Question

A. Your home computer draws approximately 7.0 amps of current when connected to a wall outlet at 120. volts.
If the computer is on for 8 hours per day and energy is sold to you at $0.20 per kilowatthour, approximately
how much will it cost you to operate the computer for an entire year when it is on for 8 hours per day?
(A) $124 (B) $251 (C) $374 (D) $491 (E) $2453

B). A conducting wire possesses a charge vs. time function given as follows;

Q(t) = 15t
4 − 12t
3 + 160

where all terms are in units of milliamps (mA). Which of the following best indicates the electric current in the
conductor at time t = 3.0 seconds?

(A) 0 mA (B) 220 mA (C) 650 mA (D) 1100mA (E) 1300 mA

Answer 1

Electrical power of computer usage = 120 V 0.7 A = 84 W

Electrical energy used per day = 84 W 8 hour = 672 W-hr

Electrical energy used in year = 672 365 W-hr = 245.28 kW-hr

Cost of Electrical energy in year = 245.28 kW-hr $0.20 $49.1

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Charge passing through wire at time t , Q(t) = 15t⁴ - 12 t³ +160

current in the wire, i(t) = dQ/dt = 60t³ - 36 t²

current at t = 3 s , i(t) = 60 27 - 36 9 = 1296 mA 1300 mA