12.07.2015 Views

Simple Nature - Light and Matter

Simple Nature - Light and Matter

Simple Nature - Light and Matter

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loss of electrical energy will just be their total charge multiplied bythe voltage difference across which they move:∆U elec = q∆V= (6.5 × 10 3 C)(1.2 V)= 7.8 kJUnits of volt-amps example 5⊲ Doorbells are often rated in volt-amps. What does this combinationof units mean?⊲ Current times voltage gives units of power, P = I∆V , so voltampsare really just a nonst<strong>and</strong>ard way of writing watts. They aretelling you how much power the doorbell requires.Power dissipated by a battery <strong>and</strong> bulb example 6⊲ If a 9.0-volt battery causes 1.0 A to flow through a lightbulb, howmuch power is dissipated?⊲ The voltage rating of a battery tells us what voltage difference∆V it is designed to maintain between its terminals.P = I ∆V= 9.0 A · V= 9.0 C s · JC= 9.0 J/s= 9.0 WThe only nontrivial thing in this problem was dealing with the units.One quickly gets used to translating common combinations likeA · V into simpler terms.Section 9.1 Current <strong>and</strong> Voltage 517

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