Practice Test (40 mins)
1. A wire made of brass and a wire made of silver have the same length, but the diameter of the brass wire is 4 times the diameter of the silver wire. The resistivity of brass is 5 times greater than the resistivity of silver. if Rb denotes the resistance of the brass wire and Rs denotes the resistance of the silver wire, which of the following is true?
2. For an ohmic conductor, doubling the voltage without changing the resistance will cause the current to
(A) decrease by a factor of 4
(B) decrease by a factor of 2
(C) remain unchanged
(D) increase by a factor of 2
(E) increase by a factor of 4
3. If a 60-watt light bulb operates at a voltage of 120 V, what is the resistance of the bulb?
(A) 2 Ω
(B) 30 Ω
(C) 240 Ω
(D) 720 Ω
(E) 7200 Ω
4. A battery whose emf is 40 V has an internal resistance of 5 Ω. If the battery is connected to a 15 Ω resistor R, what will the voltage drop across R be?
(A) 10 V
(B) 30 V
(C) 40 V
(D) 50 V
(E) 70 V
5. How much energy is dissipated as heat in 20 S by a 100 Ω resistor that carries a current of 0.5 A?
(A) 50 J
(B) 100 J
(C) 250 J
(D) 500 J
(E) 1000 J
1. A hair dryer draws 7.5 A when plugged into a 120-V line. (a) What is its resistance? (b) How much charge passes through it in 15 min? (Assume direct current.)
2. An electric clothes dryer has a heating element with a resistance of 9.6 Ω. (a) What is the current in the element when it is connected to 240 V? (b) How much charge passes through the element in 50 min?
3. A 12-V battery causes a current of 0.60 A through a resistor. (a) What is its resistance, and (b) how many joules of energy does the battery lose in a minute?
4. Compute the voltage drop along a 26-m length of household no. 14 copper wire (used in 15-A circuits). The wire has diameter 1.628 mm and carries a 12-A current.
5. A 115-V fish-tank heater is rated at 110 W. Calculate (a) the current through the heater when it is operating, and (b) its resistance?
6. A 5.8-m length of 2.0-mm-diameter wire carries of 750-mA current when 22.0 mV is applied to its ends. If the drift speed is 1.7x10^-5 m/s, determine (a) the resistance R of the wire, (b) the resistivity p, and (c) the number n of free electrons per unit volume.
7. A length of wire is cut in half and the two lengths are wrapped together side by side to make a thicker wire. How does the resistance of this new combination compare to the resistance of the original wire?
8. In an automobile, the system voltage varies from about 12 V when the car is off to about 13.8 V when the car is on and the charging system is in operation, a difference of 15%. By what percentage does the power delivered to the headlights vary as the voltage changes from 12 V to 13.8 V? Assume the headlight resistance remains constant.