# Applying Kirchoff's law for a circuit with 3 DC supplies

• Engineering
• EddieH
EddieH
Homework Statement
The question is from DC circuits:
Calculate the current through the 11V battery, and the dissipated power in R1, R2, and R4.

I have made an attempt at the question and would appreciate feedback before I submit please.
Relevant Equations
Kirchoff's laws.
P=I^2R
See my attached working out.

[Images of attached PDF pasted into post by the Mentors]

#### Attachments

• TMA1 - Q1.pdf
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Last edited by a moderator:
I didn't go through the detailed calculations, but it looks like your approach is okay. Can you back-annotate the voltages in the circuit and check yourself that way? Or another good trick is to re-solve the problem using KCL node equations to see if you get the same answers.

EddieH and scottdave

One thing you can do to check your currents: start at the bottom node (ground) and work through each path to the top and check that the voltage at the node at top middle is the same regardless of the path taken.

EddieH and berkeman
EddieH said:
Homework Statement: The question is from DC circuits:
Calculate the current through the 11V battery, and the dissipated power in R1, R2, and R4.

I have made an attempt at the question and would appreciate feedback before I submit please.
Relevant Equations: Kirchoff's laws.
P=I^2R

See my attached working out.

View attachment 346685
View attachment 346686
View attachment 346687
[Images of attached PDF pasted into post by the Mentors]
The calculations and the method, both of them are correct.

EddieH
DeBangis21 said:
Seeing other comments, this means I did not get the basics, or the circuit above is a different case, for what I learnt was 'when traversing around loops, the direction should be same.'

Pls enlight me more.

DeBangis21 said:
Seeing other comments, this means I did not get the basics, or the circuit above is a different case, for what I learnt was 'when traversing around loops, the direction should be same.'

Pls enlight me more.
These polarity questions don't really have a correct answer. You can assign polarities (current directions, etc.) at random if you like. What you then have to do, every time, is make sure that the equations you derive are consistent with your definitions, as well as how the real world works.

DeBangis21, scottdave and Joshy
Hi all,

Thanks for your input, it cheers me up knowing that not only am I using the correct interpretation of the laws, I somehow managed to get the correct answers.

And I was flapping to think I was bashing my face against the keyboard, course notes and google for nothing lol

Ed.

DeBangis21
DaveE said:
These polarity questions don't really have a correct answer. You can assign polarities (current directions, etc.) at random if you like. What you then have to do, every time, is make sure that the equations you derive are consistent with your definitions, as well as how the real world works.
Thank you very much.

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