PHYS 1112 CHAPTER 20 HOMEWORK PROBLEMS Problem 1: Over the course of an 8-hour day, 3.8x10 C of charge pass through a typical computer (presuming it is in use the entire time). Determine the current for such a computer. Problem 2: The heating element of an electric toaster is typically made of nichrome wire (an alloy of nickel and chromium). As current passes through the wires, the wires heat up, thus toasting the toast. Estimate the overall resistance of a heating element which is 220 cm long and consists of nichrome wire with a diameter of 0.56 mm. The resistivity of nichrome is 110x10 Qem. Problem 3: Determine the overall resistance of a 100-meter length of 14 AWA (0.163 cm diameter) wire made of the following materials. a. copper (resistivity = 1.67x10 Qem) b. silver (resistivity = 1.59x10 Qem) c. aluminum (resistivity = 2.65x10 Qem) d. iron (resistivity = 9.71x10 Qem) Problem 4: Defibrillator machines are used to deliver an electric shock to the human heart in order to resuscitate an otherwise non-beating heart. It is estimated that a current as low as 17 mA through the heart is required to resuscitate. Using 100,000 Q as the overall resistance, determine the output voltage required of a defibrillating device. Problem 5: Determine the amount of electrical energy (in J) used by the following devices when operated for the indicated times. a. Hair dryer (1500 W) - operated for 5 minutes b. Electric space heater (950 W) - operated for 4 hours Problem 6: John sleeps with a 7.5-Watt night light bulb on. He turns it on before getting in bed and turns it off 8 hours later. a. Determine the amount of energy used during one evening in units of kiloWattehours. b. Electrical energy costs 13 cents/kWehr where John lives. Determine the annual (365 days) cost of this practice of using a 7.5-Watt night light. c. Determine the annual savings if John replaced his 7.5-Watt incandescent night light by a 0.5-Watt LED night light. Problem 7: A central air conditioner in a typical American home operates on a 220-V circuit and draws about 15 A of current. a. Determine the power rating of such an air conditioner. b. Determine the energy consumed (in kWehr) if operated for 8 hours per day. c. Determine the monthly cost (31 days) if the utility company charges 13 cents per KWehr. Problem 8: An overhead high voltage (4.0x10 V) power transmission line delivers electrical energy from a generating station to a substation at a rate of 1500 MW (1.5x10 W). Determine the resistance of and the current in the cables