NCERT Solutions For Class 10 Science Chapter 13 Magnetic Effects Of Electric Current

Magnetic Effects Of Electric Current Long Answer Questions Answer The Following Questions

Question 1. Explain with the help of the ring system of wiring in domestic wiring
Answer: The ring system is as shown in the figure below:

It consists of a ring-circuit. Wires starting from the main fuse box run around all the main rooms ofthe house and then come back to the fuse box again. The fuse box contains a fuse of about 30 A.

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A separate connection is taken from the live wire ofthe ring for each appliance. The terminal of the appliance is connected to the live wire through a separate fuse and a switch. If the fuse of one appliance burns, it does not affect the other appliance. For each appliance, the wires used for connection should be of proper current carrying capacity.

Question 2. The figure shows a closed coil connected to a galvanometer G. The galvanometer shows a deflection to the right when the N-pole of the bar magnet is brought closer to the coil. Why does the deflection occur in the galvanometer?

1. State the observation when
2. The coil is moved away from the N-pole.
3. Both the coil and magnet are moved to the right with the same speed

Answer: As the magnet is moved closer to the coil, the flux linked with the coil increases. This sets up an induced current in the coil as shown by deflection in the galvanometer.

The direction of induced current in the soil AB is reversed and the galvanometer shows a deflection to the left.

When both the magnet and the coil are moved towards the right with the same speed, the flux linked with the coil does not change. No induced current is set up. Hence no deflection is shown by the galvanometer.

Question 3. A coil made of insulated copper wire is connected to a galvanometer.

What will happen to the deflection ofthe galvanometer if the coil is moved towards a stationary bar magnet and then moved away from it? Give a reason for your answer and name the phenomenon involved.

Answer: When the coil is moved towards the bar magnet, the galvanometer shows deflection in one direction.

Reason: When the coil is moved towards the stationary bar magnet, the magnetic flux linked with it increases and a current is induced in it, as is shown by deflection in the galvanometer.

When the coil is moved away from the coil, the galvanometer shows deflection in
opposite direction.

Reason: When the coil is moved away from the magnet, the magnetic flux linked with it decreases and a current is induced in the coil in the opposite direction, as shown by deflection in the galvanometer in the opposite direction. This phenomenon is called electromagnetic induction.

Question 4. Draw the magnetic field lines of the field produced by a current-carrying circular loop. Explain with reason whether the field will be stronger at a point at the center loop or near the circumference of the loop.
Answer: Magnetic field due to a current through a circular loop-. The given figure shows the magnetic field lines of a circular wire carrying a current. The lines of force near the wire are almost concentric circles. As we move toward the center of the loop, the concentric circles become larger and larger.

Near the center of the loop, the arcs ofthese big circles appear as parallel straight lines. Thus the magnetic field is almost uniform at N the centre of the loop. By applying the right-hand rule, we can see that the magnetic field lines due to all sections of the wire are in the same direction within the loop. That is why the field is stronger at the center or near the circumference of the loop.

Question 5. Name two electrical appliances of daily use in which an electric motor is used.

Name and state the principle on which an electric motor works.

Answer: Electric motor is commonly used in fans, washing machines, mixers, etc. An electric motor works on Fleming’s left-hand rule.

Fleming’s left-hand rule: When we stretch our thumb, forefinger, and middle finger so that they are mutually perpendicular to one another, the forefinger points in the direction of the magnetic field, and the middle finger points in the direction of the current; the thumb gives the direction of the force acting on the conductor.

Question 6. What is an electromagnet? What decides its polarity’? How does it differ from a permanent magnet? List the three factors and explain how the strength ofan electromagnet depends on these.
Answer: When a soft iron is placed inside a solenoid and current is passed through it, then the soft iron gets magnetized. The magnet so formed is called an electromagnet. Its polarity is decided by the direction of current flowing through the solenoid.

1. Permanent magnets have a constant magnetic field around them whereas the magnetic field of the electromagnet is temporary.
2. The strength of an electromagnet depends on:
3. The number of turns in the solenoid.
4. Strength of current flowing through the solenoid.
5. Position of the soft iron core within the solenoid.

Question 7. 1 Draw a magnetic field, lines of a bar magnet. “Two magnetic field lines never intersect each other”. Why? An electric oven of 1.5 kW is operated in a domestic circuit (220 V) that has a current rating of 5 A. What result do you expect in this case? Explain.

Answer: Magnetic field lines around a bar magnet.

If two magnetic field lines intersect at a point, then at the point of intersection, there must be two directions ofthe same field, which is not possible.

Here, P = 1.5 kW = 1500 W.

Current \(I=\frac{P}{V}=\frac{1500}{220}=7 \mathrm{~A}\)

1. Thus, the current flowing through the circuit when the oven is ON is nearly 7 A which is higher than the current rating (5 A) of the circuit.
2. The wiring ofthe circuit may burn, fuse wire will also blow off breaking the circuit and stopping the current supply.
3. Explain any three properties of magnetic field lines.
4. Give two uses of the magnetic compass.

Answer: Field lines emerge from the north pole and merge at the south pole outside the magnet.

1. They are closed curves.
2. Magnetic force is greater where the field lines are crowded.
3. No two field lines are found to cross each other.

Uses of magnetic compass:

1. A magnetic compass can be used to find directions.
2. It can be used to test if a substance has magnetic properties.

Question 9. Write the values ofthe following physical quantities in connection with the domestic power supply in our country:

1. The potential difference between a live wire and a neutral wire is the frequency.
2.  Explain the role of the following as safety measures in domestic electric appliances! circuits
   1. Earth Wire
   2. Fuse
   3. 220v
   4. 50Hz

Answer: 3. 220 V

2. (1) Earth wire: The earth wire is a thick metal wire connected to a metal plate dug deep into the earth and is used as a safety measure for an appliance with metallic bodies, for example, toasters, etc. The metallic body is connected to the earth wire which provides a low resistance conducting path for the current.

It also ensures that any leakage of current in the metallic body keeps its potential to that of the earth and the user does not get a severe electric shock if touched.

(2) Fuse: It is a piece of wire made of an alloy with a low melting point. If a current larger than the specified value flows through the circuit, the temperature of the fuse wire increases and it melts. This breaks the circuit and thus prevents the flow of undue high electric current in the device. In this way, a fuse of proper rating protects the appliance and the circuit from damage.

Question 10.

1. The given figure shows a domestic electric circuit. Study this circuit carefully. List any two errors in the circuit and justify your answer.
2. Give one difference between the wires used in the element ofan electric heater and a fuse.
3. List two advantages of parallel connection over series connection.

Two errors are:

1. The fuse is incorrectly connected to the neutral wire (N), it must be connected to the live wire (L).
2. Bulb B2 is not connected to the neutral wire.
3. Element of an electric heater – high melting point. Element of a fuse wire – low melting point.
4. Each appliance has an equal potential difference.
5. Each appliance has a separate switch to ON/OFF the flow of current through it.

Question 11. What is electromagnetic induction?

1. Explain the various methods of producing induced current.
2. State the rule which gives the direction of induced current.
3. Name two devices that work on the principle of electromagnetic induction.

Answer: The process by which a changing magnetic field in a conductor induces a current in another conductor is called electromagnetic induction.

1. By moving a magnet towards or away from a coil.
2. By varying, the current in one coil, induced emf is produced in the coil.

Fleming’s right-hand rule:

• AC and DC electric generator.

Question 12. What is the direction of magnetic field lines outside a bar magnet?

1. Why two magnetic field lines cannot intersect each other?
2. What is indicated by crowding of magnetic field lines in a given region?
3. What is the frequency of AC in India?
4. State one advantage of AC over DC.

Answer: North pole to South pole.

1. Because it would mean that at the point of intersection, the compass needle would point in two directions which is not possible.
2. The strength of the magnetic field is higher in that region.
3. 50 Hz
4. AC can be transmitted over long distances without much loss of energy.

Question 13. Draw a labeled diagram ofan electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?
Answer: An electric motor is a rotating device that converts electrical energy into mechanical energy.

Principle: An electric motor works on the principle that a current-carrying conductor placed in a magnetic field experiences a force, the direction of force is given by Fleming’s left-hand rule.

Construction: As shown in the given figure, an electric motor consists of the following main parts:

Field magnet: It is a strong horse-shoe type magnet with concave poles.

Armature: 2 is n rectangular coil ADCD having n large number of turns of thin insulated copper wire wound over a soil iron core.

Split ring commutator: It consists of a cylindrical metal ring split into two halves S1 and S2. The two ends A and D ofthe armature coil are connected to the split rings S( and S., respectively.

Brushes: Two graphite or flexible metal rods maintain a sliding contact with split rings S, and S2, alternately.

Battery: A battery of a few cells is connected to the brushes. The current from the battery flows to the armature coil through the brushes and the split rings.

Working: Initially, suppose the plane ofthe coil is horizontal. The split ring SL touches the brush Bx and split ring S2 touches the brush B2. The current in the coil flows in the direction ABCD, as shown in the figure.

The currents in arms AB and CD are in opposite directions. The magnetic force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards. Thus, the armature coil along with the axle rotates anticlockwise.

After half a rotation, the split ring Sx comes in contact with brush B9 and S2 in contact with brush Br Therefore, the current flows along the path DCBA. Thus, the arm AB is now pushed up and the arm CD is pushed down.

Therefore, the coil and the axle rotate half a turn more in the same direction. The reversing ofthe current is repeated at each half-rotation, giving rise to a continuous rotation of the coil and the axle.

The function of the split ring commutator is to reverse the direction of current in the armature coil after every half rotation.

Question 14. Explain the underlying principle and working of an electric generator by drawing a labeled diagram. What is the function of brushes?
Answer: AC Generator: It is a device that converts mechanical energy into alternating forms of electrical energy.

Principle: It works on the principle of electromagnetic induction. When a closed coil is rotated in a uniform magnetic field with its axis perpendicular to the magnetic field, the magnetic field lines passing through the coil change, and an em is induced and hence a current is set up in it.

Construction: It consists of the following main parts:

Field magnet: It is a strong horseshoe-type permanent magnet with concave poles.

Armature: ABCD is a rectangular armature coil. It consists of a large number of turns of insulated copper wire wound on a soft iron cylindrical core.

Slip rings: These are two brass rings S1 and S2 rigidly connected to the two ends of the armature coil.

Brushes: These are two graphite rods B1 and B2 which are kept pressed against the slip rings S and S2.

Working: Suppose the armature coil ABCD is in the horizontal position. Now the coil is rotated clockwise. The coil cuts the magnetic lines of force. The arm AB moves upwards while the arm CD moves downwards.

According to Fleming’s right-hand rule, the induced current flows from A to B in arm AB and C to D in arm CD i.e., the induced current flows along ABCD. The induced current flows in the circuit through brush B2 to Br After half the rotation of the armature, the arm CD moves upwards and AB moves downwards.

The induced current now flows in the reverse direction i.e., along DCBA. The current flows from B1 to B2.

Thus the direction of current in the external circuit changes after every half rotation. Such a current that changes its direction after equal intervals of time is called alternating current. This device is called an AC generator.

Question 15. The diagram below shows two ways of connecting three bulbs A, B, and. C to 220 Vmains in a room. Name the two arrangements. Which of them would you prefer for household electricity? Give two reasons.

Answer: The arrangement shown in the figure is a series arrangement. The arrangement in the figure is parallel.

We would prefer the parallel arrangement. The reasons are:

1. In a series arrangement, if one of the bulbs is fused, the other bulbs also fail to glow. But in a parallel arrangement, if one bulb fuses, the other bulbs continue to glow.
2. In a series arrangement, while one bulb glows if the other bulb is switched on, the resistance ofthe circuit increases, and hence the bulbs glow less brightly. But in a parallel arrangement, each bulb glows at the same voltage, therefore, the glow of a bulb is unaffected if another bulb is switched on.

Question 16. Have you just paid the electricity bill for your house? If yes, What was it that your family consumed for which you had to pay? In what unit was it measured?
Answer: Yes;

1. The family consumed electrical energy.
2. Kilowatt-hour (kWh)

The figure shows a long solenoid, a cylindrical coil of several turns of insulated copper wire, connected to a battery through an ammeter A and a rheostat Rh.

Which end of the solenoid is an N-pole and which end is an S-pole?

Draw the magnetic field lines inside the solenoid and indicate their directions. Are magnetic field lines closed?

How can you increase the strength ofthe magnetic field inside the solenoid? Give two methods.

Answer: P is the south pole and Q is the north pole.

1. Yes, the magnetic field lines are closed.
2. By increasing the current flow.
3. By increasing the number of turns ofthe solenoid.

The substances that have the property of attracting small pieces of iron, nickel, cobalt, etc. are called magnets. Pieces of iron and other magnetic materials can be made to acquire the properties of natural magnets.

1. What is the origin of the word magnetism?
2. Give two properties of magnets.
3. Agnesia Where The Natural Magnets Were First Found.
4. Magnets have both attractive and directive properties.

Question 17. A magnet produces a magnetic field in the space around it, which exerts a force on any other magnet placed in it. Field lines are used to represent a magnetic field. A field line is the path along which a hypothetical free north pole would tend to move. The direction ofthe magnetic field at a point is given by the direction that a north pole placed at that point would take. Field lines are shown closer where the magnetic field is greater.

1. What is a magnetic field line?
2. Can two magnetic field lines intersect? Justify.
3. Give two properties of magnetic field lines.

Answer: A magnetic field line may be defined as the curve the tangent to which at any point, gives the direction of the magnetic field of that point.

1. Number If two magnetic field lines intersect, then there will be two tangents and hence two directions of magnetic field at the point ofintersection which is not possible.
2. The magnetic field lines are closed curves that start in air from the N-pole end at the S-pole and then return to the N-pole through the interior of the magnet.
3. Field lines are closer together in the regions of strong fields and farther apart in the regions of weak fields.

Question 18. When a wire carries more current than it can carry without overheating, it is said to be overloaded. An event in which a large current passes through the wires when the neutral wires touch accidentally is called a short circuit. An overload or a short circuit causes.

1. Excess heating ofthe wires, which may even cause a fire. To prevent this, fuses or circuit breakers are used. To prevent electric shocks in the event of the live wire touching the metallic body ofan appliance, the body is connected to the earth wire.
2. What is meant by overloading ofan electric circuit?
3. What is short-circuiting?
4. What precautions should be taken to avoid overloading of domestic electric circuits?

Answer: If the current drawn from the mains exceeds the safety limit, then this is known as overloading of the circuit.

When the live wire comes into contact with the neutral wire, the resistance becomes almost zero, and a very large current flows through the circuit. This is called short-circuiting.

The circuit must be divided into different sections and a safety fuse must be used in each section. High-power appliances like air-conditioners, refrigerators, water heaters, etc. should not be used simultaneously.

Question 19. Student experiments to study the magnetic effects of current around a current-carrying conductor. He reports that the direction of the north pole of a compass needle kept at a given point near the conductor gets reversed when the terminals ofthe battery are interchanged.

1. What is the nature of the magnetic field of, a straight current-carrying conductor?
2. Name and state a rule for finding the direction ofthis field.
3. Give two factors on which the magnitude of the field depends.

Answer: Magnetic fields are concentric circles with their centers of wire.

1. Right-hand thumb rule.
2. Current through conductor
3. Distance from the conductor

Question 20. Draw a labeled circuit diagram of a simple electric motor and explain its working. In what way these simple electric motors are different from commercial motors’?
Answer: The circuit diagram of a simple electric motor is shown below.

Working:

Let coil ABCD be in a horizontal position. When the key is switched on, the current flows in the direction of DCBA and then leaves through the brushes B2 via ring R2.

Applying Fleming’s left-hand rule, no force acts on arms BC and AD as they are parallel to a magnetic field, arm AB experiences a force in a downward direction and arm CD experiences an equal force in an upward direction. A torque acts on the coil and it rotates in the anti-clockwise direction.

While rotating, the coil reaches the vertical position, brushes loose contact with the rings and the current stops flowing. However, the coil does not stop due to the inertia of motion.

When the coil passes the vertical position, the rings automatically change their positions and come in contact with opposite brushes.

This reverses the direction of current through the coil but the direction of current on the right-hand side remains the same.

So, the force on the right-hand side is always upward and the force on the left-hand side is always in a downward direction. Thus, the coil continues in to anti-clockwise direction.

A commercial electric motor uses the following:

1. An electromagnet in place of a permanent magnet.
2. A large number of turns of conducting wire in the current carrying coil.
3. A soft iron core on which the coil is wound. The combination of soft iron core and coil is an armature. It enhances the power of motors.
4. Thus, commercial electric motors do not use permanent magnets to rotate the armature because permanent magnets are weak and do not produce a strong magnetic field in the region.

Question 21. Explain the phenomenon of electromagnetic induction. Describe an experiment to show that a current is set up in a closed loop when an external magnetic field passing through the loop increases or decreases.
Answer: This phenomenon was discovered by Michael Faraday and Joseph Henry in 1831. The phenomenon of generation of electric current in a closed circuit from magnetic effects i.e., by changing the magnetic field is called electromagnetic induction. The electric current produced due to this phenomenon is called induced current.

The potential difference set in the circuit due to induced current is called induced EMF.

Experiment: Let two different coils of copper wire, namely coil 1 and coil 2, having a large number of turns be arranged as per the circuit diagram as shown in the figure.

Coil 1, having a larger number of turns, is connected in series with a battery and a plug key whereas the other coil 2 is connected with a galvanometer as shown in the circuit diagram.

When key K is closed, the galvanometer shows a deflection in its needle that instantly jumps to one side, and just as quickly returns to zero, indicating a momentary current in coil 2.

This happens due to an external magnetic field passing through the loop increases. When the key is open, coil 1 is disconnected from the battery. The needle momentarily moves but to the opposite side.

It means that now the current flows in the opposite direction in coil 2. This happens due to an external magnetic field passing through the loop decreases.

Question 22. Describe the working of an AC generator with the help of a labeled circuit diagram. What changes must be made in the arrangement to convert it to a DC generator?
Answer: Principle: The electrical generator works on the principle that when a straight conductor is moved in a magnetic field, then current is induced in the conductor. The direction of this induced current is given by Fleming’s right-hand rule.

Construction of AC generator: It consists of a rectangular coil ABCD which can be rotated rapidly between N and S poles of a strong horse-shoe-shaped magnet having radial ends. The coil is made of a large number of turns of insulated copper wire. The ends A and D of the coil are connected to slip rings R1 and R2.

As the slip rings R1 and R2 rotate with the coil, the two carbon brushes B1 and B2 keep contact with them. Outer ends of the two brushes B1 and 132 are connected to the galvanometer to show the flow of the current in the given external circuit.

Working: When the coil starts rotating with the arm AB moving up and the arm CD moving down, cutting the magnetic lines of force, then according to Fleming’s right-hand rule an induced current is set up in these arms along the direction ofAB and CD.

So an effective induced current flowing in the direction of ABCD is obtained. If there are a large number of turns in the coil, the current generated in each turn adds up to give a large quantity of current through the coil.

After half rotation of the coil, its arm CD starts moving up and AB moving down. As a result, the direction of the induced current gets reversed in the coil in the direction of DCBA. Thus, after every half rotation the polarity ofthe current in the respective arm changes.

To convert an AC generator into a DC generator, a split ring commutator is used. This helps in producing direct current.

Question 23. Draw an appropriate schematic diagram showing common domestic circuits and discuss the importance of use. Why is it that a burnt-out fuse should be replaced by another fuse of identical rating?
Answer: A fuse in a circuit prevents damage to the appliances and the circuit due to overloading. Otherwise, the appliances or the circuit may get damaged. It stops the flow of unduly high electric current. The fuse is placed in series with the device.

A fuse wire works because of its lower melting point which is possible. If a fuse with a larger rating is used with an appliance, the fuse wire shall not melt and hence would fail to serve the required purpose. Due to this a fuse with a defined rating should not be replaced by one with a larger rating.

Question 25. Describe the activity that shows that a current-carrying conductor experiences a force perpendicular to its length and the external magnetic field. How does Fleming’s left-hand rule help us to find the direction ofthe force acting on the current-carrying conductor?
Answer: Aim: To show that force is exerted on a current-carrying conductor placed in a strong magnetic field.

Materials required:

A conducting rod (AB), connecting wires, horse-shoe magnet, battery switch, and clamp stand.

Procedure:

1. Connect the conducting rod (AB) to the battery and key
2. Place a strong horse-shoe magnet in such a way that the rod AB lines between the poles with a magnetic field directed upwards i.e. N. Pole is vertically above the rod.
3. Plug the key.

Observation: A current-carrying rod AB experiences a force perpendicular to its length and the magnetic field.

According to Fleming’s left-hand rule, stretch the thumb, the forefinger, and the middle finger of your left hand such that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle is the direction of current, then the thumb will point in the direction of motion or force acting on the conductor.