# Simple Circuits

## Homework Statement

For this problem we will consider a simple household circuit. In your house the power is supplied at constant voltage of 120-V (not quite...) and each circuit breaker can handle a maximum of 15-Amps. Light bulbs are rated assuming that a 120-V drop occurs across them.

If you are using 100-W bulbs, what is the maximum number you could have in one circuit if they are connected in parallel? What if they were connected in series - could you have more or less bulbs than in parallel (no calculation needed here!)? Which is the "Legal" way to connect them and why? Is the circuit breaker connected in series or in parallel with the "power source"?

Suppose you want to create a circuit with 3 lights and a switch to turn on all three at once. Draw a diagram of the circuit. If you are using three 60-W light bulbs, what is the total current drawn by the circuit?

Suppose you have a 60-W, 100-W and a 13-W bulb in each. What is the current through each bulb? What is the total current in the circuit? What is the power consumed in the circuit? How much energy is consumed in 1 day?

Suppose you were checking a 3-bulb circuit that your friend installed for you and you found that two of the (all 60-W) bulbs were dimmer than the other one. Furthermore when you unscrewed one of the dull bulbs, the other one goes out! What mistake do you think your friend made? Find the voltage and the current for all three bulbs and compare them with the "proper" circuit.

W= AV

## The Attempt at a Solution

I figured out how many can be connected in parallel, but am unsure how to figure out the series circuit. Also, I saw on a website that you can legally have 12 lights connected (in parallel I believe) but want to be sure about this. I also know how to draw the circuit with 3 lights and a switch and can figure out currents. From there on I'm unsure.

Last edited:

gneill
Mentor
Don't confuse 'legal' with 'possible' Calculate the number of bulbs that can be in place that will not exceed the 15A circuit breaker threshold.

As you add more bulbs to a series circuit, does the current go up or down?

You should show us more of your attempt to solve each problem so that we can help you. That is, show your calculations and thoughts behind them.

Don't confuse 'legal' with 'possible' Calculate the number of bulbs that can be in place that will not exceed the 15A circuit breaker threshold.

As you add more bulbs to a series circuit, does the current go up or down?

You should show us more of your attempt to solve each problem so that we can help you. That is, show your calculations and thoughts behind them.
The current stays the same in a series circuit, does it not?
But what would be the current? Would it be .83A, which is what I calculated as the amperage per lamp for the first part of the question. I found that 18 lamps can be connected for the parallel circuit, but for series I'm not sure how you'd figure it out.

gneill
Mentor
The current stays the same in a series circuit, does it not?
But what would be the current? Would it be .83A, which is what I calculated as the amperage per lamp for the first part of the question. I found that 18 lamps can be connected for the parallel circuit, but for series I'm not sure how you'd figure it out.
The current in a series circuit is the same for each component in the series; it doesn't "stay the same" if you add or subtract components.

In this case, in order to determine the current in a series circuit you would add up the resistances of all the components in the series and, since you have a fixed voltage supply, use Ohm's law to determine the current.

okay so I got a resistance of 144 ohms. this means a current of .83
so if the max is 15A, then you can have 18 lights.

gneill
Mentor
0.833A is the current for a single lamp, right? And the resistance for one lamp is 144Ω. What would be the resistance for two lamps in series? How about three? How does putting more lamps in series affect the total resistance?

Given a total resistance for a series string, how would you compute the current through that string when 120V is put across it?