Question 1
A car battery has emf 12 V and internal resistance 0.4 Ω. What is the maximum current it can supply?
Steps only
Imax=ε/r=12/0.4=30 A
Answer
30 A
CLASS 12 · PHYSICS · CHAPTER 3
Current electricity
Simple worked answers to all 9 current exercises on current, resistance and cells.
9 worked answersComplete chapter answers
9 worked answers · short steps first, explanations when you need them
Chapter 3 Exercises
Textbook pages 105–106
Question 2
A cell of emf 10 V and internal resistance 3 Ω gives a current of 0.5 A. Find the external resistance and the cell's terminal voltage.
Steps only
R=ε/I−r=20−3=17 Ω; V=IR=0.5×17=8.5 V
Answer
17 Ω; 8.5 V
Question 3
A heater's resistance is 100 Ω at 27°C and 117 Ω at a higher temperature. If its temperature coefficient is 1.70×10⁻⁴ °C⁻¹, find the higher temperature.
Steps only
117=100[1+1.70×10⁻⁴(T−27)] ⇒ T=1027°C
Answer
1027°C
Question 4
A 15 m wire has cross-sectional area 6.0×10⁻⁷ m² and resistance 5 Ω. Find its resistivity.
Steps only
ρ=RA/L=5(6.0×10⁻⁷)/15=2.0×10⁻⁷ Ω m
Answer
2.0×10⁻⁷ Ω m
Question 5
A silver wire has resistance 2.1 Ω at 27.5°C and 2.7 Ω at 100°C. Find its temperature coefficient near 27.5°C.
Steps only
α=0.6/[2.1(72.5)]≈3.94×10⁻³ °C⁻¹
Answer
3.9×10⁻³ °C⁻¹
Question 6
A 230 V nichrome heater draws 3.2 A at 27°C and 2.8 A after it becomes hot. If α=1.70×10⁻⁴ °C⁻¹, find its operating temperature.
Steps only
Rc=71.875 Ω; Rh=82.143 Ω; T=27+[(Rh/Rc−1)/α]≈868°C
Answer
≈868°C
Question 7
In Fig. 3.20, a 10 V battery and a 10 Ω resistor are connected to a bridge with AB=10 Ω, BC=5 Ω, AD=5 Ω, DC=10 Ω and BD=5 Ω. Find the current in each resistor.
Steps only
Bridge voltage=70/27 V. Branch currents: AB=DC=4/27 A; BC=AD=2/9 A; BD=2/27 A from D to B.
Answer
AB 4/27 A; BC 2/9 A; AD 2/9 A; DC 4/27 A; BD 2/27 A (D→B)
Question 8
An 8 V cell with internal resistance 0.5 Ω is charged from a 120 V source through a 15.5 Ω resistor. Find the terminal voltage and explain the resistor's purpose.
Steps only
I=(120−8)/(15.5+0.5)=7 A; V=ε+Ir=8+3.5=11.5 V
Answer
7 A; 11.5 V; resistor limits current
Question 9
Copper has 8.5×10²⁸ free electrons per cubic metre. How long does an electron take to drift through a 3 m wire of area 2×10⁻⁶ m² carrying 3 A? Take the electron charge as 1.6×10⁻¹⁹ C.
Steps only
vd=I/(neA)≈1.10×10⁻⁴ m/s; t=3/vd≈2.72×10⁴ s≈7.6 h
Answer
≈2.72×10⁴ s (7.6 h)