Effects of electric current
Short Questions
1. What determines if a material is a good conductor of electricity?
Answer : A material with low resistance, allowing easy flow of electrons, is a good conductor.
2. Why don’t we get an electric shock from a piece of iron on the ground?
Answer : It’s not connected to a live circuit, so no current flows through it.
3. What is the source of energy in an electric circuit with a resistor?
Answer : The electric cell provides the energy to drive the current through the resistor.
4. How is electrical power defined in a circuit?
Answer : Electrical power is the rate of energy transfer, given by P = V × I (volt × ampere).
5. What is Joule’s law of heating?
Answer : Heat produced in a resistor is H = I² × R × t, where I is current, R is resistance, and t is time.
6. Why is nichrome used in electric heaters?
Answer : Nichrome has high resistivity and can withstand high temperatures without melting.
7. What unit is used for electrical energy consumption in households?
Answer : Kilowatt-hour (kWh), where 1 kWh = 3.6 × 10⁶ J.
8. What causes a short circuit in a home electrical system?
Answer : Contact between live and neutral wires due to faulty insulation causes excessive current flow.
9. What is the function of a fuse wire in a circuit?
Answer : It melts at high current to break the circuit, preventing fire or damage.
10. What produces a magnetic field around a current-carrying wire?
Answer : The flow of electric current generates a magnetic field, as discovered by Oersted.
11. What does the right-hand thumb rule predict?
Answer : It determines the direction of magnetic field lines around a current-carrying conductor.
12. How does the magnetic field vary with distance from a current-carrying wire?
Answer : The magnetic field strength decreases as distance from the wire increases.
13. What is a solenoid, and how does it behave magnetically?
Answer : A solenoid is a coil of wire that acts like a bar magnet when current flows through it.
14. What does Fleming’s left-hand rule describe?
Answer : It relates the directions of current, magnetic field, and force on a conductor in a motor.
15. What is the principle behind an electric motor?
Answer : It converts electrical energy to mechanical energy via force on a current-carrying conductor in a magnetic field.
16. What is electromagnetic induction, as discovered by Faraday?
Answer : It is the production of current in a conductor due to a changing magnetic field.
17. When is the induced current maximum in a conductor?
Answer : When the conductor’s motion is perpendicular to the magnetic field.
18. What does Fleming’s right-hand rule indicate?
Answer : It shows the direction of induced current relative to the magnetic field and conductor motion.
19. How does an AC generator differ from a DC generator?
Answer : An AC generator produces alternating current, while a DC generator produces direct current.
20. What is the frequency of AC power supply in India?
Answer : The frequency of alternating current in India is 60 Hz.
Long Questions
1. Explain how energy is transferred in an electric circuit with a resistor.
Answer : The cell converts chemical energy into electrical energy, driving charge Q through the resistor. The work done, VQ, is converted to heat energy in the resistor, following H = I²Rt. This heat increases the resistor’s temperature, as per Joule’s law.
2. Why does a short circuit lead to fire hazards, and how is it prevented?
Answer : A short circuit occurs when live and neutral wires touch, causing a large current to flow. This generates excessive heat, igniting nearby flammable materials like wood or plastic. Fuse wires or MCBs melt or trip to break the circuit, preventing fires.
3. How does the heating effect of electric current power household appliances?
Answer : Current through high-resistivity materials like nichrome or tungsten produces heat (H = I²Rt). In appliances like heaters, this heat is used for cooking or warming; in bulbs, it causes light emission. The choice of material ensures high heat output without melting.
4. Describe the magnetic field produced by a current-carrying straight conductor.
Answer : A straight conductor carrying current produces a magnetic field in concentric circles around it. The field’s direction is given by the right-hand thumb rule, with the thumb pointing along the current. Field strength decreases with distance and increases with current.
5. How does a solenoid’s magnetic field resemble that of a bar magnet?
Answer : A solenoid’s magnetic field has parallel lines inside, with one end as a north pole and the other as a south pole, like a bar magnet. The field is uniform inside the solenoid, unlike the curved external field of a bar magnet. The right-hand rule determines pole orientation.
6. Explain the force on a current-carrying conductor in a magnetic field.
Answer : A conductor in a magnetic field experiences a force perpendicular to both the current and field directions, per Fleming’s left-hand rule. The force is maximum when the current is perpendicular to the field, causing motion in devices like motors. Reversing current or field direction reverses the force.
7. Describe the working principle of an electric motor with a diagram.
Answer : A rectangular coil in a magnetic field rotates due to forces on opposite sides, per Fleming’s left-hand rule. Split rings reverse current every half-turn, ensuring continuous anticlockwise rotation. This converts electrical energy into mechanical energy for applications like fans.
8. How does electromagnetic induction produce current in a conductor?
Answer : A changing magnetic field, caused by moving a magnet or altering current in a nearby coil, induces current in a conductor. Faraday’s law states this occurs when magnetic flux through the coil changes. The induced current’s direction follows Fleming’s right-hand rule.
9. Why is alternating current (AC) preferred for long-distance power transmission?
Answer : AC can be stepped up to high voltages, reducing current and minimizing power loss (I²R) in wires. It can be easily transformed to lower voltages for household use. This efficiency makes AC more practical than DC for long-distance transmission.
10. Explain the construction and working of an AC generator.
Answer : A coil rotates in a magnetic field, inducing current via electromagnetic induction, with ends connected to slip rings. As the coil rotates, the magnetic flux changes, producing alternating current, per Fleming’s right-hand rule. Slip rings ensure continuous AC output to external circuits.
11. How does a DC generator differ from an AC generator in construction and output?
Answer : A DC generator uses a split ring (commutator) instead of slip rings, ensuring current flows in one direction externally. The coil’s rotation in a magnetic field induces current, but the commutator reverses connections every half-turn. This produces direct current, unlike the oscillatory AC output.
12. Calculate the resistance of a circuit producing 100 W with a 3 A current, and explain its significance.
Answer : Using P = I²R, R = P/I² = 100/(3²) = 100/9 ≈ 11 Ω. This resistance determines the heat generated in the circuit, critical for appliance design. Higher resistance increases heat output, as per Joule’s law, affecting efficiency and safety.
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