# Did I count this current in a circuit correctly?

• Duc Anh Le
In summary: I think it would be a good idea to learn about equivalent circuit transformations before continuing. Alright, thanks for the verification. So far we've only been taught ohm's and kirchhoff's laws. I think it would be a good idea to learn about equivalent circuit transformations before continuing.
Duc Anh Le
This i a test question and we are not allowed to use a calculator, so I was wondering whether I did everything correctly, since the I1 = - 1/7. I am supposed to find out I3, but stopped at I1, since the result is a bit tricky.

1. Homework Statement

R1 = 10 ohm, R2 = 20 ohm, R3 = 40 ohm
I3 = ?

## Homework Equations

I3 = I2 + I1
V1 = I1 * R1 + I3 * R3
V2 = R2 * I2 + I3 * R3

## The Attempt at a Solution

10 = 20I1 + 40 I2 + 40I1
20 = 20I2 + 40I1 + 40I2
-10 = 70I1
I1 = -1/7

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#### Attachments

• 5ly1os8imkw11.jpg
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The circuit in the image is very difficult to read the component labels. Please post a closeup or draw and photograph a larger version.
Duc Anh Le said:
3. The Attempt at a Solution
10 = 20I1 + 40 I2 + 40I1
20 = 20I2 + 40I1 + 40I2
-10 = 70I1
I1 = -1/7
From what I can tell, the first term on the RHS should be 10I1 rather than 20I1. This is likely a transcription typo since your value for the current I1 looks good to me.

gneill said:
The circuit in the image is very difficult to read the component labels. Please post a closeup or draw and photograph a larger version.

From what I can tell, the first term on the RHS should be 10I1 rather than 20I1. This is likely a transcription typo since your value for the current I1 looks good to me.

Here's a better picture

#### Attachments

• 6uVlA3E.png
6 KB · Views: 232
It is indeed much better! Thanks.

Duc Anh Le said:
This i a test question and we are not allowed to use a calculator, so I was wondering whether I did everything correctly, since the I1 = - 1/7. I am supposed to find out I3, but stopped at I1, since the result is a bit tricky.
...

I will show that ##I_1## is negative:

$$50 I_1 + 40 I_2 = 10;\ 40 I_1 + 60 I_2 = 20 \\ 5 I_1 + 4 I_2 = 1; 2 I_1 + 3 I_2 = 1\\ 3 I_1 + I_2 = 0$$
Note that ##I_1## and ##I_2## have different signs and that ##I_3 = -2 I_1##, so ##I_3## and ##I_1## have different signs. And since ##I_2## is clearly positive, we know all the signs and ##I_1## is indeed negative.

You are on the right track, good work so far!
Do you know about Thevinin & Norton equivalent circuit transformations yet? I think they make problems like this much easier to do.

DaveE said:
You are on the right track, good work so far!
Do you know about Thevinin & Norton equivalent circuit transformations yet? I think they make problems like this much easier to do.
Alright, thanks for the verification. So far we've only been taught ohm's and kirchhoff's laws.

## 1. What is the purpose of counting current in a circuit?

Counting current in a circuit allows you to determine the flow of electricity and identify any potential issues or imbalances in the circuit. It is an essential step in troubleshooting and ensuring the proper functioning of the circuit.

## 2. How do I count current in a circuit?

To count current in a circuit, you will need to use a multimeter or ammeter. The device should be set to measure current and then connected in series with the circuit. The reading on the device will indicate the amount of current flowing through the circuit.

## 3. What is the unit of measurement for current?

The unit of measurement for current is ampere (A), which is defined as the amount of electric charge passing through a given point in one second.

## 4. Can I use a digital multimeter to count current in a circuit?

Yes, a digital multimeter (DMM) can be used to count current in a circuit. Most DMMs have a setting specifically for measuring current, and they typically have a higher accuracy than analog multimeters.

## 5. What should I do if the current reading is too high?

If the current reading is unusually high, it could indicate a potential problem with the circuit. You may need to check for any loose connections or damaged components and make necessary repairs. If you are unsure, it is best to consult a professional electrician.

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