# Homework Help!

Discussion in 'Homework Help' started by fishguts, Sep 3, 2007.

1. ### fishguts Thread Starter Member

Sep 3, 2007
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Hello, I am new to electronics. I am taking EE 111 and this is the first time I have ever dealt with this stuff. I have a few questions. The first question on my assignment that hs been troubling me is

1.what can be sid about currents flowing through series elements (Think of the point as a node and use KCL to prove your theory)

my second question is how would a schematic for a cars headlight circuit look like with a correct voltage design ensures that if one lamp burns out the other will stay lit?

I would appreciate any help.

2. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Your answer to the first bit is in paragraph 1 of this link.

For KCL, it states that: "The algebraic sum of all currents entering and exiting a node must equal zero", i.e. the current entering your series arrangement of components must be the same as the current leaving your arrangement of components. Given the information in the above paragraph (see link) you should then be able to use reason to state why this condition is true.

Post back if you have further questions or wish to clarify your answer.

Dave

3. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
What do you think concerning those questions?

4. ### recca02 Senior Member

Apr 2, 2007
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if one light burns out and others have to stay lit then they shud be in parallel right?

5. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Absolutely.

The OP needs to decide what detail to go into in order to create the schematic asked for in the question. Is it a general description or a detailed layout with component values - if its the later, I think more details are required.

Dave

6. ### fishguts Thread Starter Member

Sep 3, 2007
22
0
for my first questions I was thinking it would be The algebraic sum of all currents entering and exiting a node must equal zero, but was not too sure about it.

As for the second question I was thinking the headlight circuit would not be wired in parallel but in series.

Thanks for all the help

7. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
A point to make about series connected components, is that at each node there are only two connections: one where current flows into the node and one where current flows out of the node - by KCL, these two currents must be equal.

They must be wired in parallel, otherwise if one light blows, effectively becomes an open circuit, then if the lights are in series all lights will go out because current flow (which is common to all lights) will become zero: Ohm's Law - I = V/R, when R $\rightarrow$ $\infty$, then I $\rightarrow$ 0

Dave

8. ### fishguts Thread Starter Member

Sep 3, 2007
22
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One more question. How would I determine the power absorbed by a resistor whose resistance is 5 ohms an the current going to it is 2amps? Would I use the formula r=v/a?

9. ### recca02 Senior Member

Apr 2, 2007
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you really must learn some circuit theories if u are to deal with these.
the power dissipated across a resistor is square(current) x resistance.
in this case it would be power=2^2*5.

10. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
There are 3 power equations that you can use depending on what information you have:

1) P = V*I

2) P = V^2/R

3) P = I^2*R

In your case you want number as shown by recca02.

Dave

11. ### fishguts Thread Starter Member

Sep 3, 2007
22
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I have another question and did not want to create another thead for it. When calculating equivalent resistance of a circuit with resistors both in parallel and series do you calculate the resistance of the parallel circuits first and then add them with the series circuits to find the equivalent resistance?

12. ### recca02 Senior Member

Apr 2, 2007
1,211
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yes,
is there any other alternative approach to that ?