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The Vault
RowQ
The Vault
CBSE Class 10 Physics · 11 questions · 26 marks
Every time current flows, it quietly creates a magnetic field around it, and that one fact powers motors, generators, and household safety circuits alike. This chapter links current and magnetism through the thumb rule and Fleming's rules, then shows how those same ideas run in reverse inside a generator. It closes the loop between electricity and magnetism that the rest of Class 10 Physics builds on.
Which rule gives the direction of the magnetic field around a straight current-carrying conductor?
Answer
Right-hand thumb rule (Maxwell's rule) — if a straight current-carrying conductor is held in the right hand with the thumb pointing along the direction of current flow, the curled fingers show the direction of the magnetic field lines circling the conductor.
Which rule is used to find the direction of the force on a current-carrying conductor placed in a magnetic field, and what device is this force the basis of?
Answer
Fleming's left-hand rule; the basis of an electric motor — with the forefinger along the field and the central finger along the current, the thumb gives the direction of the force on the conductor, which is exactly how an electric motor produces rotation.
What is the main difference between the working principles of an electric motor and an electric generator?
Answer
A motor converts electrical energy into mechanical energy using the force on a current-carrying conductor, while a generator converts mechanical energy into electrical energy using electromagnetic induction — a motor's direction of force is found using Fleming's left-hand rule, while a generator's induced current direction is found using Fleming's right-hand rule.
Why is a fuse always connected in series in the live wire, close to the point where electric supply enters a household circuit?
Answer
So it melts and disconnects the entire circuit from the supply the instant an unsafe, excessive current flows due to a short circuit or overload — being in series in the live wire near the point of entry, the fuse breaks the whole circuit as soon as unsafe current flows, protecting the household wiring and appliances from damage or fire.
Assertion (A): A current-carrying solenoid, when suspended freely, behaves like a bar magnet and settles along the north-south direction. Reason (R): The magnetic field pattern outside a current-carrying solenoid closely resembles that of a bar magnet, with one end acting as a north pole and the other as a south pole.
Answer
Both A and R are true and R is the correct explanation of A — because the external field of a current-carrying solenoid is essentially identical to that of a bar magnet, with one end behaving as a north pole and the other as a south pole, a freely suspended solenoid aligns itself along the north-south direction just as a bar magnet does.
State Fleming's left-hand rule and mention what it is used to find.
Answer
Fleming's left-hand rule: stretch the thumb, forefinger, and central finger of the left hand so that they are mutually perpendicular to one another. If the forefinger points in the direction of the magnetic field and the central finger points in the direction of the current, then the thumb points in the direction of the force (motion) experienced by the conductor. It is used to find the direction of the force on a current-carrying conductor placed in a magnetic field, and it explains the working of an electric motor.
State two ways in which the strength of the magnetic field produced by a current-carrying solenoid can be increased.
Answer
(i) Increasing the number of turns of wire in the solenoid. (ii) Increasing the current flowing through the solenoid. (Placing a soft iron core inside the solenoid also greatly increases the field strength, converting it into an electromagnet.)
Explain why it is dangerous to touch an electrical appliance with wet hands, in terms of the resistance of the human body. Also name the household circuit wire that must be connected to the earth for safety.
Answer
Dry human skin has fairly high electrical resistance, which limits the current that can pass through the body if it accidentally touches a live wire. Wet skin has much lower resistance, since water is a relatively good conductor, so touching a live appliance with wet hands allows a far larger current to pass through the body, which can cause a severe and potentially fatal electric shock. For safety, the metal body/casing of electrical appliances is connected through the earth wire (usually with green insulation) to a metal plate buried in the ground; this earthing provides a safe, low-resistance path for any leakage current, causing the fuse to blow and preventing the appliance's casing from becoming dangerously live.
Describe, in words, the pattern of magnetic field lines around (i) a straight current-carrying conductor and (ii) a current-carrying circular loop. State the factors on which the magnetic field at the centre of a current-carrying circular loop depends.
Answer
(i) Around a straight current-carrying conductor: the magnetic field lines form concentric circles centred on the wire, lying in planes perpendicular to it. Their direction is given by the right-hand thumb rule, and the circles spread farther apart (the field weakens) with increasing distance from the wire. (ii) Around a current-carrying circular loop: each small segment of the loop produces circular field lines as above. At the centre of the loop, the field contributions from all segments add up and appear as nearly straight, parallel lines perpendicular to the plane of the loop, while closer to the wire of the loop the lines remain nearly circular. One face of the loop behaves like a north pole (field lines emerge from it) and the opposite face behaves like a south pole (field lines converge into it). Factors affecting the field at the centre of a current-carrying circular loop: (i) it is directly proportional to the current flowing through the loop; (ii) it is inversely proportional to the radius of the loop, so a smaller loop gives a stronger field at its centre for the same current; (iii) it is directly proportional to the number of turns, so a coil of N turns produces N times the field of a single loop.
Explain the phenomenon of electromagnetic induction with a simple example. State Fleming's right-hand rule and explain how it gives the direction of an induced current. Briefly describe the working principle of an AC generator.
Answer
Electromagnetic induction is the production of an induced electromotive force (and an induced current, if the circuit is closed) in a conductor due to a change in the magnetic flux linked with it. For example, if a bar magnet is quickly pushed into a coil connected to a galvanometer, the galvanometer needle deflects, showing an induced current while the magnet is moving; the needle deflects the opposite way when the magnet is withdrawn, and shows no deflection when the magnet is held still relative to the coil — showing that it is the change in flux, not merely the presence of a magnetic field, that induces the current. Fleming's right-hand rule: stretch the thumb, forefinger, and central finger of the right hand mutually perpendicular to each other. If the forefinger points along the magnetic field and the thumb points along the direction of motion of the conductor, the central finger gives the direction of the induced current in the conductor. It is applied whenever a conductor moves through a magnetic field, to determine which way the induced current flows. AC generator: an AC generator converts mechanical energy into electrical energy through electromagnetic induction. A rectangular armature coil is rotated, by some mechanical means, between the poles of a strong field magnet. As the coil rotates, the magnetic flux linked with it keeps changing, inducing an alternating emf and current in the coil; this current is carried out of the generator through slip rings and brushes and reverses direction twice every rotation, producing alternating current (AC) — in India, this AC has a frequency of 50 Hz.
Read the following and answer the questions that follow: In many Indian homes, domestic electric supply is delivered through three wires: a live wire (red insulation), a neutral wire (black insulation), and an earth wire (green insulation), with a potential difference of 220 V between the live and neutral wires. Ramesh plugs an electric heater, a toaster, and an iron into the same extension socket and switches all three on together, after which the main fuse of the house blows and every appliance stops working. (a) Explain, in terms of current, why the fuse blew when Ramesh ran all three high-power appliances together. (b) What term describes this situation, where too many high-power appliances are drawn from a single circuit at once? (c) Explain how the earth wire specifically protects a person from electric shock. (d) Suggest one precaution Ramesh should follow to avoid this problem in future.
Answer
(a) Each high-power appliance (heater, toaster, iron) individually draws a large current. When all three are switched on together through the same socket/circuit, their currents add up, and the total current drawn far exceeds the safe current-carrying capacity of that circuit's wiring and fuse. This excess current heats the thin fuse wire past its melting point, so it melts and breaks the circuit, cutting off supply to all connected appliances. (b) This situation is called overloading of the electric circuit. (c) The earth wire connects the metal casing of an appliance to a metal plate buried in the ground. If a fault causes the live wire to touch the appliance's metal body, the earth wire offers a safe, low-resistance path for this leakage current to flow into the ground rather than through a person touching the appliance; the resulting surge of current also blows the fuse, cutting off the supply and preventing electric shock. (d) Ramesh should avoid plugging several high-power appliances into the same socket or extension at the same time, spreading them across separate circuits/sockets with appropriately rated fuses instead.
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