The total current delivered to a number of devices connected in parallel is the

Question

The total current delivered to a number of devices connected in parallel is the sum of the individual currents in each device. Circuit breakers are resettable automatic switches that protect against a dangerously large total current by "opening" to stop the current at a specified safe value. A 1700 W toaster, a 1050 W iron, and a 1250 W microwave oven are turned on in a kitchen. As the drawing shows, they are all connected through a 20 A circuit breaker to an ac voltage of 120 V.

(a) Find the equivalent resistance of the three devices.
(b) Obtain the total current delivered by the source.
(c) Determine whether the breaker will "open" to prevent an accident.

Explanation / Answer

1. P = v2/r

1700 = 1202/R R = 8.47

1250 = 1202/R R = 11.52

1050 = 1202/R R = 13.71

equivalent resistance = 1/(1/8.47 + 1/11.52 + 1/13.71) = 3.6

2. total current = 120 V / 3.6 = 33.33 A

3. the breaker will trip, 33 > 20

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