Susan Brown

[Susan Brown]

Yes, and because of Kirchhoff’s Law that we taught you in Unit 8.

In series circuits, the sum of the voltage drops across all the loads will equal the source voltage.

Note that this is only for loads in series, not parallel or series-parallel.

Okay – you’ve gotten #3 and 4 handled, unless you have any further questions.

Two suggestions:
1. Make note of your answers so far if you haven’t already. I’ll need to hide them from future students!
2. Start a new topic for subsequent questions, since this one is getting so long.

[Susan Brown]

Yep – do you know why that is?

[Susan Brown]

Yes, those are correct.

Do you notice what those numbers add up to (approximately)?

[Susan Brown]

Yes, V (or E) = I x R is the right formula. But it doesn’t give you 20, 40, and 80. You have to do the calculations using the current and each resistance.

[Susan Brown]

Specifically, as the current flows through one or more loads.

So – you’ve got current flowing through each of these 3 loads. You know the resistance of each load.

Do the same procedure as before – what are you trying to find? what info do you have? which formula fits best?

• This reply was modified 3 years, 7 months ago by Susan Brown.

[Susan Brown]

Okay, first of all, do you know what voltage drop is? Can you express it in a short sentence? (See Unit 8 if you need help on this.)

[Susan Brown]

That’s correct. I hope that makes sense now.

Since Question 4 is based on the same circuit… now that you know the circuit current, how will you calculate the voltage drop across each load?

[Susan Brown]

Yes. That is total resistance. (FYI – this is only for loads in series. Loads in parallel have a different relationship.)

What is your answer for current in this circuit?

[Susan Brown]

Remember, you are only calculating one current for the whole circuit. So you need one value for R. What will that be? (Look back a few comments where we were discussing what determines the current in a series circuit.)

[Susan Brown]

Yes. What are values are you going to put in for E and R?

[Susan Brown]

Please reread my description of how to choose a formula a few comments back

[Susan Brown]

Yes. More specifically, the source voltage and the total resistance in the circuit.

What Ohms Law formula describes this?

• This reply was modified 3 years, 7 months ago by Susan Brown.

[Susan Brown]

Yes, there is one current.

We aren’t talking about light bulbs or batteries here. This is an AC circuit.

What two characteristics of a circuit determine what the current will be?

[Susan Brown]

2. Circuit current is I
3. You are given E (120v) and R (10, 20, and 40 ohms)

So you are looking for a formula that starts with “I =” and uses E and R in the calculation.

But… are you going to calculate one current, or three values for current?

Can you put into words what determines the current in a series circuit?

[Susan Brown]

Here are the basic steps to do on any question like this.

1. Identify what type of circuit you are dealing with, and make sure you understand the basic behavior of this type of circuit.
2. Identify what you are being asked to calculate.
3. Identify what information you are given.
4. Identify the Ohm’s Law formula that is appropriate.

I’ll answer number 1.
We have a series circuit with 3 loads in it and a power supply of 120v. We know that in a series circuit, the current will be the same throughout, and the voltage drop will be divided up between the 3 loads. The sum of the voltage drops will equal the source voltage.

What are your answers for the other questions?

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