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Practice Problems 0502 The current through a lamp is 0.5 amps (amperes). The lamp is turned on for a total time of 30 seconds. The potential difference across the lamp is 1.2 volts. Calculate the electric potential energy that is transferred to the lamp in 30 seconds. I = 0.5 amps = 0.5 coulombs/sec t = 30 sec ∆V = 1.2 volts Step 1: Solve for Q (charge) I= Q t 0.5 = Q 30 30(0.5) = Q 15 Coulombs = Q Step 2: Solve for E (electric potential energy) V= E Q 1.2 = E 15 1.2(15) = E 18 Joules = E Batteries used in cell phones or notebooks sometimes have a number on them, which for example may be 650 milliamperes-hours. What is a milliampere-hour a measure of? Milliampere is a measure of current Hours is a measure of time (I) (t) When these quantities are multiplied to produce the unit milliamperes-hours, we have the following product (I)(t) Charge Q is the product of current and time Q = (I)(t) Therefore a milliampere-hour is a measure of charge. For cell phone batteries, the larger the number of milliampere-hours, the longer the battery lasts before it needs to be re-energized. Check your cell phone battery for this number.