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Review CH -27
Q 1. Find the drift velocity for electrons in a wire which has a radius of 1 mm and carries a current of 2 A. Assume that the number of atoms per unit volume in the wire is 6x10 28 m and there are two free electrons per atom. (ANS: 3.32x10 )
Q 2. A conducting wire of uniform radius 0.05 cm is 80 m long. A potential difference of 2 V produces a current of 2 A in the wire. What is the resistivity of this conductor?
(ANS: 9.8x10)
Q 3. Nine copper wires each of length L and diameter 0 are connected in parallel. What must be the diameter of a single copper wire of length L if it is to have the same resistance as the parallel combination of nine wires? (ANS: 30)
Q 4. An electric field of 300 V/m is applied to a length of material of uniform cross-section. What is the current if the cross-sectional area is 0.3 cm and the resistivity of the material is 1 .6x1 ohm.m? (ANS: 5.63x10)
Q 5. The current I (in A) in a wire depends on time t (in s) as sin ( 0.32 t+ 5.).
What amount of charge moves across a section through the wire during the interval from t = 1.0 s to 2.0 s ? (ANS:5.46)
Q 6. A copper wire and an aluminium wire of same cross-sectional area are found to have the same resistance. If the length of the copper wire is 2 m, find the length of the aluminium wire ? ( ANS: 1.31 m)
( resistivity : for Cu = 1 .72x1 0-8 ohm.m for Al = 2.63x1 8 ohm.m )
Q 7. A silver wire is 50 cm long and has a radius of 1 mm. Find the potential difference required to send a current of 10 A through this wire. (ANS: 2.55x1 0-2 V)
( resistivity of silver = 1 .6x1 0-8 ohm.m )
Q 8. An electric heater is rated at 1500 W, a toaster is rated at 1200 Wand a dishwasher is rated at 800W. The three appliances are connected to a common 120-V circuit. What is the total current drawn by three appliances? (ANS:29.2 A)
Q 9. What is the required resistance of an immersion heater that will increase the temperature of 1 kg of water from 20C to 80C in 3 mm while operating at 220 V? (ANS: 34.7 ohms)
Q 10. A resistance thermometer using a platinum wire is used to measure the temperature of a liquid. The resistance is 2.42 ohm at 0 C; and when immersed in the liquid it is 2.98 ohm. The temperature coefficient of resistivity of platinum 0.0038 / C. What is the temperature of the liqid? (ANS: 60.9 C)
Q 11. The average temperature coefficient of resistivity of iron is 5x10 / C. If the resistivity of iron is
1 xl 0 ohm.m at 20 C., at what temperature will the resistivity be twice this value? (ANS: 220 C)
Q 12. A resistor is constructed by forming a material of resistivity rho into the shape of a hollow cylinder of length L and inner and outer radii ra and rb, respectively. (See Fig.1 ). In use, a potential difference is applied between the ends of the cylinder, producing a current parallel to the axis. Calculate the resistance of this resistor when L = 14 cm, ra = 0.5 cm, rb =1.5 cm and rho = 3.5x1 ohm.m (ANS: 7.80x107 ohm)
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