induced in the coil. So, no deflection in the galvanometer. Q14 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: Two circular coils A and B are placed close to each other. When the current in coil A is changed, the magnetic field associated with it also changes. As a result, the magnetic field around coil B also changes. This change in magnetic field lines around coil B induces an electric current in it. This is called electromagnetic induction. Q15 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, is applied to determine the magnetic field produced around a straight conductor carrying current. (ii) Fleming’s left hand rule, determine the direction of force experienced by a current-carrying straight conduct placed in a magnetic field which is perpendicular to it. (iii) Fleming’s right hand rule, determine the direction of current induced in a coil due to its rotation in a magnetic field. Q16 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 is an electric device used to convert mechanical energy (kinetic energy) into electrical energy (electricity). Principle: Electric generator based on the principle of an electromagnetic induction. When the coil of an electric generator rotates in a magnetic field it induces the current flows in the circuit connected with the coil. There are two types of electric generator: i. AC generator ii. DC generator AC generator: AC generator is a device that converts mechanical energy into electrical energy in the form of alternating current or AC. DC generator: DC generator is a device that converts mechanical energy into electrical energy in the form of direct current or AC. AC Generator Construction: The components of AC generator are: Armature: Large number of turns of insulated copper wire wound over a soft iron core form  armature coil (ABCD). Strong field magnet: A strong permanent magnet or an electromagnet whose poles are cylindrical in shape is a field magnet. The armature coil rotates between the poles. The uniform magnetic field is provided by the field magnet perpendicular to the axis of rotation of the coil. Slip Rings: Two brass slip rings R1 and R2 are connected to the two ends of the armature coil. Rings rotate with the armature coil. Rings R1 and R2 are at different heights. Brushes: These brushes are connected to the external circuit across which the output is obtained. Two carbon brushes (B1 and B2), are pressed against the slip rings. The brushes are fixed while slip rings rotate along with the” armature. Working: The armature coil ABCD when rotates in the magnetic field that is provided by the strong field magnet, it cuts the magnetic field lines.therefore, the change in  magnetic field produces induced current in the coil. The direction  of the induced current in the coil is determined by Fleming's right hand rule. The current flows out through the brush B1 and brush B2 is in the opposite direction. The brush B1 in one direction in the first half of the revolution and by the brush B2 in the next half revolution in the opposite direction. This process is repeated several times. Hence, induced current produced is of alternating current. DC generator or Dynamo Construction: Armature coil. Large number of turns of insulated copper wire wound over iron core in a form of a rectangle coil. Strong field magnet. Strong field magnets provide a strong magnetic field. When an armature coil is kept between two pole pieces of a strong magnet. Split-ring Type Commutator consists of two halves (R1 and R2) of a metallic ring. Two halves of the ring are connected to the two ends of the armature coil. Carbon brushes B1 and B2 are pressed against the commutator. The output is shown by the glowing bulb connected across the carbon brushes. Working: When the coil of d.c. the generator rotates in the magnetic field, potential difference is induced in the coil. This induced potential difference gives rise to the flow of current through the bulb and therefore the bulb glows. In d.c. generator, current flows  in the circuit in the same direction for long as the coil rotates in the magnetic field. This is one brush is always in contact with the arm of the armature moving up and the other brush is in contact with the arm of the armature moving downward in the magnetic field. Q17 When does an electric short circuit occur? Ans: When neutral wire and live wire touch each other that they come in direct contact, the resistance of the circuit becomes small and a large amount of current flows through it. As a result of which, a large amount of heat is produced and the circuit catches fire. Q18 What is the function of an earth wire? Why is it necessary to earth metallic appliances? Ans: Earth wire is a safety measure that prevents short circuits and shock. When a live wire touches the metallic case of an electric gadget, the electric current flows from the casing of the appliance to the earth through the copper wire. As the earth offers very low or almost no resistance to the flow of current, so large current passes through the copper wire instead of the human body. Due to this large current heat is produced in the circuit and hence the fuse in the circuit melts. So, the circuit is switched off automatically and the electric appliance is saved from burning and no electric shock to the human body.