You're a safety engineer reviewing plans for a university's new high-rise dorm. The elevator motors draw 20 A and behave electrically like 2.6-H inductors. You're concerned about dangerous voltages developing across the switch when a motor is turned off, and you recommend that a resistor be wired in parallel with each motor.
1) What should be the resistance in order to limit the emf to 70 V?
2) How much energy will the resistor dissipate?

(a) The resistance that should be used to limit the voltage  is 3.5 ohms.

(b) The energy dissipated in the resistor is 520 J.

The given parameters;

The resistance that should be used to limit the voltage  is calculated as follows;

V = IR

The energy dissipated in the resistor is calculated as follows;

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a) R `= 3.5 ohms

b) energy decipated =   560J

Explanation:

V = I . R

R = V / I

R `= 70 / 20

R `= 3.5 ohms

2)energy decipated = 1/2ij²

energy decipated = 1/2 x 2.8 x (20)²

energy decipated =   560J

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