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Electric Current, Drift Velocity and Ohm's Law · Class XII / Second PUC

Name: ____________________
Date: ____________________
Mode: BalancedQuestions: 18Difficulty: MixedCheck code: 879944781
Question 1Challenge

1. A current of 5 A flows through a wire for 2 minutes, then increases to 8 A for the next 3 minutes. Find the total charge that has flowed.

  1. 600 C
  2. 1040 C
  3. 2040 C
  4. 1440 C
Question 2Foundation

2. A wire of cross-sectional area 4×10⁻⁶ m² carries a current of 20 A. Find the current density.

  1. 1×10⁷ A/m²
  2. 5×10⁵ A/m²
  3. 5×10⁶ A/m²
  4. 2.5×10⁶ A/m²
Question 3Application

3. Inside a battery's external circuit, electrons flow from the negative terminal, through the wire, to the positive terminal. In which direction is conventional current considered to flow through this EXTERNAL circuit?

  1. From the negative terminal to the positive terminal, matching electron flow
  2. There is no conventional current in this case
  3. Conventional current only exists inside the battery
  4. From the positive terminal, through the wire, to the negative terminal
Question 4Application

4. A single wire carries the same current through a wide section (A=18×10⁻⁶ m²) and a narrow section (A=2×10⁻⁶ m²). Find the ratio v_d(narrow)/v_d(wide).

  1. 18
  2. 9
  3. 1/9
  4. 4.5
Question 5Application

5. Two wires, X and Y, made of the same metal (same n) carry the same current, but wire X has twice the cross-sectional area of wire Y. Compare their drift velocities.

  1. Drift velocity in X is half that in Y
  2. Drift velocity in X is double that in Y
  3. They are equal
  4. Cannot be determined without more information
Question 6Application

6. A resistor's resistance is found to increase noticeably as more current is passed through it, due to self-heating. Strictly speaking, over this range of increasing current, is the resistor's V-I behavior perfectly ohmic?

  1. Yes, as long as V and I are both positive
  2. Cannot be determined without knowing the exact resistance values
  3. No — R must stay constant for the behavior to be strictly ohmic, and here it's changing due to heating
  4. Yes, since V=IR can still be computed at each point
Question 7Foundation

7. A charge of 104 C flows through a wire in 8 seconds. Find the current.

  1. 13 A
  2. 0.077 A
  3. 832 A
  4. 96 A
Question 8Challenge

8. A wire's free electron density is 5×10²⁸ per m³ and its cross-sectional area is 2.5×10⁻⁶ m². If the drift velocity is measured to be 4×10⁻⁴ m/s, find the current flowing, then find how many electrons pass a given cross-section every second. Use e=1.6×10⁻¹⁹ C.

  1. I=8 A, 1.25×10¹⁹ electrons/s
  2. I=1.25×10²³ A, 5×10¹⁹ electrons/s
  3. I=8 A, 8×10¹⁹ electrons/s
  4. I=8 A, 5×10¹⁹ electrons/s
Question 9Challenge

9. A wire tapers uniformly so that its cross-sectional area varies with position x as A(x)=A₀(1+x) (x in metres, A₀=2×10⁻⁶ m²), carrying a steady current of 8 A. Find the current density at x=0 and at x=3 m.

  1. J(0)=1×10⁶ A/m², J(3)=4×10⁶ A/m²
  2. J(0)=4×10⁶ A/m², J(3)=4×10⁶ A/m² (unchanged)
  3. J(0)=8×10⁶ A/m², J(3)=2×10⁶ A/m²
  4. J(0)=4×10⁶ A/m², J(3)=1×10⁶ A/m²
Question 10Challenge

10. A wire carries a current that grows linearly with time: I(t)=2t (in amperes, t in seconds), starting from t=0. Find the total charge that flows between t=0 and t=5 s.

  1. 50 C
  2. 5 C
  3. 25 C
  4. 10 C
Question 11Challenge

11. A current of 4 A flows in one direction for 90 s, then reverses direction and flows at 3 A for the next 60 s. Taking the original direction as positive, find the NET charge transferred.

  1. −180 C
  2. 180 C
  3. 540 C
  4. 360 C
Question 12Challenge

12. A component's resistance R(T) increases with temperature: at 20°C, R=8 Ω; at 70°C, R=13 Ω. If it carries exactly 2 A at 70°C, find the voltage across it at that temperature, and state whether the device behaves as strictly ohmic across this temperature range.

  1. 13 V; not ohmic
  2. 26 V; not strictly ohmic across this temperature range, since R itself changes
  3. 16 V; ohmic across the range
  4. 26 V; ohmic across the range, since V=IR was still used correctly
Question 13Application

13. How many electrons flow past a cross-section per second in a wire carrying exactly 3.2 A? Use e=1.6×10⁻¹⁹ C.

  1. 6.25×10¹⁸ electrons/s
  2. 3.2×10⁻¹⁹ electrons/s
  3. 2×10¹⁸ electrons/s
  4. 2×10¹⁹ electrons/s
Question 14Foundation

14. A wire has n=1×10²⁸ per m³, A=4×10⁻⁶ m², carrying a current of 3.2 A. Find the drift velocity. Use e=1.6×10⁻¹⁹ C.

  1. 5×10⁻⁵ m/s
  2. 5×10⁻⁴ m/s
  3. 1×10⁻⁴ m/s
  4. 2.5×10⁻⁴ m/s
Question 15Challenge

15. A copper wire (n=8.5×10²⁸ per m³) has cross-sectional area 1×10⁻⁶ m² and carries a current of 3.4 A. Find the drift velocity, then find the time taken for an electron to drift 1 m along the wire (assume the drift velocity stays constant). Use e=1.6×10⁻¹⁹ C.

  1. v_d=2.5×10⁻⁴ m/s; time≈4000 s (≈66.7 min)
  2. v_d=2.5×10⁻⁴ m/s; time≈400 s
  3. v_d=4×10³ m/s; time≈2.5×10⁻⁴ s
  4. v_d=2.5×10⁻⁴ m/s; time≈40000 s
Question 16Challenge

16. A conductor's resistance is measured to be 6 Ω. As current is increased from 2 A to 10 A, the voltage across it is measured at several points, and the V-I graph turns out to be a PERFECT straight line through the origin throughout this entire range. What can be concluded?

  1. R must be changing even though the graph looks straight
  2. More data is needed regardless of the straight-line result
  3. The conductor IS ohmic across this entire current range, with R=6 Ω throughout
  4. The conductor cannot be ohmic since current varies so much
Question 17Challenge

17. A wire narrows uniformly from area A₁=12×10⁻⁶ m² to area A₂=3×10⁻⁶ m² over its length, carrying a steady current of 6 A throughout. Find the current density J₂ at the narrow end, and the ratio J₂/J₁.

  1. J₂=2×10⁶ A/m², ratio J₂/J₁=4
  2. J₂=5×10⁵ A/m², ratio=1
  3. J₂=2×10⁶ A/m², ratio=0.25
  4. J₂=8×10⁶ A/m², ratio=16
Question 18Challenge

18. A wire carries a current that varies with time as I(t)=3t² (in amperes, t in seconds). Find the total charge that flows between t=0 and t=4 s.

  1. 192 C
  2. 64 C
  3. 48 C
  4. 16 C
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