Class 10 Science Chapter 12 Previous Year Questions - Magnetic Effects of Current

Ans: (i) If they intersect then at the point of intersection, there would be two directions of magnetic field or compass needle would point towards two directions, which is not possible.
(ii) Uniform magnetic field is represented by equidistant parallel straight lines

Ans: (b)
The strength of the magnetic field in a solenoid depends on factors like the number of turns of wire and the radius of the solenoid. However, the direction of the current (whether it flows one way or the opposite) does not affect the strength of the magnetic field itself; it only changes the direction of the field. Therefore, the correct answer is (b).

Ans: (d)
The assertion states that the deflection of a compass needle decreases as the current increases, which is false. The reason explains that the magnetic field strength increases with more current, which is true. Therefore, the correct answer is (d), as the assertion is false while the reason is true.

Ans:

Right-Hand Thumb Rule: When a current-carrying straight conductor is being held in right-hand such that the thumb points towards the direction of current, then fingers will wrap around the conductor in the direction of the magnetic field lines.

Ans: (a) 

(b) Permanent magnet / Current carrying solenoid/ Electromagnet

Ans: (b)
A rectangular loop ABCD carrying a current I is situated near a straight conductor XY, such that the conductor is parallel to the side AB of the loop and is in the plane of the loop. If a steady current I is established in the conductor as shown, the conductor XY will move towards the side AB of the loop.

Ans: (a)

The assertion states that magnetic field lines never intersect, which is true. The reason explains that if they did intersect, a compass needle would point in two different directions at that point, which is also true. Since the reason correctly explains why the assertion is true, the correct answer is (a).

Ans: (i) The iron filings arrange themselves in a particular pattern because they align with the magnetic field lines created by the bar magnet. The filings act like tiny magnets and are attracted to the lines of force, forming a pattern that shows the shape of the magnetic field around the magnet.

(ii) The crowding of iron filings at the ends of the magnet indicates the location of the magnetic poles. The magnetic field is stronger at the poles of the magnet, which is why the filings are more concentrated in these regions. This shows the magnetic field lines emerging from the north pole and entering the south pole.

(iii)The lines along which the iron filings align represent the magnetic field lines. These are the paths along which the magnetic force is exerted. The pattern formed by the iron filings shows the direction and shape of the magnetic field around the magnet.

(iv) If the student places a cardboard horizontally in a current-carrying solenoid and repeats the activity, the iron filings would arrange themselves in concentric circles around the solenoid, with the pattern showing loops around the solenoid. The magnetic field of a solenoid is similar to that of a bar magnet, with distinct field lines forming a pattern resembling that of a bar magnet's poles.

Conclusion: The magnetic field inside a solenoid is uniform and parallel to its axis, while outside the solenoid, the magnetic field resembles that of a bar magnet, with a clear direction from one end (north) to the other (south)

Ans: (c)
A solenoid is a coil of wire that, when carrying an electric current, creates a magnetic field similar to that of a bar magnet, with distinct north and south poles. This means that a solenoid can generate a uniform magnetic field inside it, making it behave like a bar magnet. Therefore, the correct answer is (c).

Ans: (a)

The direction of the current is taken as opposite to the flow of electron or in the direction of the flow of protons.

In the given figure, the proton and electron are moving in the opposite direction to each other and perpendicular to the direction of the magnetic field. So, current due to both electron and proton are in the same direction. Hence, the forces acting on both will be in the same direction. By Fleming's left-hand rule the direction of force is pointing into the plane of the paper

Ans: (a) 

• It gets magnetised.
• Electromagnet. 
• It behaves as a magnet only when current passes through the solenoid.

(b)

This pattern indicates that the magnetic field is uniform.

Ans: (a)
The pattern of the magnetic field inside a solenoid is uniform and parallel, resembling straight lines. This means that the magnetic field strength is consistent throughout the solenoid, making it effective for applications requiring a stable magnetic field. Therefore, the correct answer is (a).

Sol: Assertion is true and reason is also true, but reason is not the correct explanation of assertion. Magnetic field lines around a current carrying straight wire do not intersect each other because at the point of intersection there will be two directions which is not possible. Also, the strength of magnetic held increased by increasing the magnitude of the current in the wire.

Ans: The pattern of magnetic field produced around a vertical current carrying straight conductor passing through a horizontal cardboard is shown in figure. The magnitude of magnetic field produced is
(i) ∝ I (ii) ∝ 1/r

Right hand thumb rule is used to determine the direction of magnetic field associated with a current carrying conductor.
It states that you are holding a current carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then your finger will wrap around the conductor in the direction of the field lines of the magnetic field.

Sol: According to the Fleming’s left hand rule, the I direction of force is out of the page.

Ans: (a)
Sol: According to right hand thumb rule, the point is ; at the direction above the wire.

Ans: (b)
Sol: Magnetic force, F = qvBsinθ
In current carrying wire, net charge is zero because of equal number of electrons and protons present in the wire, but only electrons are moving.

Ans: (i) It is because that over a long distance to a distant places, the loss of electric power is very less in case of A.C. as compared to D.C.
(ii) The current used in household supply is alternating in nature while the current given by battery is direct in nature.
(iii) Electric fuse protects circuits and appliances by stopping the flow of any unduly high electric current. It consists of a piece of wire made of a metal or an alloy of appropriate melting point, for example aluminium, copper, iron, lead etc. If a current larger than the specified value flows through the circuit, the temperature of the fuse wire increases. This melts the fuse wire and breaks the circuit.

Ans: (A) Fleming’s left-hand rule is used to determine the direction of force experienced by a current carrying conductor placed in a uniform magnetic field. Fleming’s Left Hand Rule: When a current carrying conductor is placed in a magnetic field, it experiences a force, whose direction is given by Fleming’s left hand rule, which states that “Stretch the forefinger, the central finger and the thumb of your left hand mutually perpendicular to each other. If the forefinger shows the direction of the field and the central finger that of the current, then the thumb will point towards the direction of motion of the conductor, i.e., force.”

(B) Force on electron is maximum in (i) case because the electron direction show that it is moving at right angle to the direction of a magnetic field. Force on electron is minimum in (iii) case as the electron shown is moving parallel to the direction of a magnetic field. The direction of maximum force acting on an electron in (i) case which is into the plane of paper in accordance with Fleming’s left hand rule.

Ans: (A) 

(i)

(ii)

(B) 

Ans: (i) There is either a convergence or a divergence of magnetic field lines near the ends of a current carrying straight solenoid because it behaves similar to that of a bar magnet and has a magnetic field line pattern similar to that of a bar magnet. Thus the ends of the straight solenoid behaves like poles of the magnet, where the converging end is the south pole and the diverging end is the north pole.
(ii) The current carrying solenoid behaves similar to that of a bar magnet and when freely suspended aligns itself in the north-south direction.

Ans: (a) The pattern of iron filings is shown below.

 (b) This pattern of iron filings demonstrate that the magnet exerts its influence in the region surrounding it. Therefore, the iron filings experience a force. The lines along which the iron filings align themselves represent magnetic field lines.
(c) (i) The direction of magnetic field is determined by placing a small compass needle in the magnetic field. The N-pole of the compass indicates the direction of magnetic field at that point.
Magnetic field lines never intersect each other because it is not possible to have two directions of magnetic field at the same point.

Ans: (i) A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
(ii)

Ans: The magnitude of force experienced by a current carrying conductor placed in a uniform magnetic field is
(i) maximum when the conductor is placed perpendicular to the magnetic field,
(ii) minimum when the conductor is placed parallel to the magnetic field.

Ans: (i) Fleming's left-hand rule is used to determine the direction of magnetic force experienced by a current carrying straight conductor placed perpendicularly in a uniform magnetic field.
Fleming's left-hand rule, states that when left hand’s thumb, forefinger and centre finger are held mutually perpendicular to one another and adjusted in such a way that the forefinger points in the direction of magnetic field, and the centre finger points in the direction of the current, then the direction in which thumb points, gives the direction of force acting on the conductor.
(ii) As we know that, an alpha particle is positively charged. It is given that an alpha particle while passing through a magnetic field gets deflected (projected) towards north. Since an electron is negatively charged, it will deflect in ODDOsite direction i.e.. south.

Ans: (a) On passing current, the rod gets displaced because of a magnetic force exerted on the rod when it is placed in the magnetic field.
(b) Fleming’s left hand rule is used to determine the direction of magnetic force exerted on the conductor AB.
(c) The rod will be displaced towards left according to Fleming’s left-hand rule.

Ans: The direction of magnetic field (B) at any point is obtained by drawing a tangent to the magnetic field line at that point. In case, two magnetic field lines intersect each other at the point P as shown in figure, magnetic field at P will have two directions, shown by two arrows, one drawn to each magnetic field line at P, which is not possible.

Ans: Galvanometer is used to detect the current in a circuit.

Ans: An electromagnet is a current-carrying solenoid coil which is used to magnetise steel rod inside it.

Ans: (a) In Figure 'a', poles P and Q of the magnet represents north pole and south pole respectively. In figure 'b', poles R and S of the magnet also represents north pole and south pole respectively.
(b) Magnetic field lines are closed continuous curves directed from north pole to south pole outside the magnet but from south pole to north pole inside the magnet.

Ans: (i) Strength of magnetic field produced by a straight current-carrying wire at a given point is directly proportional to the current passing through it. inversely proportional to the distance of that point from the wire.
(ii) Right-hand thumb rule: The straight thumb of right hand points in the direction of electric current. The direction of the curl of fingers represents the direction of magnetic field.

Ans: (a) An electromagnet is a current-carrying solenoid coil which is used to magnetise steel rod inside it. Electromagnets are used in electric bells and buzzers, loudspeakers and headphones etc.
(b) 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.

(c) The soft iron core placed in an electromagnet increases the strength of the magnetic field produced. Thus increasing the strength of electromagnet.
(d) The strength of electromagnet can be increased by
(i) Increasing the current passing through the coil.
(ii) Increasing the number of turns in the coil.

Ans: (A) There is a divergence of magnetic field lines near the ends of a current carrying straight solenoid as the ends of the solenoid behave as poles. So, lines emerge and enter the ends, crowding the space and appearing divergent. 
(B) The current carrying solenoid when suspended freely rests along a particular direction because a current carrying solenoid behaves like a bar magnet with fixed polarities at its ends. The magnetic field lines are exactly identical to those of a bar magnet with one end of solenoid acting as a south-pole and its other end as north-pole

Ans: (A)

(B) The rule used to find the direction of magnetic field lines is right hand thumb rule, which states that if we had the loop wire in our hand such that our thumb points in direction of current then our curled finger show the direction of field at that point.

Ans: A fuse in a circuit prevents damage to the appliances and the circuit due to overloading and short-circuiting. Overloading can occur when the live wire and the neutral wire come into direct contact. In such a situation, the current in the circuit abruptly increases. This is called short-circuiting. The use of an electric fuse prevents the electric circuit and the appliance from a possible damage by stopping the flow of unduly high electric current. 
According to Joule's Law of heating that takes place in the fuse melts it to break the electric circuit. So, a fuse is always connected in series with an appliance. If it is connected in parallel, then it will not be able to break the circuit and the current keeps on flowing. Overloading can also occur due to an accidental hike in the supply voltage. Sometimes, overloading is caused by connecting too many appliances to a single socket. 
The fuse with defined rating means the maximum current that can flow through the fuse without melting it and blows off when a current more than the rated value flows through it. If a fuse is replaced by one with larger ratings, then large current will flow through the circuit without melting the fuse. This large current may damage the appliances.

Ans: (A) An electromagnet is a temporary strong magnet. Its magnetism is only for the duration of current passing through it. The polarity and strength of an electromagnet can be changed. 
Uses of electromagnet:
Electromagnets are used:
(1) in electrical appliances like electric bell, electric fan etc. 
(2) in electric motors and generators.
(B) 

(C) Soft iron increases the strength of the electromagnet. 
(D) Ways of increasing the strength of an electromagnet:      
(1) If we increase the number of turns in the coil, the strength of electromagnet increases.      
(2) If the current in the coil is increased, the strength of electromagnet increases

Ans: (a) If current in the coil P (Fig. 13.24) 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.

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

Ans: Three possible reasons of short-circuiting of an electrical circuit are as follows:

• The insulation of electrical wirings is damaged.
• The electrical appliance used in the circuit is defective.
• An appliance of higher power rating is being run on an electrical line of lower power rating.

Short-circuiting can be prevented by the use of electrical fuse of appropriate capacity.

Ans:

Right-Hand Thumb Rule. This rule is used to find the direction of the magnetic field due to a straight current-carrying wire.
It states that if we hold the current-carrying conductor in the right hand in such a way that the thumb is stretched along the direction of the current, then the curly finger around the conductor represents the direction of the magnetic field produced by it.
Using a compass needles, we can determine the strength of the magnetic field at various points. If the needle is moved away from the straight wire carrying constant current, the deflection in the needle decreases. It implies that magnetic field strength decreases as the distance increases.

Ans: (a) It is defined as the path along which the hypothetical unit N-pole tends to move in a magnetic field, if free to do so.
By drawing a tangent at that point on the magnetic field lines one can find the direction of magnetic field at that point.
(b) Yes, of the current in coil X is changed, the magnetic field associated with it also changes around the coil 'Y' placed close to 'X'. This change in magnetic field lines linked with 'Y', according to Faraday law of electric magnetic induction, induces a current in the coil Y.
(c) Right-Hand Thumb Rule. This rule is used to find the direction of magnetic field due to a straight current carrying wire.
It states that if we hold the current carrying conductor in the right hand in such a way that the thumb is stretched along the direction of current, then the curly finger around the conductor represent the direction of magnetic field produced by it. This is known as right-hand thumb rule.
Direction of Field Lines due to current carrying straight conductor as shown in figure.

Ans: (a) Fleming’s Left-Hand Rule: Stretch the thumb, forefinger and middle finger of the left hand such that they are mutually perpendicular to each other. If the forefinger pointed towards the direction of magnetic field and middle finger in the direction of current, then the thumb will indicate the direction of motion or force experienced by the conductor. It is to be applied only when the current and magnetic field, both are perpendicular to each other.
(b) Principle of an electric motor: It works on the principle of magnetic effect of current. When a current carrying conductor is placed perpendicular to the magnetic field, it experiences a force. The direction of this force is given by Fleming’s left hand rule.
(c) (i) Armature: It consists of a large number of turns of insulated current-carrying copper wires wound over a soft iron core and rotated about an axis perpendicular to a uniform magnetic field supplied by the two poles of permanent magnet.
(ii) Brushes: Two conducting stationary carbon or flexible metallic brushes constantly touches the revolving split rings or commutator. These brushes act as a contact between commutator and terminal battery.
(iii) Split ring: The split ring acts as a commutator in an electric motor. With the help of split ring, the direction of current through the coil is reversed after every half of its rotation and make the direction of currents in both the arms of rotating coil remains same. Therefore, the coil continues to rotate in the same direction.

Ans: (b)
A fuse is a safety device that protects electrical appliances and circuits from damage caused by overloading or short circuits. It contains a metal wire that melts when excessive current flows through it, breaking the circuit and stopping the flow of electricity, which prevents potential damage to appliances and reduces fire risk.
Here’s why the other options are not as suitable for preventing short circuits or overloading:
(a) Earthing: Protects against electric shocks but does not prevent overloading.
(c) Use of stabilizers: Helps maintain a stable voltage but doesn’t protect against overloading or short circuits.
(d) Use of electric meter: Measures electricity usage but doesn’t offer any protective function.
Therefore, the most important safety method is the use of a fuse.

Ans: The force acting on a current-carrying conductor placed in a magnetic field is maximum when the direction of current is at right angles to the direction of the magnetic field.

Ans: An electric fuse is a device that is used ahead of 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 tin-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 a 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.

Ans: (a) The magnitude of the magnetic force acting on a moving charge in a magnetic field depends on:

• the magnitude of charge
• speed of moving charge
• strength of magnetic field
• 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.

Ans: 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:

• Magnetic field lines inside the solenoid are nearly straight and parallel to its axis. It shows that magnetic field inside a solenoid is uniform.
• 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.
• A current-carrying solenoid exhibits the directive and attractive properties of a bar magnet.

Ans: 

• Take a long straight thick copper wire and insert it through the centre of a plain 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 points X and Y. Sprinkle some iron filings uniformly on the cardboard. Put the plug in key K so that a current flows 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 circles represent the magnetic field lines. Direction of field lines is determined by the use 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.

Ans: (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 compass 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 as shown in Fig.

(b) The degree of closeness of magnetic field lines signifies the strength of magnetic field.

Ans: Direct current (D.C.).

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

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

Ans: Magnetic field lines form concentric circles around the conductor with conductor at the centre.

Ans: A magnetic field line around a magnet is the path along which north pole of a magnetic compass needle points. A magnetic field line gives the direction of magnetic field at a point.

Ans: (a) Take a straight, thick copper wire X Y and connect it to an electric circuit consisting of a battery, key and resistor. Place a small compass near the copper wire. Now put the plugin key K so that a current begins to flow. The compass needle is deflected.
It shows that the current-carrying copper wire is behaving like a magnet.

(b) The direction of the magnetic field around a straight current-carrying conductor is given by the “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 the current. Then the fingers of the right hand wrap around the conductor in the direction of the field lines of the magnetic field.

Ans: The live wire.

Ans: 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.

Ans: 50 Hz.

Ans: 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 as shown in Fig.
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 the 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 electromagnetic induction.

Ans: (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
(iii) On reversing the direction of current the direction of force is reversed and conductor is displaced towards right instead of left direction.

Ans: Outside the solenoid magnetic field is minimum. At the ends of solenoid, magnetic field strength is half that of inside it. So Minimum - at point B; Maximum - at point A

Ans: Solenoid: A coil of many circular turns of insulated copper wire wound on a cylindrical insulating body (i.e. cardboard etc.) such that its length is greater than its diameter is called solenoid.

When current is flowing through the solenoid, the magnetic field line pattern resemble exactly with those of a bar magnet with the fixed polarity North and South pole at its ends and it acquires the directive and attractive properties similar to bar magnet. Hence the current carrying solenoid behaves a bar magnet.
Use of current carrying solenoid: It is used to form a temporary magnet called electromagnet as well as permanent magnet.

Ans: Magnetic field lines: It is defined as the path along which the unit North pole (imaginary) tends to move in a magnetic field if free to do so.
(a) The magnetic lines of force do not intersect (or cross) one another. If they do so then at the point of intersection, two drawn tangents at that point indicate that there will be two different directions of the same magnetic field, i.e. the compass needle points in two different directions which is not possible.
(b) Magnetic field lines are closed continuous curves. They emerge out from the north pole of a bar magnet and go into its south pole. Inside the magnet, they move, from south pole to north pole.

Ans: (i) The magnitude of force experienced by the current-carrying conductor when placed in a magnetic field depends on current flowing, length of the conductor, the strength of magnetic field, orientation of conductor in the magnetic field.
(ii) Magnitude of force is maximum when current-carrying conductor is placed at right angles to the direction of magnetic field.
(iii) (a) Direction of force is reversed that is now the force acts from left to right.
(b) Direction of force is reversed that is now the force acts from left to right.

Ans:  (i) Current shown in Fig. (a) is direct current (D.C.) but current shown in Fig. (b) is an alternating current (A.C.).
(ii) A cell/battery produces D.C. but an A.C. generator produces A.C.
(iii) In India frequency of A.C. is 50 Hz.
(iv) A.C. is used in transmitting electric power over long distances. It is so because transmission loss of electric power can be minimised for A.C. by employing suitable transformers at generating stations and consuming centres.

Ans: 
(i) The student observes the magnetic field due to the given bar magnet. The pattern of the magnetic field is shown here.
(ii) There is a magnetic field around the given bar magnet. The iron filings experience a force due to the magnetic field and thus align themselves along the magnetic field lines.

(iii) The crowding of the iron filings at the ends of the magnet indicates the position of two magnetic poles N and S of a given bar magnet.

Ans: (c)
In the arrangement described, if the key is taken out and the circuit is open, no current flows through the wire. This means that no magnetic field is generated by the wire.
However, if magnetic field lines are drawn over the horizontal plane ABCD in the absence of a current (or any other magnetic source), they would represent the Earth's magnetic field, which appears as straight, parallel lines over a small horizontal area.
Thus, the correct answer is (c) straight lines parallel to each other.

Ans: The straight current carrying conductor should be placed in the paper plane, passing through A, to produce a magnetic field perpendicular to the paper plane. This ensures the compass needle remains undeflected due to the vertical magnetic field produced by the wire. The maximum deflection in the compass needle occurs when the conductor is perpendicular to the paper plane and the magnetic field is in the paper plane.