If an electrical doorbell has a resistance of 70 ohms and requires a current of 2 amps, what is the necessary voltage to operate the bell?

To determine the necessary voltage to operate the electrical doorbell, Ohm’s Law is applied, which states that Voltage (V) is equal to the current (I) multiplied by the resistance (R). In mathematical terms, this is expressed as:

[ V = I \times R ]

In this case, the current required by the doorbell is 2 amps, and the resistance is 70 ohms. Using these values:

[ V = 2 \text{ A} \times 70 \text{ ohms} ]

Calculating this gives:

[ V = 140 \text{ volts} ]

To express this voltage in kilovolts, which is the unit for options A through D, you convert volts to kilovolts by dividing by 1,000:

[ 140 \text{ volts} = 0.14 \text{ kV} ]

This conversion shows that the voltage necessary to operate the doorbell indeed is 0.14 kV, making it the correct answer. The other options present much higher voltage levels that would not be appropriate or necessary for typical electrical doorbell operation.