Electricity · Class 10 Science: 88 flashcards
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1. Define electric current and write its formula.
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Electric current is the rate of flow of electric charge through a conductor: I = Q/t.
2. What is the SI unit of electric charge? How many electrons make it?
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The coulomb (C). 1 C is the charge of about 6 × 10¹⁸ electrons.
3. What is the charge on one electron?
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1.6 × 10⁻¹⁹ coulomb (negative).
4. Define 1 ampere.
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The current when 1 coulomb of charge flows through a conductor in 1 second: 1 A = 1 C/s.
5. Express 1 mA and 1 μA in amperes.
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1 mA = 10⁻³ A; 1 μA = 10⁻⁶ A.
6. What is the direction of conventional current?
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From the positive terminal to the negative terminal of the cell (in the external circuit) — the direction a positive charge would move.
7. In which direction do electrons actually flow in a circuit?
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From the negative terminal to the positive terminal — opposite to conventional current.
8. Define potential difference between two points.
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The work needed to carry one unit of charge across from the first point to the second: V = W/Q.
9. Define 1 volt.
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The potential difference between two points when 1 joule of work is done to move 1 coulomb of charge between them: 1 V = 1 J/C.
10. What maintains the potential difference in a circuit?
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The cell or battery — it converts chemical energy into electrical energy to keep its terminals at different potentials.
11. How is an ammeter connected in a circuit and why?
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In series, so the entire current passes through it; its resistance is kept very low so it doesn't reduce that current.
12. How is a voltmeter connected and why?
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In parallel across the component, so it compares the potential at two points; its resistance is very high so it draws almost no current.
13. What happens if an ammeter is wrongly connected in parallel?
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Its near-zero resistance short-circuits the component — a dangerously large current flows and can damage the ammeter.
14. What is an electric circuit?
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A continuous and closed conducting path along which electric current can flow — typically cell, wires, devices and a switch.
15. Why does no current flow when the switch is open?
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The conducting path is broken; without a closed loop, charges cannot flow continuously anywhere in the circuit.
16. A current of 2 A flows for 30 s. How much charge passes?
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Q = I × t = 2 × 30 = 60 C.
17. How much work moves 5 C of charge across a 12 V battery's terminals?
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W = V × Q = 12 × 5 = 60 J.
18. How many electrons flow per second in a wire carrying 1 A?
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n = Q/e = 1/1.6×10⁻¹⁹ ≈ 6.25 × 10¹⁸ electrons every second.
19. In a cell symbol, which line is the positive terminal?
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The longer, thinner line; the shorter, thicker line is the negative terminal.
20. What is the difference between a cell and a battery?
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A cell is a single unit that produces a potential difference; a battery is a combination of two or more cells (usually in series).
21. State the water analogy for current and potential difference.
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Current is like water flow in a pipe; potential difference is like the pressure difference that drives the flow; the cell acts like the pump maintaining that pressure.
22. Why doesn't current get 'used up' around a series circuit?
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Charge is conserved — the same charges keep circulating. What the circuit components use is the ENERGY carried by the charges, not the charges themselves.
23. State Ohm's law.
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At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends: V = IR.
24. What condition must hold for Ohm's law to apply?
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The temperature and other physical conditions of the conductor must remain constant.
25. Define resistance and state its SI unit.
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Resistance is the opposition offered by a conductor to the flow of current, R = V/I. Its SI unit is the ohm (Ω).
26. Define 1 ohm.
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A conductor has a resistance of 1 ohm if a current of 1 ampere flows through it when a potential difference of 1 volt is applied across its ends.
27. What causes resistance in a conductor at the microscopic level?
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The drifting electrons repeatedly collide with the atoms/ions of the conductor, which opposes their motion.
28. What does the slope of a V–I graph (V on y-axis) give?
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The resistance R of the conductor. A straight line through the origin confirms ohmic behaviour.
29. List the four factors on which the resistance of a wire depends.
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(1) Length — R ∝ l; (2) area of cross-section — R ∝ 1/A; (3) nature of the material (resistivity); (4) temperature.
30. Write the relation between R, ρ, l and A.
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R = ρl/A.
31. Define resistivity and give its SI unit.
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Resistivity is the resistance of a conductor of unit length and unit area of cross-section, ρ = RA/l. SI unit: ohm-metre (Ω·m).
32. Does resistivity change if the wire is cut in half?
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No — resistivity depends only on the material and temperature. Only the RESISTANCE halves (R ∝ l).
33. How does resistance of a metal change with temperature?
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It increases — hotter atoms vibrate more and collide more often with the drifting electrons.
34. A wire's length is doubled (same thickness). What happens to its resistance?
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It doubles — R ∝ l.
35. A wire is replaced by one of the same material and length but double the diameter. What happens to R?
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Area ∝ diameter², so A becomes 4× and R becomes one-fourth.
36. A wire is stretched to double its length. What happens to its resistance?
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Volume stays constant, so the area halves while length doubles: R' = 4R.
37. Give the resistivity ranges of conductors and insulators.
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Conductors (metals): about 10⁻⁸ to 10⁻⁶ Ω·m. Insulators: about 10¹² to 10¹⁷ Ω·m.
38. Why are heating elements made of nichrome (an alloy) and not a pure metal?
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Nichrome has high resistivity (strong heating), a very high melting point, does not oxidise even at red heat, and its resistance barely changes with temperature.
39. Why are copper and aluminium used for electrical wiring?
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They have very low resistivity, so very little energy is wasted as heat when current flows through them; aluminium is also light and cheap for long lines.
40. What is a rheostat and what is it used for?
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A variable resistance with a sliding contact, used to change the current in a circuit without changing the voltage source.
41. A 6 V battery drives 0.3 A through a wire. Find its resistance.
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R = V/I = 6/0.3 = 20 Ω.
42. What current flows through a 44 Ω device on a 220 V supply?
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I = V/R = 220/44 = 5 A.
43. Which material is the best conductor of electricity?
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Silver — resistivity about 1.6 × 10⁻⁸ Ω·m; copper is used instead mainly because silver is expensive.
44. Name two alloys used for standard resistors and why alloys suit them.
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Manganin and constantan — their resistivity is almost unchanged by temperature, so the resistance value stays reliable.
45. What quantity is the same for all resistors in SERIES?
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The current — there is only one path, so the same current flows through each resistor.
46. What quantity is the same for all resistors in PARALLEL?
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The potential difference — every branch is connected between the same two points.
47. Write the equivalent resistance formula for three resistors in series.
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Rs = R₁ + R₂ + R₃.
48. Write the equivalent resistance formula for three resistors in parallel.
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1/Rp = 1/R₁ + 1/R₂ + 1/R₃.
49. State the product-over-sum rule.
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For exactly two resistors in parallel: Rp = R₁R₂/(R₁ + R₂).
50. Find the equivalent of 3 Ω and 6 Ω in parallel.
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Rp = (3 × 6)/(3 + 6) = 18/9 = 2 Ω.
51. Find the equivalent of 5 Ω, 10 Ω and 15 Ω in series.
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Rs = 5 + 10 + 15 = 30 Ω.
52. What are the equivalents of n equal resistors R in series and in parallel?
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Series: nR. Parallel: R/n.
53. Is the series equivalent bigger or smaller than the largest resistor?
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Bigger — series resistances add, so Rs exceeds every individual resistor.
54. Is the parallel equivalent bigger or smaller than the smallest resistor?
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Smaller — every extra branch adds another path, so Rp drops below the smallest branch.
55. In series, which resistor gets the largest share of the voltage?
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The largest resistance — V = IR with the same I, so voltage divides in direct proportion to R.
56. In parallel, which branch carries the largest current?
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The smallest resistance — I = V/R with the same V, so current divides in inverse proportion to R.
57. Why does breaking one bulb in a series string switch off all the others?
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There is only one conducting path; a break anywhere opens the whole loop and stops the current everywhere.
58. Give three reasons domestic circuits are wired in parallel.
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(1) Every appliance gets the full supply voltage (220 V). (2) Each appliance can be switched on/off independently. (3) A fault in one branch doesn't stop the others (and total resistance stays low for heavy loads).
59. Two 8 Ω resistors are connected in parallel. Find the equivalent.
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R/n = 8/2 = 4 Ω.
60. What is the maximum resistance obtainable from resistors of 4 Ω, 8 Ω and 12 Ω?
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All in series: 4 + 8 + 12 = 24 Ω.
61. What is the minimum resistance obtainable from 4 Ω, 8 Ω and 12 Ω?
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All in parallel: 1/Rp = 1/4 + 1/8 + 1/12 = 11/24 → Rp = 24/11 ≈ 2.18 Ω.
62. A 2 A current enters a parallel pair of 3 Ω and 6 Ω. How does it split?
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Inversely to resistance — 2:1. The 3 Ω branch carries 4/3 A and the 6 Ω branch 2/3 A.
63. In a mixed circuit, what should you simplify first?
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The innermost pure-series or pure-parallel group; replace it with its equivalent, redraw, and repeat outward.
64. Why does adding one more parallel branch always increase the total current from the battery?
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The new branch lowers the circuit's total resistance while V is unchanged, so I = V/R rises — the battery feeds the new road on top of the old ones.
65. Three 2 Ω resistors: list every equivalent you can build using all three.
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All series: 6 Ω. All parallel: 2/3 Ω. Two in parallel + one in series: 1 + 2 = 3 Ω. Two in series + one in parallel: 4×2/6 = 4/3 Ω.
66. A voltmeter connected across one of several series resistors reads what exactly?
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The potential difference across that one resistor only — its share V = IR of the battery voltage.
67. State Joule's law of heating.
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The heat produced in a resistor is directly proportional to the square of the current, the resistance, and the time: H = I²Rt.
68. Why does a conductor heat up when current flows through it?
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Drifting electrons collide with the conductor's atoms and transfer energy to them; the increased atomic vibration appears as heat.
69. If the current through a resistor is doubled, how does the heat change (same R, t)?
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It becomes four times — heat is proportional to the SQUARE of the current.
70. Why is the heating element of appliances made of nichrome?
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High resistivity (more heat), very high melting point, and it does not oxidise even when red hot.
71. Why don't the connecting wires of a heater glow while its element does?
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The same current flows through both, but heat ∝ resistance. The copper wires have negligible resistance compared to the element, so nearly all the heat appears in the element.
72. What is an electric fuse and where is it connected?
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A safety wire of low-melting-point alloy connected in SERIES with the live wire; it melts and breaks the circuit when current exceeds its rating.
73. What property must fuse-wire material have?
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A LOW melting point (and suitable resistance) — so that it melts quickly when the current becomes excessive.
74. Why should a fuse never be replaced by a thick copper wire?
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Copper's high melting point and low resistance mean it would not melt during a fault — dangerous currents would flow unchecked, risking fire.
75. Define electric power and its SI unit.
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The rate at which electrical energy is consumed or dissipated: P = W/t = VI. SI unit: watt (W); 1 W = 1 J/s.
76. Write the three forms of the power formula.
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P = VI = I²R = V²/R.
77. Which power formula suits series problems, and why?
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P = I²R — in series the current is common to all components, so comparing I²R across them is direct.
78. Which power formula suits appliances on the mains, and why?
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P = V²/R — appliances in parallel all share the same mains voltage.
79. A bulb is rated 220 V, 100 W. Find its working resistance.
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R = V²/P = (220)²/100 = 48400/100 = 484 Ω.
80. Find the normal current of a 220 V, 1100 W appliance and a suitable fuse.
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I = P/V = 1100/220 = 5 A; use a fuse rated just above, e.g. 6 A (or the 5 A line's next standard rating).
81. Define 1 watt and 1 kilowatt.
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1 watt is the power when 1 joule of energy is consumed per second (1 W = 1 J/s); 1 kilowatt = 1000 watts.
82. Define the kilowatt-hour.
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The energy consumed by a 1 kW device operating for 1 hour — the commercial unit of electrical energy: 1 kWh = 3.6 × 10⁶ J.
83. Show that 1 kWh = 3.6 × 10⁶ J.
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1 kWh = 1000 W × 3600 s = 3,600,000 J = 3.6 × 10⁶ J.
84. A 500 W TV runs 6 hours. How many units does it consume?
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E = 0.5 kW × 6 h = 3 kWh = 3 units.
85. Why is electric power transmitted at high voltage over long distances?
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For fixed power P = VI, high V allows small I; line losses are I²R, so a smaller current wastes far less energy as heat in the wires.
86. Which has more resistance: a 60 W or a 100 W bulb (both 220 V)?
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The 60 W bulb — R = V²/P, so lower power rating means higher resistance (807 Ω vs 484 Ω).
87. In a series connection of a 60 W and 100 W bulb, which glows brighter?
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The 60 W bulb — same current through both, P = I²R, and the 60 W bulb has the higher resistance.
88. A 4 Ω resistor carries 2.5 A for 2 minutes. Find the heat produced.
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H = I²Rt = (2.5)² × 4 × 120 = 6.25 × 4 × 120 = 3000 J.
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