CHAPTER 3: ELECTRIC CURRENT AND DIRECT-CURRENT CIRCUITS UPS SEMESTER 2 2011/2012 1. (a) (i) What is meant by electrical resistivity? (ii) Calculate the resistance of an iron wire of uniform diameter 0.8 mm and length 3.0 m. (ρiron = 10.2 10-8 Ω m) [Answer: 0.61 Ω] [3 marks] A battery of emf ε and internal resistance r is connected across a variable resistor. When the resistor is set to 21 Ω, the current through it is 0.48 A. When it is set at 36 Ω, the current through it is 0.30 A. Calculate (i) the emf ε and internal resistance r. [Answer: 12.0 V and 4 Ω] (ii) the power consumed in the internal resistance of the battery r when the current is 0.50 A. [Answer: 1 W] [7 marks] 2. (a) Define potential difference and electromotive force. Give the unit for both quantities. [3 marks] FIGURE 1 (i) (ii) State Kirchhoff s second law. Calculate the current through the 12 Ω resistor in FIGURE 1 using Kirchhoff s law. [Answer: 0.013 A] [7 marks] 2012/2013 3. (a) (i) Explain the effect of the resistance of a metal wire if the length is increased. PCH 1
4. (a) (ii) State TWO factors which affect the resistivity of a metal wire. (i) A 75 watt tungsten light bulb has a resistance of 190 Ω when lighted and 15 Ω when turned off. Calculate the temperature of the filament when the bulb is lighted. (Temperature coefficient of resistivity of tungsten, α = 4.5 10-3 C -1 at 20 C) [Answer: 2612 C] (ii) A light bulb draws 450 ma of current from a 2.0 V battery. If the battery becomes weak and the voltage drops to 1.5 V, what is the change in the current if the resistance remains constant? [Answer: 0.112 A] [6 marks] FIGURE 2 FIGURE 2 shows four identical resistors connected to a battery. When the switch is opened, the current through the battery is I0. (i) When the switch is closed, will the current through the battery increase, decrease or remain unchanged? (ii) Calculate the current that flows through the battery when the switch is closed. Give your answer in term of I0. [6 marks] FIGURE 3 PCH 2
2013/2014 FIGURE 3 shows a Wheatstone bridge where AB is a uniform wire of length 100 cm. Two resistors RX = 5 Ω and RY = 4 Ω are connected as shown. The bridge is balanced when the length from points A to C is l1. (i) What is the reading of the galvanometer? Explain your answer. (ii) Determine l1. [Answer: 55.6 cm] 5. (a) (i) Define electric current. (ii) How to reduce power loss when current is flowing through a wire? FIGURE 4 FIGURE 4 shows an arrangement of four resistors connected to a 20 V battery. Calculate the (i) Effective resistance. [Answer: 6. 75 Ω] (ii) Current that flows in the 3 Ω resistor. [Answer: 0. 74 A] 6. (a) FIGURE 5 Calculate current I 3 for the circuit in FIGURE 5. [Answer: 3. 83 A] PCH 3
(i) State TWO applications of potentiometer. (ii) FIGURE 6 FIGURE 6 shows a balanced Wheatstone bridge consisting of four resistors with resistances 3 Ω, 10 Ω, 2 Ω and an unknown resistance R X. Calculate R X. [Answer: 0. 6 Ω] 2014/2015 7. (a) (i) Describe the microscopic model of current. (ii) A generator produces 250 kw power at potential difference 15 kv. If a cable with resistance 10 Ω is used for the power transmission, calculate the power dissipated in the cable. [Answer: P dissipated = 2. 8 kw] A bird stands on wire with its feet 3 cm apart. The wire has diameter 1.5 cm and carries a current of 3 ma. If the resistivity of the wire is 1.77 10 8 Ω m, calculate the (i) resistance of the wire between the bird s feet. [Answer: R = 3 10 6 Ω] (ii) potential difference between the bird s feet. [Answer: V = 9 10 9 V] 8. (a) FIGURE 7 PCH 4
FIGURE 7 shows an arrangement of resistors connected to a battery with emf 6 V and internal resistance 0.5 Ω. Calculate the: (i) Effective resistance. [Answer: R eff = 2. 5 Ω @ 3. 0 Ω] (ii) Current through the battery. [Answer: I = 2 A] FIGURE 8 FIGURE 8 shows a circuit consisting of a 12 V battery, a switch and three resistors, R 1 = 20 Ω, R 2 = 30 Ω and R 3 = 40 Ω. If a voltmeter is connected across PQ and the switch is opened, determine the reading shown on the voltmeter. [Answer: V voltmeter = 5. 1 V] 2015/2016 3. (a) (i) State Ohm s law. (ii) What is the effect of increasing temperature on electrical resistance in metal? Explain briefly. FIGURE 4 PCH 5
FIGURE 4 shows a circuit with three resistors connected to 12V battery. Calculate the (i) effective resistance and the total current flows in the circuit. [Ans: Reff 5.12 ; Itotal 2.34 A] (ii) rate of energy supplied by the battery. [Ans: P 28.1W ] [6 marks] 4. (a) (i) Define electromotive force emf without using mathematical equation. (ii) A battery with an emf and internal resistance r is connected to a variable resistor. The current in the circuit is 0.50 A when the resistor is set to 19.5. When it is set to 24.5, the current reduces to 0.40 A. Calculate and r. [Ans: 10V ; r 0.50 ] FIGURE 5 FIGURE 5 shows three emf 1, 2 2.0V and 3 0.5V connected to R1 3 and R2 5 to form a complete circuit. Calculate 1 and I 1. [Ans: 1 1.3V ; I1 0.46 A] PCH 6