Problem 39.to lose PE while gaining KE.
√39 Find the current drawn from the battery.
√40 It’s fairly common in electrical circuits for additional, undesir-
able resistances to occur because of factors such as dirty, corroded,
or loose connections. Suppose that a device with resistanceRnor-
mally dissipates powerP, but due to an additional series resistance
rthetotal power is reduced toP′. We might, for example, detect
this change because the battery powering our device ran down more
quickly than normal.
(a) Find the unknown resistancer.
√(b) Check that the units of your result make sense.
(c) Check that your result makes sense in the special casesP′=P
andP′= 0.
(d) Suppose we redefineP′ as the useful power dissipated inR.
For example, this would be the change we would notice because a
flashlight was dimmer. Findr.
√Key to symbols:
√easy typical challenging difficult very difficult
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Problems 571