Class 10 Science Chapter 12 Board Questions

(a) Alternating current.
(b) Direct current.

Electric generator.

In Fleming’s right hand rule
Thumb indicates- direction of motion of the conductor.
Forefinger- direction of magnetic field.

Room heater and geyser.

The region surround a magnet where the force of attraction or repulsion by the magnet can be detected constitutes the magnetic field and is represented by magnetic field lines.

This can be shown by continuous decreases in the deflection of magnetic compass needle as it continuously shifts away from the current carrying conductor.

When steel bar is kept inside a current carrying solenoid along its axis, steel bar can be magnetised.

The magnetic field of a bar magnet is the region around the magnet in which force due to magnet can be felt.

The direction in which N pole of a small compass needle placed at that point sets itself is the direction of magnetic field at that point.

The strength and direction of magnetic field at the given point.

Near the poles of the magnet.

From N pole toward S pole outside the magnet and from S pole towards N pole inside a magnet.

Every current carrying conductor has a magnetic field around it.

A magnetic field is produced around a current carrying conductor.

Magnetic field lines forms concentric circles around the conductor with conductor at the center.

Strength of magnetic field is directly proportional to the strength of current flowing in the wire.

The strength of the magnetic field increases on increasing the current flowing through the conductor.

The direction of the magnetic field gets reserved on changing the direction of flow of current in a straight conductor.

At the center of current carrying circular loop.

Along the axis of solenoid.

A solenoid coil.

The force experienced by a current carrying conductor placed in a magnetic field is (i) maximum when direction of current is at right angles to the direction of magnetic field, (ii) minimum (zero) when direction of current is parallel or antiparallel to the direction of magnetic field.

A current carrying conductor produces a magnetic field around it. This magnetic field interacts with the externally applied magnetic field and as a result the conductor experiences a force.

A galvanometer is used to detect the flow of current, if any, in a circuit.

Electric current set up in a closed coil/circuit, whenever, magnetic field near it is changing, is called an induced current.

Whenever magnetic field around a coil is changed, a current is induced in the coil.

Thumb indicate the direction of motion of the conductor and forefinger indicates the direction of magnetic field.

An electric generator.

Magnitude of induced current in a secondary coil is directly proportional to the rate of change of current in a primary coil.

Direct current (D.C.).

An alternating current (A.C.).

50 Hz.

Change its direction in one second
= 50 × 2
= 100 times.

An electric generator.

Electric power in A.C. can be easily transmitted over long distances without much loss of energy.

Red for live wire insulation, black for neutral wire insulation and green for earth wire insulation.

Wire of green insulation cover is conventionally used for earth wire.

When an earth wire is connected to metallic parts of appliances, any leakage of current to the metallic parts is safely conducted to the ground. Thus, severity of shock received by a person on touching the metallic part is very much reduced.

The live wire.

With live wire.

Electric fuse.

AS a safety measure for the user person. User will not get a severe electric shock even in case of leakage, if proper earth wire has been installed.

It provides a safety measure to the user person and saves him from a severe shock even when there is leakage of current.

Short circuiting means that the two wire, live and neutral of the domestic electric circuit have common in contact with each other.

(i) Too many electrical appliances should not be operated using a single socket.
(ii) Too many large power rating appliances should not be switched on at a time.

(a) Observation: Deflection of needle increases
Reason: Magnetic field strength due to current carrying wire increases as current in wire increases.
(b) Observation: The deflection in the compass needle decreases as its displacement from the current carrying wire increases.
Reason: Strength of magnetic field reduces with the increase in the distance from the wire.

For earth wire green or yellow colour insulation is used. It provides the low resistance conducting path for the current and maintains the potential of appliances body with that of the earth. So earth wire is used as safety measures.

Role of fuse: It is a safety device connected in series with live wire or with any electrical appliances in an electric circuit. It stops the flow of unduly high electric current in the circuit by melting itself due to rise in temperature as per Joule’s law of heating High rating fuse wire has the larger capacity. So it will not stop the flow of any unduly high current. Therefore, electric devices cannot be protected from the possible damage.

Direction of current- east to west is determine by Right hand thumb rule.
Right Hand Thumb Rule: If we hold a current carrying conductor by right hand in such a way that the stretch thumb is along the direction of current then the curly fingers around the conductor represents the direction of current then the curly fingers around the conductor represents the direction of field lines of magnetic field.

(i) Deflection in the galvanometer needle will be more on right side.
(ii) Larger deflection in opposite direction as compared to the case (i) will be seen
(iii) No deflection.

In both the given case galvanometer shows momentary deflection but in opposite direction. IN coil A magnetic field lines [increases in case (i) and decreases in case (ii)] induces a potential difference across the coil A which set up induced electric current in coil A. It is shown by the deflection in the galvanometer. This is known as electromagnetic induction.

Frequency of the Alternating current is equal to the number of cycles complete in one second. In India frequency of AC is 50 Hz. An alternating current is considered to be advantageous over direct current for long range transmission of electric energy because it can be transmitted over long distances to distinct places without much loss of electric power as compared to direct current.

The temporary magnet which retain its magnetism as long as the current passes through the coil wound around the piece of soft iron core.

Magnetic field: The region surrounding a magnet in which another magnet experience a force of attraction or repulsion is called magnetic field. Compass needle is a small magnet. When it is placed in a magnetic field of a bar magnet, it experiences magnetic force due to which it gets deflected.

(a) Right hand thumb rule is used to find the direction of magnetic field in a coil of wire and the electric current in a straight conductor.
(b)Fleming’s left hand rule is used to find the direction of force exerted on a current carrying conductor in a magnetic field as in electric motor.
(c) Fleming’s right hand rule is used to find the direction of induced current in a closed circuit in changing magnetic field as in electric generator.

Short circuiting: When electric current offers very low resistance to the flow of current through it, the current increases heavily and the circuit is said to be ort circuited. It occurs when live wire touches the neutral wire. This happen due to damage in insulation of the power lines.
Safety measures device: Fuse.
Working principal of fuse: It works on heating effect of electric current or Joules law of heating.
According to this law the heat produced in a resistor is directly proportional to the
(i) square of current for a given resistance.
(ii) resistance for a given current and
(iii) time for which current flows through the resistor.

Difference between AC and DC The alternating current (AC) reverse its direction periodically whereas the direct current (DC) always flows in one direction AC is preferred over DC because it can be transmitted over long distance without much loss of energy.
DC source: Battery
AC source: AC generator

Magnetic field B at the current of the circular coil
(i) increases the current is increased B∝ I
(ii) reverses on reversing of current.
(iii) increases if the number of turns in the coil increases as field is directly proportional to the number of turns.

(a) The direction of force experienced by the current carrying conductor depends on
(i) direction of current and
(ii) direction of magnetic field
(b) When the direction of current is at to the direction of magnetic field the force is maximum.
(c) No forced is experienced by the proton beam As proton beam moving along the direction of magnetic field.

Magnetic field lines are used to represent a magnetic field. A field line is the path along which the north pole of a small compass tends to move. The direction of the magnetic field at a point is given by the direction in which north pole of compass placed at that point would take.
Two properties of magnetic field lines are as follows:
(i) In air field lines start from N pole and end at S pole.
(ii) No two magnetic field lines can ever intersect at any one point.

(a) Take a bar magnet and place it on a sheet of white paper fixed on a drawing sheet. Mark the boundary of magnet. Take a small compromise and place it near the north pole of the magnet. Mark the position of two ends of compass needle. Move the needle to a new position so that its south pole occupies the position previously occupied by its north pole. In this way proceed step by step till we reach the south pole of magnet. Join the points marked on the paper by a smooth curve. This curve represents a magnetic field line.
(b) The degree of closeness of magnetic field lines signifies the strength of magnetic field.

Take a long straight thick copper wire and insert it through the center of a plane cardboard. Cardboard is fixed so that it is perfectly horizontal and the thick wire is in vertical direction.
Prepare an electric circuit consisting of a battery a variable resistance an ammeter and a plug key. Connect the copper wire in the circuit between the point X and Y. Sprinkle some iron-fillings uniformly on the cardboard. Put the plug in the key K so that a current flow in the circuit. Gently tap the cardboard using a finger. We observe that the iron filings align themselves forming a pattern of concentric circles around the copper wire. These concentric circle represent the magnetic field lines. Direction of field lines is determined by the uses of a compass needle. If current is flowing vertically downward then the magnetic field lines are along clockwise direction. If current is flowing vertically upward then the field lines are along anticlockwise direction.

Take a rectangular cardboard sheet. Make two hole in it at a suitable distance. Insert a circular coil having large number of turns of enameled copper wire passing through these holes so that the coil is normal to the plane to the cardboard. Complete the electric circuit by joining ends of the coil with a battery and a key K. Sprinkle iron-fillings uniformly on the cardboard. Plug the key and gently tap the cardboard. The iron filings arrange themselves along the magnetic field lines. The pattern of iron filings gives the magnetic field lines. The pattern of iron filings gives the magnetic field lines due to current carrying circular coil.

(i) The magnetic field produced in a current carrying circular coil increases on increasing the value of current.
(ii) On increasing the distance from the coil the magnetic field strength decreases.
(iii) The value of magnetic field increases on increasing the number of turn of the circular coil.

A solenoid is a coil of large number of circular turns of wire wrapped in the shape of a cylinder. On passing electric current a magnetic field is developed. Magnetic field lines are drawn below. The field is along the axis of solenoid such that one end of solenoid behaves as north pole and the other south pole. Thus field of a solenoid is similar to that of a bar magnet
Important features of magnetic field due to a current carrying solenoid are:
(i) Magnetic field lines inside the solenoid are nearly straight and parallel to its axis. It shows that magnetic field inside a solenoid is uniform.
(ii) Magnetic field of solenoid is identical to that due to a bar magnet with one end of solenoid behaving as a north pole and other end as a south pole.
(iii) A current carrying solenoid exhibits the detective and attractive properties of a bar magnet.

A solenoid is a coil of large number of circular turns of wire wrapped in the shape of a cylinder. On passing electric current a magnetic field is developed. Magnetic field lines are drawn below. The field is along the axis of solenoid such that one end of solenoid behaves as north pole and the other south pole. Thus field of a solenoid is similar to that of a bar magnet.
The strength of magnetic field of a solenoid can be increased by (i) increasing the current flowing through solenoid coil and (ii) increasing the number of turns per unit length of the solenoid coil.

Magnets having constant magnetic field around them (e.g. a bar magnet) are called permanent magnets. Permanent magnets are used in electric generators, speakers, refrigerator door etc.
Material behaving as magnet only when a current is passed through a coil wound around the material are called electromagnet. Electromagnets are used in electric bell, transformer, electric cranes etc.

(a) An electromagnet is a rod of magnetic material (say soft iron) placed inside a solenoid coil. On passing current in solenoid coil, iron rod begins to behave as a magnet. ON stopping the current flow, magnetization of iron is lost. Thus, a soft iron piece behaves as an electromagnet.
(b) The strength of an electromagnet can be increased by
(i) Increasing the number of turn of windings of the solenoid coil wrapped around the electromagnet, or
(ii) Increasing the amount of electric current flowing through the coil.

Important characteristics of magnetic force are as given below:
(i) Magnetic force acts only on moving charges (or flowing current) and does not act as stationary charges.
(ii) Force acts on moving charge when its motion is not parallel to the direction of magnetic field. Similarly, magnetic force acts when current carrying conductor is in any direction other than the direction of magnetic field.
(iii) Magnitude of magnetic force is maximum when current carrying conductor is set perpendicular to magnetic field or when charge is moving at right angle to magnetic field.
(iv) The direction of force F is given by Fleming’s left hand rule.
(v) The magnitude of magnetic force depends upon (a) the amount of current I, (b) the magnetic field B and (c) the length of the conductor l. On increasing anyone or all of these factors force increases proportionately.

(i) On increasing the current flowing through metallic conductor, the force experienced by it is proportionately increased because F∝ I.
(ii) On using a stronger horse-shoe magnet the magnetic force increases because F∝ B.
(iii) On reversing the direction of current the direction of force is reversed and conductor is displaced towards right instead of left direction.

(a) Magnitude of magnetic force acting on a moving charge in a magnetic field depends on (i) The magnitude of charge (ii)speed of moving charge (iii) strength of magnetic field (iv) the angle between the direction of motion of charge and the direction of magnetic field.
(b) The electron streaming from west to east is equivalent to a current from east to west. The magnetic field B is vertically upwards and shown by cross marks (x). Hence, in accordance with Fleming’s left hand rule, the electron will experience a force in north direction and deflected in that direction.

(a) 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. As a result, galvanometer gives a deflection (say towards left).
(b)When the bar magnet is withdrawn from inside the coil, again an induced current is set up in the coil. However now direction of induced current is opposite to that in earlier case. Hence, deflection in galvanometer is in reverse direction (say towards right).
(c) If the bar magnet is held stationary inside the coil, then there is no induced current in the coil. As a result, galvanometer does not show any deflection.

Take a coil AB of insulated copper wire having a number of turns. Connect the ends of coil to a sensitive galvanometer G. Now take a bar magnet NS and rapidly bring the magnet towards the end B of coil. The galvanometer gives momentary deflection in one direction. Now take the magnet away from the coil, the galvanometer again gives momentary deflection but in the opposite direction. It clearly shows that motion of magnet induces, a current in the coil.
Again keep the magnet fixed and gently move the coil AB either towards the magnet or away from the magnet. We get deflection in galvanometer even now. Thus, an induced current is produced in a coil whenever there is relative motion between the coil and the magnet. This phenomenon is known as the electromagnetic induction.

Take a coil AB of insulated copper wire having a number of turns. Connect the ends of coil to a sensitive galvanometer G. Now take a bar magnet NS and rapidly bring the magnet towards the end B of coil. The galvanometer gives momentary deflection in one direction. Now take the magnet away from the coil, the galvanometer again gives momentary deflection but in the opposite direction. It clearly shows that motion of magnet induces, a current in the coil.
Again keep the magnet fixed and gently move the coil AB either towards the magnet or away from the magnet. We get deflection in galvanometer even now. Thus, an induced current is produced in a coil whenever there is relative motion between the coil and the magnet. This phenomenon is known as the electromagnetic induction.

Take two different coils (say P and Q) of insulated copper wire having large number of turns. Insert the two coils over a non-conducting cylindrical thick paper roll.
Connect a battery and a plug key K in series of coil-1 (P). With coil-2 (Q) connect a sensitive galvanometer G. Now put the plug in key K. The galvanometer joined with coil-2 (Q) also gives a momentary deflection and then the pointer quickly returns to its mean position.
Now remove the plug from key K. The galvanometer in coil-2 (Q) again gives a momentary deflection but in the reverse direction.
Moreover, we observe that as soon as current in coil I (P) becomes either steady or zero, there is no deflection in galvanometer.
From the above experiment we conclude that an induced is produced in coil-2 (Q) on account of electromagnetic induction whenever current in coil-1 (P) is changing.

(a) If current in the coil P is increased we get momentary deflection in galvanometer connected across the coil Q in one direction. However, if current in the coil P is decreased, the deflection in galvanometer is in opposite direction. Due to change of current in coil P, the magnetic field of coil Q also changes and so as induced current is set up in coil Q and the galvanometer shows a deflection.
(b) If both the coils P and Q are moved in the same direction with the same speed, the magnetic field of both the coils remain unchanged. Hence no induced current is set up in coil Q and there is no deflection in the galvanometer.

An electric fuse is device which is used ahead and 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 low. Generally, wire of tin or tin-lead alloy or thin-copper alloy is used as a fuse wire. 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 gets melted. As a result, the electric circuit is broken and current flow stops. Thus, possible damage to the circuit and appliances is avoided.

Important features of domestic electric supply lines are as given below:
(i) Different circuits and different appliances in each circuit are connected in parallel.
(ii) Each appliances are provided with an independent on/off switch which is always joined to the live wire.
(iii) Two separate circuits are used, one of 15 A for appliances with higher power ratings and another of 5 A for bulbs, tubes, fans, etc.
(iv) Electric fuses of appropriate capacities are used ahead of each electric circuit as a safety measure. Moreover, proper earthing must be ensured.

(i) The student observes the magnetic field due to the given bar magnet.
(ii) There is the magnetic field around the given bar magnet. The iron filings experience force due to the magnetic field and thus align themselves along the magnetic field lines.
(iii) The crowding of the iron filing at the ends of the magnet indicate the position of the two magnetic poles N and S of given bar magnet.

(a) The direction magnetic field around a straight current carrying conductor is given by right hand thumb rule. According to the 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 the magnetic field.
(b) The strength of magnetic field due to current carrying straight conductor is inversely proportional to the normal distance between the conductor and the point where magnetic field is to be determined. So as the point as moved away the strength of the magnetic field goes on decreasing.

(a) A solenoid is a coil of large number of circular turns of wire wrapped in the shape of a cylinder. On passing electric current a magnetic field is developed. Magnetic field lines are drawn below. The field is along the axis of solenoid such that one end of solenoid behaves as north pole and the other south pole. Thus field of a solenoid is similar to that of a bar magnet
Important features of magnetic field due to a current carrying solenoid are:
(i) Magnetic field lines inside the solenoid are nearly straight and parallel to its axis. It shows that magnetic field inside a solenoid is uniform.
(ii) Magnetic field of solenoid is identical to that due to a bar magnet with one end of solenoid behaving as a north pole and other end as a south pole.
(iii) A current carrying solenoid exhibits the detective and attractive properties of a bar magnet.
(b) The pattern of filed lines inside the solenoid indicate that solenoid is now behaving as a bar magnet with one end behaving as the north pole and the other end as the south pole of the magnet. If a piece of soft iron is placed inside the solenoid and a current is passed through the solenoid coil the iron piece is magnetised and begins to behave as an electromagnet.

(a) Fleming’s left hand rule states that stretch the forefinger the central finger and the thumb of your left hand in mutually perpendicular to each other. If the forefinger shows the direction of the magnetic field and the central finger that of the current, then the thumb will point toward the direction of motion of conductor.
(b) An electric motor works on the principal that a coil carrying current when placed in a uniform magnetic field experience a force whose direction is given by Flemings left hand rule. As a result the coil start rotating about its own axis.
(c) Function of various parts of motor is given below
(i) Armature coil carries current. As a result, two long arms of armature coil experience equal force in mutually opposite direction and under their influence the coil begins to rotate.
(ii) Brushes draw current from split rings and supply it to the coil.
(iii) Split rings draws current from the battery. However, split rings change their contacts with brushes after every half rotation. As a result, current supplied to the armature coil through brushes changes its direction after every half rotation.

(a) Fleming’s left hand rule states that stretch the forefinger the central finger and the thumb of your left hand in mutually perpendicular to each other. If the forefinger shows the direction of the magnetic field and the central finger that of the current, then the thumb will point toward the direction of motion of conductor.
(b) Electric fan, refrigerator, electric mixer, washing machine etc., make use of interaction between magnetic field and a current carrying conductor.

(i) The earth wire function as a safety measure for electric appliances having a metallic body. Due to use of earth wire the user does not get a severe shock even when there is some leakage of current in metallic body of appliances.
(ii) Short circuiting of an electric supply means direct contact of the live wire and the neutral wire of supply line without any load in between. Short circuiting occurs either due to some fault in electric circuit or due to damage to insulation of live wire.
(iii) (a) Usual current rating of the fuse wire in the line to feed lights and fans is 5A.
(b) Usual current rating of the fuse wire in the line to feed appliances of 2kW or more power is 15 A.