CBSE Class 10 Science Magnetic Effects of Electric Current VBQs Set 02

Question. (a) An electric fuse of rating 3 A is connected in a circuit in which an electric iron of power 1.5 kW is connected which operates at 220 V. What would happen? Explain. 
(b) Which effect of electric current is utilised in the working of an electrical fuse?
(c) Name two safety measures taken in domestic circuits.

Answer: (a) Power, \( P = VI \)

\( \implies \) \( I = \frac{P}{V} = \frac{1.5 \text{ kW}}{220 \text{ V}} = \frac{1500 \text{ W}}{220 \text{ V}} = 6.8 \text{ A} \)
Thus, current drawn by the electric iron is much more greater than the fuse rating 3 A. So, fuse will be blown.
(b) Heating effect of electric current.
(c) Earthing and fuse.

Question. (a) State two point difference between short circuit and overloading.
(b) An electric sandwich maker of 1.0 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: (a)
Short circuit:
1. It occurs when live wire comes in direct contact with neutral wire.
2. Current in the circuit abruptly increases.
3. Voltage drops to zero.
Overloading:
1. It occurs by connecting too many appliances to a single socket and turned on simultaneously.
2. Less amount of current flows as compared to short circuit.
3. Voltage becomes low but cannot be zero.
(b) Current drawn by electric oven,
\( I = \frac{P}{V} = \frac{1.0 \text{ kW}}{220 \text{ V}} = \frac{1000 \text{ W}}{220 \text{ V}} = 4.54 \text{ A} \)
which is less than the current rating 5 A. So fuse will not blow off and oven will work continuously.

Question. (a) Why the current abruptly increases during short circuit? Explain.
(b) When you connect the live wire to the neutral wire through an electric bulb a short circuit does not occur. Why?

Answer: (a) During the short circuit, at the point of contact, the resistance becomes zero. So as per Ohm’s law,
\( I = \frac{V}{R} = \frac{V}{0} = (\text{infinite}) \)
Hence, infinite or maximum current will flow from that path which is called short circuit.
(b) The filament of an electric bulb has some resistance. When a current passes through it, there is some potential drop across the resistance. This will limit the flow of current and ensure that a short circuit does not occur.

Question. (a) Explain why there are two separate circuits one for high power rating appliances and other for low power rating appliances.
(b) A domestic circuit has 5 A fuse. How many bulbs of rating 100 W, 220 V can be safely used in this circuit? Justify your answer. 

Answer: (a) Two separate circuits are formed in the domestic wiring, one for high power rating appliances called power circuit and other for low power rating appliances called lightning circuit.
Power circuit: The circuit which draw heavy current (15 A) from mains and used for high power rating devices such as microwave, oven, air conditioners, geysers, washing machine, etc. is known as power circuit.
Lightning circuit: The circuit which draw small amount of current from the mains and used for low power rating devices such as bulb, tube light, fans, T.V., Computer, etc. having a current rating of 5A is known as lightning circuit.
(b) Power rating of bulb = 100 W, 220 V
Electric current flowing in the circuit is given by \( I = \frac{P}{V} \)
Hence, \( I = 5 \text{ A} \), \( V = 220 \text{ V} \) and \( P = n \times 100 \text{ W} \)
Where, \( n = \) number of bulbs
\( \implies \) \( 5 = \frac{n \times 100}{220} \)

\( \implies \) \( n = \frac{5 \times 220}{100} = \frac{1100}{100} = 11 \)
So, 11 bulbs of 100 W can be used in the domestic circuit having potential difference of 220 V.

Question. State the various advantages and disadvantages of AC over DC.

Answer: Advantages of AC over DC.
(i) The generation of AC is cheaper than that of DC.
(ii) AC voltage can be stepped up or stepped down with the help of transformers.
(iii) The magnitude of AC can be controlled by using inductor (coil) without any appreciable loss of energy.
(iv) AC can easily be converted into DC with the help of rectifiers.
(v) AC can be transmitted over a long distance without much loss of energy as compared to a DC transmission.
(vi) AC devices are highly efficient, more durable, less expensive and are simple in their functions.
Disadvantages of AC.
(i) It is dangerous to use due to its high peak value. Therefore, good insulation of wire is required.
(ii) As AC is transmitted near the surface of a conductor, it needs several strands of thin wire insulated from each other.
(iii) Any electrical equipment which need DC cannot run on AC.
(iv) At higher voltage of AC, more safety measures are required.
(v) AC attracts a person who touches it unlike DC which gives a repelling shock.

Question. Although electric kettle and electric toaster were used simultaneously in the kitchen to prepare breakfast for the family, yet the two devices could work efficiently due to ‘fuse’ used in the electric circuit.
(i) What is a fuse? Write the material used in fuse wires. How is a fuse connected in an electric circuit?
(ii) State the ratings of fuse used in electric circuits.
(iii) What is the function of a fuse? How does it perform its function?
(iv) A device uses 1 kW electric power when operated at 220 V. Calculate the rating of the fuse to be used. 

Answer: (i) • A fuse is a safety device / used in domestic electric circuits to prevent damages from short circuiting or overloading.
• An alloy / metal of appropriate (lower) melting point / aluminium / copper / iron / lead etc.
• In series
(ii) 1A, 2A, 3A, 5A, 10A (any other appropriate value)
(iii) • To protect the circuits and appliances by stopping the flow of unduly high electric current.
• If current larger than the specified / rated value flows through the circuit, the temperature of the fuse wire increases, this melts the fuse wire and breaks the circuit.
(iv) Power = 1 kW; \( V = 220 \text{ V} \); \( I = ? \)
Formula used \( P = VI \)

\( \implies \) \( I = \frac{P}{V} = \frac{1000}{220} = 4.54 \text{ A} \)
Rating of fuse will be 5A and above.

Question. Overloading is due to
(a) direct contact between live wire and neutral wire.
(b) accidental hike in supply voltage.
(c) connecting too any appliances to a single socket.
(d) All of the options

Answer: (d) All of the options

Question. A fuse is normally a
(a) current rating device
(b) voltage rating device
(c) power rating device
(d) All of the options

Answer: (a) current rating device

INTEGRATED (MIXED) QUESTIONS

Question. The instrument that use to detect electric current in the circuit is known as 
(a) electric motor
(b) A.C generator
(c) galvanometer
(d) None of the options

Answer: (c) galvanometer

Question. State and explain the rule to determine the direction of a
(a) magnetic field produced around a straight conductor-carrying current.
(b) force experienced by a current-carrying straight conductor placed in a magnetic field that is perpendicular to it. 

Answer: (a) Right Hand Thumb Rule: Imagine 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 fingers will wrap around the conductor in the direction of the field lines of the magnetic field.
(b) Fleming's Left-Hand Rule: Stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.

ASSERTION AND REASON QUESTIONS

In the following Questions, the Assertion and Reason have been put forward. Read the statements carefully and choose the correct alternative from the following:
(a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.
(b) Both the Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) The statement of the Assertion is false but the Reason is true.

Question. Assertion: At high temperature, metal wires have a greater chance of short circuiting.
Reason: Both resistance and resistivity of material vary with temperature.
(a) a
(b) b
(c) c
(d) d

Answer: (b) Both the Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Question. Assertion: A stationary charged particle placed in a magnetic field experiences a force.
Reason: A stationary charged particle does not produce a magnetic field.
(a) a
(b) b
(c) c
(d) d

Answer: (d) The statement of the Assertion is false but the Reason is true.

Question. Assertion: It is easier to bring North pole of a magnet to South pole of other magnet.
Reason: There is a force of attraction between unlike poles of magnet.
(a) a
(b) b
(c) c
(d) d

Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: A compass needle gets deflected when electric current is passed through the metallic conductor.
Reason: Current carrying conductor produces magnetic field which exerts the force on the compass needle.
(a) a
(b) b
(c) c
(d) d

Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: The magnetic field inside a solenoid is uniform.
Reason: The magnetic field lines inside a solenoid are parallel. 
(a) a
(b) b
(c) c
(d) d

Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: The magnitude of the magnetic field at a point on the axis of a current carrying solenoid is inversely proportional to the current through the solenoid.
Reason: The magnitude of the magnetic field at a point on the axis of a current carrying solenoid is directly proportional to the number of turns per unit length of a solenoid. 
(a) a
(b) b
(c) c
(d) d

Answer: (d) The statement of the Assertion is false but the Reason is true.

Question. Assertion: On freely suspending a current – carrying solenoid, it comes to rest in Geographical N-S direction.
Reason: One end of current carrying straight solenoid behaves as a North pole and the other end as a South pole, just like a bar magnet.
(a) a
(b) b
(c) c
(d) d

Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: A current carrying straight conductor experience a force when placed perpendicular to the direction of magnetic field.
Reason: The net charge on a current carrying conductor is always zero. 
(a) a
(b) b
(c) c
(d) d

Answer: (b) Both the Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Question. Assertion: When the direction of velocity of moving charge is perpendicular to the magnetic field, it experience a maximum force.
Reason: Force on the moving charge does not depends on the direction magnetic field in which it moves.
(a) a
(b) b
(c) c
(d) d

Answer: (c) Assertion is true but the Reason is false.

Question. Assertion: It is fatal to touch a live electric wire as the person gets a severe electric shock. In some cases, electric shock can even kill a person.
Reason: The electric current passes through the body to the earth forming a circuit and burns the blood.
(a) a
(b) b
(c) c
(d) d

Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: Fuse is a safety device which is installed to prevent electrical circuits and possible fires.
Reason: Fuse consist of tin-plated copper wire having low melting point, which melts and breaks the circuit if the current exceeds a safe value.
(a) a
(b) b
(c) c
(d) d

Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: When a battery is short circuited, the terminal voltage is zero.
Reason: In short circuit, the current is zero.
(a) a
(b) b
(c) c
(d) d

Answer: (c) Assertion is true but the Reason is false.

Question. Assertion: Magnetic field lines do not intersect each other.
Reason: Magnetic field lines are imaginary lines, the tangent to which any point gives the direction of the field at that point. 
(a) a
(b) b
(c) c
(d) d

Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: The strength of the magnetic field produced at the centre of a current carrying circular coil increases on increasing the number of turns in it.
Reason: The current in each circular turn has the same direction and the magnetic field due to each turn then just adds up. 
(a) a
(b) b
(c) c
(d) d

Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

CASE-BASED QUESTIONS

The following questions are source-based/case-based questions. Read the case carefully and answer the questions that follow.

A student fixes a sheet of white paper on a drawing board using some adhesive materials. She places a bar magnet in the centre of it and sprinkles some iron filings uniformly around the bar magnet using a salt sprinkler. On tapping the board gently, she observes that the iron filings have arranged themselves in a particular pattern. 

Question. (a) Draw a diagram to show this pattern of iron filings.
(b) Draw the magnetic field lines of a bar magnet showing the poles of the bar magnet as well as the direction of the magnetic field lines.
(c) (i) How is the direction of magnetic field at a point determined using the field lines?
Why do two magnetic field lines not cross each other?
Or
(ii) How are the magnetic field lines of a bar magnet drawn using a small compass needle? Draw one magnetic field line each on both sides of the magnet. 

Answer: (a) [Diagram should show iron filings forming curved loops around a bar magnet, highly concentrated at the N and S poles].
(b) [Diagram of a bar magnet with arrows on field lines emerging from North and entering South].
(c) (i) The direction of the magnetic field at a point is given by the tangent drawn to the field line at that point. Two magnetic field lines do not cross because if they did, at the point of intersection, the compass needle would point in two different directions, which is not possible.
Or
(ii) Place a small compass needle near the North pole. Mark the positions of its ends. Move the needle so its tail is at the previous head position. Repeat this until the South pole is reached. Joining these marks gives a field line.

Question. Why does a compass needle get deflected when brought near a bar magnet?
Answer: Compass needle is also a tiny pivoted magnet and experience a force exerted by a magnetic field set up due to a bar magnet. Due to this magnetic force, compass needle gets deflected when brought near a bar magnet.

Question. List the properties of magnetic field lines.
Answer: (i) Magnetic field lines have both direction and magnitude. The direction of the magnetic field is indicated by the tangent drawn at any point on the curve.
(ii) The field lines emerge from the North pole and merge at the South pole while inside the magnet, they move from south to north. So they form a continuous closed curve or loop.
(iii) The strength of magnetic field is indicated by the closeness of the field lines. Closer the lines, more will be the strength and farther the lines, lesser will be the field strength.
(iv) No two field lines intersect each other.

Question. Why don’t two magnetic field lines intersect each other?
Answer: The two magnetic field lines do not intersect each other. If they intersect at the point of intersection, then there will be two different directions of magnetic field, which is impossible.

Question. 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.
Answer: (d) is the same at all points.

Question. Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer).
(a) mass
(b) speed
(c) velocity
(d) momentum
Answer: (c) and (d) because when charged particle enter in the magnetic field, then it moves in a circular path so velocity is changing at every point due to change in its direction and hence momentum is also changed.

Question. Name two safety measures commonly used in electric circuits and appliances.
Answer: Fuse: For protecting circuits due to short circuit or overloading of the circuit.
Earthing: For protecting us from a severe electric shock if there is any leakage of current in metallic body appliance.

Question. An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.
Answer: \( P = 2 \text{ kW} = 2000 \text{ W}, V = 220 \text{ V} \)
\( I = \frac{P}{V} = \frac{2000}{220} \approx 9 \text{ A} \)
The current rating of the circuit is 5 A and the current flowing is 9 A so the fuse melts. The circuit is broken and the electric oven is saved from damaging.

Question. What precaution should be taken to avoid the overloading of domestic electric currents?
Answer: We should not connect many appliances with a single socket to avoid overloading.

EXERCISES

Question. Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire.
(b) The field consists of straight lines parallel to the wire.
(c) The field consists of radial lines originating from the wire.
(d) The field consists of concentric circles centred on the wire.
Answer: (d) The field consists of concentric circles centred on the wire.

Question. At the time of short circuit, the current in the circuit
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.
Answer: (c) Increases heavily as the resistance becomes almost zero at the time of short circuit.

Question. State whether the following statements are true or false.
(a) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(b) A wire with a green insulation is usually the live wire of an electric supply.
Answer: (a) True: The magnetic field at the centre of the coil can be taken to be uniform.
(b) False: Green wire is used as earth wire.

Question. List two methods of producing magnetic fields.
Answer: Bar Magnet, straight wire carrying current and circular coil carrying current. (any two)

Question. When is the force experienced by a current carrying conductor placed in a magnetic field largest?
Answer: When the length of the conductor and magnetic field are perpendicular to each other, the force experienced is maximum.

Question. When does an electric short circuit occur?
Answer: When live wire touches the neutral wire, circuit offers very low resistance to the flow of current due to which current increases heavily in the circuit, electric short circuit occurs.

Question. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer: Earth wire offers low resistance path to the current and ensure that the body of the electrical appliance is at the same potential (electric) as that of earth (zero). Any leakage of current in an electrical appliances to their metallic body is immediately transferred to the earth through the earth wire and user gets protected from possible and dangerous electrical shock. Hence, it is necessary to ground the metallic appliances through the earth wire which acts as a safety measure.

SELECT NCERT EXEMPLAR PROBLEMS

Question. For a current in a long straight solenoid N-pole and S-pole are created at the two ends. Among the following statements, the incorrect statement is
(a) The field lines inside the solenoid are in the form of straight lines which indicates that the magnetic field is the same at all points inside the solenoid.
(b) The strong magnetic field produced inside the solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the coil.
(c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet.
(d) The N-pole and S-pole exchange position when the direction of current through the solenoid is reversed.
Answer: (c) A solenoid behaves like a bar magnet. Hence the pattern of magnetic field associated with solenoid and around the bar magnet is same.

Question. What is the role of the two conducting stationary brushes in a simple electric motor?
Answer: The two stationary conducting brushes make the contact in between the two halves of the split rings and the source battery. Current in the coil enters from the source battery through one conducting brush and flows back to the battery through another brush.

Question. It is established that an electric current through a metallic conductor produces a magnetic field around it. Is there a similar magnetic field produced around a thin beam of moving (i) alpha particles, (ii) neutrons? Justify your answer.
Answer: (i) Yes, similar magnetic field produced around a thin beam of moving alpha-particle, \( \alpha \)-particle being positively charged constitutes a current in the direction of beam motion. (ii) No, as neutron being electrically neutral and does not constitute a current.

Question. Meena draws magnetic field lines of field close to the axis of a current carrying circular loop. As she moves away from the centre of the circular loop, she observes that the lines keep on diverging. How will you explain her observation?
Answer: Strength of magnetic field at a point decreases as the distance increases \( B \propto \frac{1}{r} \). Therefore as she moves away from the centre of circular coil, magnetic field decreases. This is indicated by the decrease in degree of closeness of the field lines and appears diverging in nature.

Question. Inside the magnet, the field line moves:
(a) from north to south
(b) from south to north
(c) towards south pole
(d) away from north pole
Answer: (b) from south to north

Question. A magnetic field directed in north direction act on an electron in east direction. The magnetic force on the electron will act:
(a) vertically upwards
(b) towards east
(c) vertically downwards
(d) towards north
Answer: (c) vertically downwards

Question. At the time of short circuit, the current in the circuit
(a) reduces substantially
(b) remains same
(c) increases heavily
(d) vary continuously
Answer: (c) increases heavily

Question. The purpose of the Fleming’s left hand rule is used for finding the direction of the
(a) electric current
(b) magnetic field
(c) motion of conductor
(d) all of the options
Answer: (c) motion of conductor

Question. Assertion: A compass needle is placed near a current carrying wire. The deflection of the compass needle decreases when it is displaced away from the wire.
Reason: Strength of magnetic field decreases as one move away from a current carrying straight conductor.
(a) a
(b) b
(c) c
(d) d

Answer: (a) Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Question. Assertion: All electrical devices with metallic body have three pins excluding electric bulb which has two pin connection.
Reason: Three pin socket increases its cost of manufacturing.
(a) a
(b) b
(c) c
(d) d

Answer: (c) Assertion is true but the Reason is false.

Question. State the rule to determine the direction of a
(a) magnetic field produced around a straight conductor carrying current and
(b) force experienced by a current carrying straight conductor placed in a magnetic field which is perpendicular as to it.
Answer: (a) Right Hand Thumb Rule: If we hold a current carrying conductor in our right hand such that the thumb points in the direction of current, then our fingers wrap around the conductor in the direction of the magnetic field.
(b) Fleming's Left-Hand Rule: Stretch the thumb, forefinger and middle finger of your left hand mutually perpendicular. If the forefinger points in the direction of magnetic field and middle finger in the direction of current, the thumb gives the direction of force.

Question. State the two factors on which the magnitude of magnetic field due to current carrying solenoid depends.
Answer: (i) The amount of current flowing through the solenoid. (ii) The number of turns per unit length of the solenoid.

Question. What is the role of fuse, used in series with any electrical appliance?
Answer: The role of a fuse is to act as a safety device that protects the appliance from excessive current due to short circuiting or overloading by melting and breaking the circuit.

Question. How is the magnetic field at the centre of a circular coil affected if
(a) radius of the coil is increased.
(b) the amount of current flowing in the coil is increased.
(c) number of turns in the circular coil increases.
Answer: (a) Decreases, as \( B \propto \frac{1}{r} \). (b) Increases, as \( B \propto I \). (c) Increases, as the field due to each turn adds up.

Question. A current carrying conductor is placed in a magnetic field. Now answer the following:
(i) List the factors on which the magnitude of a force experienced by conductor depends.
(ii) When is the magnitude of this force maximum?
(iii) If initially this force was acting from right to left, how will the direction of force change, if:
(a) direction of magnetic field is reversed?
(b) direction of current is reversed?
Answer: (i) Strength of magnetic field, magnitude of current, and length of conductor in the field. (ii) When the conductor is perpendicular to the magnetic field. (iii) (a) Reverses (left to right). (b) Reverses (left to right).