# Electrical circuits

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Posted (edited)

My 13 year old nephew has asked me to help with his science homework on electricity, which I'm not familiar with. Can you explain what I need to know. It would have helped if i knew what book he was working from but doesn't have one apparently. Click image to enlarge.

Edited by StringJunky

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My 13 year old nephew has asked me to help with his science homework on electricity, which I'm not familiar with. Can you explain what I need to know. It would have helped if i knew what book he was working from but doesn't have one apparently. Click image to enlarge.

You need to know the current law, see here https://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws

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You need to determine R1, R2 and R3 by Ohm's Law. Then find current magnitude in ammeters A4 and A5.

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You need to determine R1, R2 and R3 by Ohm's Law. Then find current magnitude in ammeters A4 and A5.

This is not correct, I already explained what needs to be done.

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Mate, the current splits up for each branch at the right and then comes together on the left. The current is different depending on the size of the resistance.

As an easy example see the following:

http://www.bbc.co.uk/bitesize/standard/physics/using_electricity/useful_circuits/revision/3/

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Mate, the current splits up for each branch at the right and then comes together on the left. The current is different depending on the size of the resistance.

As an easy example see the following:

http://www.bbc.co.uk/bitesize/standard/physics/using_electricity/useful_circuits/revision/3/

so, A4 is A2 + A3 and A5 is A1 + A2 + A3?

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so, A4 is A2 + A3 and A5 is A1 + A2 + A3?

Yeah.

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Posted (edited)

Yeah.

Cheers.

What is the resistance of R2 when the voltage is 12v? How do I work that one out?

Edited by StringJunky

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Posted (edited)

For future reference.

The first thing you need to know is the circuit sign convention.

A battery is drawn with the longer of the two lines denoting the positive and the shorter the negative.

Conventional current flows from negative to positive. Oops Edit positive to negative

(There was recently an ill advised movement to change this convention which still causes much confusion to many people). (Edit serious confused me anyway)

You circuit is drawn (thankfully) using conventional current so the readings on the ammeters are positive when convention current flows (from positive to negative through them).

The next thing to note is that all the battery current flows through ammeter 5 and since it has no other path from the negative to the positive the current in A5 must equal the sum of the currents in all the available paths from negative to positive.

It is quite simple in this case to see that this is equal to A1 + A2 + A3

It is also fairly obvious that the only way current can flow through A2 and A3 is what is left after A1 has left the main flow. ie the current through A4 = A2 + A3

Edited by studiot

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Posted (edited)

For future reference.

The first thing you need to know is the circuit sign convention.

A battery is drawn with the longer of the two lines denoting the positive and the shorter the negative.

Conventional current flows from negative to positive.

(There was recently an ill advised movement to change this convention which still causes much confusion to many people).

You circuit is drawn (thankfully) using conventional current so the readings on the ammeters are positive when convention current flows (from positive to negative through them).

The next thing to note is that all the battery current flows through ammeter 5 and since it has no other path from the negative to the positive the current in A5 must equal the sum of the currents in all the available paths from negative to positive.

It is quite simple in this case to see that this is equal to A1 + A2 + A3

It is also fairly obvious that the only way current can flow through A2 and A3 is what is left after A1 has left the main flow. ie the current through A4 = A2 + A3

Yes, Thanks for the clarification. Do I need to take R2, R3 and A4 into consideration to find the resistance of R2?

Edited by StringJunky

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Yes, Thanks for the clarification. Do I need to take R2, R3 and A4 into consideration to find the resistance of R2?

The resistance of R2 is independent of any other variable in the circuit. That is it can be set independently of other circuit variables, although there will be a change to currents and perhaps voltages if you do change the value of R2.

Please note this is a circuit with all the resistors in parallel.

Do you know if this will change the voltage or the currents or both if you change a resistor?

Resistors as circuit components are non polarised. That is they can be connected either way round to achieve the same effect.

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Posted (edited)

What is the resistance of R2 when the voltage is 12v? How do I work that one out?

I=U/R

Reverse equation:

R=U/I

And you can determine resistance of resistor from ammeter and voltmeter.

That's how resistance meter works, passing through element well known voltage, and observing received current.

Edited by Sensei

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I=U/R

Reverse equation:

R=U/I

And you can determine resistance of resistor from ammeter and voltmeter.

That's how resistance meter works, passing through element well known voltage, and observing received current.

Where do I get the current reading from: A2 or A4?

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Posted (edited)

Where do I get the current reading from: A2 or A4?

A4 you can calculate, the way has been already presented.

(combined currents from two bottom branches)

Edited by Sensei

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You know A5(=A1+A2+A3) and V=12V , so find equivalent resistance.

Then find the value of each resistance R.

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Posted (edited)

StringJunky, do you have multimeter at home, and 3 scrap resistors from broken device dismounted?

You could take them out. Use multimeter, and measure everything, measure either resistance, voltage, amperage of each branch.

And then, after test-drive, show it to your nephew.

Edited by Sensei

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Posted (edited)

The resistance of R2 is independent of any other variable in the circuit. That is it can be set independently of other circuit variables, although there will be a change to currents and perhaps voltages if you do change the value of R2.

Please note this is a circuit with all the resistors in parallel.

Do you know if this will change the voltage or the currents or both if you change a resistor?

Resistors as circuit components are non polarised. That is they can be connected either way round to achieve the same effect.

They are designed to help understanding as well as resolving an immediate issue.

They were written in anticipation of this question for instance

What is the resistance of R2 when the voltage is 12v? How do I work that one out?

Please also note that placing theoretically perfect voltmeters and ammeters in the circuit does not change any real circuit values.

At a more advanced level the effect of real world instruments is taught.

Edited by studiot

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They are designed to help understanding as well as resolving an immediate issue.

They were written in anticipation of this question for instance

Please also note that placing theoretically perfect voltmeters and ammeters in the circuit does not change any real circuit values.

At a more advanced level the effect of real world instruments is taught.

The value of a resistor is fixed and will affect the values of the other two parameters?

I've sat down with my nephew tonight and explained what I did from the help I've had here. This is one area I'm quite ignorant about really.

Can you recommend a book on this subject that will explain from first principles? I did buy The Art of Electronics many years ago but it was too heavy on the maths for me at the time IIRC.

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The value of a resistor is fixed and will affect the values of the other two parameters?

I've sat down with my nephew tonight and explained what I did from the help I've had here. This is one area I'm quite ignorant about really.

Can you recommend a book on this subject that will explain from first principles? I did buy The Art of Electronics many years ago but it was too heavy on the maths for me at the time IIRC.

First please note my very important edit to my post#10. My apologies.

There are so many ways to teach electricity but I feel it is important to follow the route that your nephew is following with some extensions if he has the interest.

Too many traditional physics texts start with a whole list of declarations which are only semi connected and appear to serve no real purpose.

To be studying parallel circuits at 13, perhaps your nephew's syllabus starts somewhere to provide a real motivation. That would be good.

But whatever you do, my advice would be to avoid Horowitz.

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First please note my very important edit to my post#10. My apologies.

There are so many ways to teach electricity but I feel it is important to follow the route that your nephew is following with some extensions if he has the interest.

Too many traditional physics texts start with a whole list of declarations which are only semi connected and appear to serve no real purpose.

To be studying parallel circuits at 13, perhaps your nephew's syllabus starts somewhere to provide a real motivation. That would be good.

But whatever you do, my advice would be to avoid Horowitz.

That's OK, I knew the convention. I think I need to get ahead of him a bit so I'll start learning it now. I'll try Khan Academy out.