Question

3. You have a simple circuit with a light bulb (R = 50 Ω) and a power supply (you can treat it like a battery with 120 V). Unfortunately, a loose screw fell on the circuit and is connected in parallel to the light bulb. You can treat the screw as a resistor with a resistance of 1 Ω.
a. Sketch this circuit





b. What effect does the screw have on the circuit? Focus on the current and voltage.




c. This situation is also known as a “short circuit.” Why is this bad if it happens at home?

Answer 1

a. If the 12ov supply holds up, you have a parallel circuit with a 1 ohm and a 50 ohm resistor in parallel.

b. The current drawn by the lamp:
I = V / R = 120 / 50 = 2.4 Amps.

The current drawn by the 1 ohm resistor:

I = V / R = 120 / I = 120 Amps.

The total current:
I(tot) = 2.4 + 120 = 122.4 Amps.

These calculations were based on the assumption that the 120v supply remained constant This is unlikely
unless the current rating of the power supply is well over 120 Amps. A 1 ohm
resistor connected across a 120v 5 Amp
supply would definitely be a short circuit. But if it is connected across a 120v 100 Amp supply; it would be an overload but not a short circuit.


c. A short circuit can trip a circuit breaker, blow a fuse, or burn the house down.