MAGNETIC EFFECTS OF ELECTRIC CURRENT NCERT SOLUTIONS
Page 224
Q.1 Why does a compass needle get deflected when brought near a bar magnet?
Sol. When a compass needle is brought near a bar magnet, the compass needle experiences a deflection. This happens because of interaction of magnetic fields of the compass needle and the bar magnet.
Page 228
Q.1 Draw magnetic field lines around a bar magnet.
Sol. Magnetic field line
Q.2 List the properties of magnetic lines of force.
Sol. Properties of magnetic field lines:
(a) Magnetic field lines follow the direction from the north pole to the south pole.
(b) Magnetic field lines always show concentric pattern.
(c) Magnetic field lines do not cross one another.
(d) Closer the field lines; stronger is the magnetic field and vice-versa is also true.
(e) Magnetic field lines are closer near the poles; which shows greater strength of magnetic field near the poles.
Q.3 Why don’t two magnetic lines of force intersect each other?
Sol. The direction of magnetic field lines is always from the north pole to the south pole. If the field lines would cross each other then the direction of field line would change at the point of crossing which is practically impossible. Hence, two magnetic field lines never intersect each other.
Page 229
Q.1 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.
Sol. As the given figure shows; current is flowing clockwise through a circular loop. The direction of magnetic field around the conductor can be known by using the right hand thumb rule. As the figure shows, the magnetic field would be towards the plane of the paper when it is inside the loop. On the other hand, the magnetic field would be away from the paper when it is outside the loop.
Q. 2 The magnetic field in a given region is uniform. Draw a diagram to represent it.
Sol.
Page 230
Q.1 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.
Sol. (d) Is the same at all points.
Page 231
Q.2 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?
Sol. In this case, the displacement would vary directly as the strength of current, strength of magnetic field and length of the conductor. Due to this, the displacement of conductor would be increased in all the three cases.
Q.3 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
Sol. This question can be solved by using Fleming’s Left Hand Rule. We know that the direction of current is opposite to the direction of electron’s movement and hence it would be same as the direction of proton’s movement. So, the direction of current is towards west. As per Fleming’s Left Hand Rule; the middle finger shows the direction of current, the forefinger shows the direction of magnetic field and the thumb shows the direction of motion. Here, the deflection is towards north, i.e. in north westerly direction and hence, the direction of magnetic field would be towards north, i.e. upward.
Page 233
Q.1 State Fleming’s Left Hand Rule.
Sol. Fleming’s Left Hand Rule states that if the left hand is stretched in a way that the index finger, the middle finger and the thumb are in mutually perpendicular directions; then the index finger and middle finger of a stretched left hand show the direction of magnetic field and direction of electric current respectively and the thumb shows the direction of motion or force acting on the conductor. The directions of electric current, magnetic field and force are similar to three mutually perpendicular axes, i.e. x, y and z axes.
Q.2 What is the principle of an electric motor?
Sol. Principle of Electric Motor: The electric motor works on the principle of Fleming’s Left Hand Rule. When a rectangular coil is placed within a magnetic field and current is passed through the coil, there is deflection in the coil. The deflection changes into rotation of coil because of split ring commutator in the motor.
Q.3 What is the role of a split ring in an electric motor?
Sol. In an electric motor, after every half rotation the direction of coil gets reversed due to change in orientation of the magnetic field. To ensure a continuous rotation; a split ring is attached to the coil so that the polarity of the coil changes after every half rotation. This changes the direction of current and thus the armature keeps on rotating continuously.
Page 236
Q.1 Explain different ways to induce current in a coil.
Sol. For electromagnetic induction; the coil and the magnet should be in relative motion. This can be ensured by any of the following two ways:
(a) The coil should be moved within a magnetic field.
(b) The magnet should be moved and coil can be kept static.
Page 237
Q.1 State the principle of an electric generator.
Sol. Electric Generator works on the principle of electromagnetic induction which obeys Fleming’s Right Hand Rule. When coil is moved inside a magnetic field a current is induced in the coil. The electric current is thus ‘generated’ by electric generator.
Q.2 Name some sources of direct current.
Sol. Electrochemical cell, DC generator, photovoltaic cell, etc.
Q.3 Which sources produce alternating current?
Sol. AC generator
Q.4 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
Sol. (c) Half revolution
Page 238
Q.1 Name two safety measures commonly used in electric circuits and appliances.
Sol. Earth wire and electric fuse
Q.2 An electric oven of 2 kW is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What results do you expect? Explain.
Sol. The current drawn by the electric oven can be calculated using following formula:
P=VI Or , I=PV
We have; P = 2kW = 2000 W and V = 220 V
So, I=2000w220V=9.09A
Here, the oven is drawing 9.09 ampere of current from a 5 ampere source.
This means there would be an overload on the circuit. This can result in accidental fire in the circuit.
Q.3 What precaution should be taken to avoid the overloading of domestic electric circuits?
Sol. Precautions to avoid overloading of domestic electric circuit:
(a) Avoid using too many appliances in a single line, i.e. from a single output.
(b) Appliances should always be checked for potential faults and should be repaired in time.
(c) Fuse or MCB of proper rating should be used.
Back Exercise Questions
Q.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 centered on the wire.
Sol. (d) The field consists of concentric circles centered on the wire.
Q.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.
Sol. (c) Producing induced current in a coil due to relative motion between a magnet and the coil.
Q.3 The device used for producing electric current is called a
(a) Generator.
(b) Galvanometer.
(c) Ammeter.
(d) Motor.
Sol. (a) Generator
Q.4 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.
Sol. (d) AC generator has slip rings while the DC generator has a commutator.
Q.5 At the time of short circuit, the current in the circuit
(a) Reduces substantially.
(b) Does not change.
(c) Increases heavily.
(d) Vary continuously.
Sol. (c) Increases heavily
Q.6 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.
Sol. (a) F (b) T (c) T (d) F
Q.7 List three methods of producing magnetic fields.
Sol. Three methods of producing magnetic fields are as follows:
(a) By permanent magnet
(b) By electromagnet
(c) By current carrying conductors
Q.8 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.
Sol. A solenoid begins behaving like a magnet when electric current flows through it. We know that any current carrying conductor creates a magnetic field around it and that is what happens in case of solenoid. For determining the different poles of a solenoid, we can use a bar magnet and look for interaction between different poles of two magnets. If the north pole of the bar magnet gets repulsed by a particular pole of the electromagnet (solenoid) then it gets confirmed that the bar magnet was brought near the north pole of the electromagnet.
Q.9 When is the force experienced by a current–carrying conductor placed in a magnetic field largest?
Sol. From Fleming’s Left Hand Rule, it is clear that when the direction of current and magnetic field are in mutually perpendicular directions, the deflection in conductor is the maximum. This shows that when magnetic field and direction of current are perpendicular to each other, the force experienced by the conductor is the largest.
Q.10 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?
Sol. Here, the electron beam is moving towards the viewer, i.e. out of the plane of the page. This means that the direction of current is towards the page. This shows the direction in which the forefinger is pointing. The thumb is pointing towards right which is same as the direction of deflection. The middle finger is pointing downwards; which shows the direction of the magnetic field.
Q.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?
Sol. Working of Electric Motor:
Electrical energy is converted into mechanical energy by using an electric motor. Electric motor works on the basis of rule suggested by Marie Ampere and Fleming’s Left Hand Rule. In an electric motor, a rectangular coil is suspended between the two poles of a magnetic field. The electric supply to the coil is connected with a commutator. Commutator is a device which reverses the direction of flow of electric current through a circuit. When electric current is supplied to the coil of electric motor, it gets deflected because of magnetic field. As it reaches the half way, the split ring which acts as commutator reverses the direction of flow of electric current. Reversal of direction of current reverses the direction of forces acting on the coil. The change in direction of force pushes the coil; and it moves another half turn. Thus, the coil completes one rotation around the axle. Continuation of this process keeps the motor in rotation. In commercial motor, electromagnet; instead of permanent magnet; and armature is used. Armature is a soft iron core with large number of conducting wire turns over it. Large number of turns of conducting wire enhances the magnetic field produced by armature.
Q.12 Name some devices in which electric motors are used.
Sol. Electric fan, mixer grinder, tape recorder, CD player, hard disk drive, washing machine, cooler, toy car, vacuum cleaner, etc. are some devices in which electric motor is used.
Q.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?
Sol. When the bar magnet is pushed into the coil or withdrawn from the coil; the galvanometer needle would show deflection. When the bar magnet is kept stationary inside the coil; the galvanometer needle would show no deflection.
Q.14 Two circular coils A and B are placed close to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.
Sol. When two circular coils A and B are placed close to each other and the current in coil A is changed, it leads to induction of current in coil B. This happens because of change in magnetic field of coil A; because of change in current in this coil.
Q.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.
Sol. (a) Right hand thumb rule or Maxwell’s corkscrew rule,
(b) Fleming’s Left Hand Rule and
(c) Fleming’s Right Hand Rule
Q.16 Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
Sol.
The structure of electric generator is similar to that of an electric motor. In case of an electric generator a rectangular armature is placed within the magnetic field of a permanent magnet. The armature is attached to wire and is positioned in way that it can move around an axle. When the armature moves within the magnetic field an electric current is induced.The direction of induced current changes, when the armature crosses the halfway mark of its rotation. Thus, the direction of current changes once in every rotation. Due to this, the electric generator usually produces alternate current, i.e. AC.To convert an AC generator into a DC generator, a split ring commutator is used. This helps in producing direct current.
Q.17 When does an electric short circuit occur?/
Sol. When positive and negative wires touch each other, the resistance suddenly decreases and current increases. This leads to excessive heating of wire which manifests in the form of sparks. This is called short circuit.
Q.18 What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Sol. The earth wire transfers any leakage of electric current to the earth. The leaked current can otherwise reach the metallic body of an appliance and can lead to electric shock. Earth wire prevents from electric shock by safety transferring the leaked current to the earth.
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Class 10