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Q1. An electric bulb produces light due to
An electric bulb produces light due to the heating effect of electric current. The filament (made of tungsten) heats up to a very high temperature (about 2500°C) and starts glowing, emitting light. This is called incandescence.
Q2. Electrical energy is supplied by
Electrical energy is supplied by a battery (chemical energy to electrical) or a generator (mechanical energy to electrical). Resistors consume energy, ammeters measure current, and voltmeters measure voltage.
Q3. In practical situations, current is calculated using
In practical situations, current is calculated using Ohm’s law: I = V/R. This gives the current flowing through a conductor when voltage and resistance are known.
Q4. Heating effect of current is unavoidable because electrical energy
The heating effect of current is unavoidable because electrical energy is converted to heat in any conductor with resistance. This is due to collisions between electrons and atoms in the conductor.
Q5. Heat produced in a resistor for steady current I in time t is
Heat produced in a resistor is H = VIt = I²Rt = V²t/R. All these forms are equivalent using Ohm’s law. H = VIt is a basic form of Joule’s law.
Q6. Tungsten is used because it has a melting point of about
Tungsten has a very high melting point of about 3380°C (actually 3422°C). This allows the filament to glow at high temperatures without melting. Tungsten also has high resistivity, making it suitable for bulb filaments.
Q7. Joule’s law of heating states that heat produced is proportional to
Joule’s law states that heat produced is proportional to the square of the current (I²), resistance (R), and time (t): H = I²Rt. This means if current doubles, heat becomes four times.
Q8. Fuse wire melts due to
A fuse wire melts due to the heating effect of electric current. When excess current flows, the fuse wire heats up and melts, breaking the circuit and protecting appliances from damage.
Q9. Resistance of the electric iron at maximum heating is
Q10. Power of a device depends on
Power of a device depends on voltage and current: P = VI. It can also be expressed as P = I²R or P = V²/R, so it indirectly depends on resistance as well.
Q11. Heating effect is useful in
The heating effect is useful in electric heaters, irons, toasters, and other heating appliances. In motors and fans, the heating effect is undesirable.
Q12. The work done in moving a charge Q through a potential difference V is
Work done W = VQ, where V is the potential difference and Q is the charge. This is the energy required to move charge Q through voltage V.
Q13. Fuse wire is made of a metal having
Fuse wire is made of a metal having an appropriate (low) melting point, like tin or lead alloy. It melts quickly when excess current flows, breaking the circuit.
Q14. Joule’s law explains conversion of
Joule’s law explains the conversion of electrical energy to heat energy when current flows through a resistor. This is the heating effect of electric current.
Q15. A fuse is connected in
A fuse is connected in series with the circuit it protects. It must carry the same current as the circuit. If excess current flows, the fuse melts and breaks the circuit.
Q16. A refrigerator of 400 W used for 8 h/day for 30 days consumes
Energy = Power × Time = 400W × 8h/day × 30 days = 400 × 240 = 96,000 Wh = 96 kWh. So it consumes 96 kWh.
Q17. One kilowatt hour is equal to
1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J. This is the commercial unit of electrical energy.
Q18. Kilowatt hour is a unit of
Kilowatt hour (kWh) is a unit of energy. It is the energy consumed by a device of power 1 kW operating for 1 hour.
Q19. The filament of an electric bulb is made of
The filament of an electric bulb is made of tungsten because it has a very high melting point (3422°C) and high resistivity, allowing it to glow brightly without melting.
Q20. Electric irons and toasters work on
Electric irons and toasters work on the heating effect of electric current. They convert electrical energy into heat energy for ironing or toasting.
Q21. Heating effect is undesirable in circuits because it
Heating effect is undesirable in circuits because it converts useful electrical energy into heat, which is wasted energy. This reduces efficiency and can damage components.
Q22. The bulb filament is surrounded by nitrogen or argon to
The bulb filament is surrounded by inert gases like nitrogen or argon to prevent oxidation of the hot tungsten filament. This increases the life of the bulb.
Q23. A fuse wire melts when
A fuse wire melts when excess current flows through it. The heating effect of this excess current melts the fuse wire, breaking the circuit and protecting appliances.
Q24. An electric iron consuming 840 W at 220 V draws a current of
Using P = VI, I = P/V = 840 W / 220 V = 3.818 A ≈ 3.8 A. So the iron draws about 3.8 A.
Q25. Commercial unit of electrical energy is
The commercial unit of electrical energy is the kilowatt hour (kWh). It is used by electricity companies to bill consumers.
Q26. Most of the power consumed by a bulb appears as
Most of the power consumed by a bulb appears as heat. Only a small fraction (about 5-10%) is converted to light. The rest is wasted as heat.
Q27. Energy supplied by the source in time t is
Energy supplied by the source is E = VIt (since Q = It, so E = VQ = VIt). This is the total electrical energy delivered in time t.
Q28. Heat produced when current flows through a resistor depends on
Heat produced is H = I²Rt, so it depends on current (I), resistance (R), and time (t). This is Joule’s law.
Q29. Cost of 96 kWh at Rs 3 per unit is
Cost = 96 kWh × Rs 3 = Rs 288. This is the electricity bill for consuming 96 kWh.
Q30. Domestic fuses are rated as
Domestic fuses are rated at low currents like 1 A, 2 A, 3 A, 5 A, etc., for different circuits. Higher ratings are used for heavy appliances.
Q31. Resistance of the iron at minimum heating is
Q32. When heating is minimum at 360 W and 220 V, current drawn is
Using P = VI, I = P/V = 360 W / 220 V = 1.636 A ≈ 1.6 A. So the current drawn is about 1.6 A.
Q33. An electric motor mainly converts electrical energy into
An electric motor mainly converts electrical energy into mechanical energy (rotation of the shaft). It also produces some heat as a by-product.
Q34. Joule’s law of heating is applicable when current is
Joule’s law H = I²Rt applies for steady (direct) current. For alternating current, the RMS value of current is used.
Q35. Electrons in a circuit are
Electrons in a circuit are not consumed. They flow through the circuit and carry energy, but they are not used up. The energy is transferred from the source to the load.
Q36. According to Joule’s law, heat produced is directly proportional to
According to Joule’s law, heat produced is directly proportional to resistance (H = I²Rt). Higher resistance produces more heat for the same current.
Q37. SI unit of electric power is
The SI unit of electric power is the watt (W). One watt is one joule per second (1 W = 1 J/s). Power is the rate of doing work.
Q38. Power input to an electric circuit is given by
Power input to an electric circuit is P = VI (voltage × current). This is the rate at which electrical energy is delivered.
Q39. Rate at which energy is delivered by a current is
Power is the rate at which energy is delivered or consumed. It is measured in watts (W). Power = Energy / Time.
Q40. Heating effect increases when current
Heating effect increases when current increases because H = I²Rt. If current doubles, heat becomes four times.
Q41. We pay electricity bills for
We pay electricity bills for the energy consumed, measured in kilowatt hours (kWh). The more energy you use, the higher the bill.
Q42. For a 1 kW iron working at 220 V, the correct fuse rating is
Current I = P/V = 1000 W / 220 V = 4.5 A. A 5 A fuse is appropriate as it’s slightly higher than the normal current.
Q43. Electric power is defined as
Electric power is defined as the rate of doing work or the rate of energy transfer. P = W/t = VI.
Q44. Electric power is given by
Electric power is given by P = VI (voltage × current). This is the fundamental formula for electrical power.
Q45. Filament of a bulb must have
The filament of a bulb must have a high melting point so it can glow at high temperatures without melting. Tungsten is used because of its high melting point.
Q46. A bulb draws 0.5 A from a 220 V source. Its power is
Power P = VI = 220 V × 0.5 A = 110 W. So the bulb consumes 110 watts.
Q47. One watt is equal to
One watt is equal to 1 volt × 1 ampere (1 W = 1 V × 1 A). It is the power when 1 ampere of current flows at 1 volt.
Q48. Using Ohm’s law, the heat produced in a resistor is
Using Ohm’s law (V = IR), the heat produced is H = I²Rt. This is Joule’s law and is the standard formula for heat produced in a resistor.
Q49. A fuse protects electrical appliances by
A fuse protects electrical appliances by breaking the circuit when excess current flows. This prevents damage to appliances and reduces the risk of fire.
Q50. Heat produced in a resistor is directly proportional to
Heat produced in a resistor is directly proportional to time (H = I²Rt). The longer the current flows, the more heat is produced. It is also proportional to I² and R.