what is reason behind reading amps in a series for solving power?

[doctorm]

so i was reading in my electronic book about Power. (P=IE) and it showed a set up with a ammeter and a voltmeter how to solve for power in a simple circuit. i understand the voltmeter was read aross the resistance, its a no brainer. but when it says that the ammeter was in series with the resister, i dont quite get it. shouldnt you see the amps across the resistor as well like the voltmeter?

[joelby]

Current is the flow of electrons through a circuit. To measure it directly, you need to break the circuit and add an ammeter in series.

Adding the ammeter in series with the resistor will effectively short out the resistor (assuming your ammeter has very low impedance) and change the characteristics of the circuit. You can use the voltage over the resistor to determine the current, but only if the resistor's value is known.

[DJPhil]

As joelby says, current is measured through something, while voltage is measured across it. It makes a bit more sense if you think of the voltmeter as having as high resistance as possible and the ammeter as having as low a resistance as possible. It's a common beginner's error to use an ammeter in parallel, and often results in a blown fuse in the meter.

[Excavatoree]

Maybe it would help to use the water analogy - or air, or some other fluid.

An ammeter in an electrical system is like a flow meter - you have to put the flowmeter in series to "count" or measure all of the water or other fluid that is flowing.

I work for a company that makes hydraulic equipment, and I always have to translate the hydraulic stuff into electrical to understand it. 

Fluid flow "across" something has no meaning - fluid flows through something.  It's the same with electricity.  Current flows through wires and other components, just like the fluids do in mechanical systems.

[alm]

This should be explained in any competent electronics textbook (Kirchoff's laws), either a formal or informal proof. If it doesn't, it's either very shallow or an advanced level text. The water analogy works well for this, an ammeter can be compared to a flow meter (measures amount of water per second), a voltmeter measures pressure difference (if you're talking about pipes) or height (waterfalls).

[djsb]

[KTP]

Now THAT is a good t-shirt!

[dengorius]

Ye it's been done already

[KTP]

The first one is more subtle and therefore I like it better.

Actually I would change it slightly to have the /\/\/\ symbol for the resistor instead of the box with R in it.  I don't like spelling out exactly what a shirt means...part of the fun is having people with a small clue about electronics recognize it.

[Simon]

to measure amps (the quantity of electrons flowing) you need to pass the same current through the amp meter, the amp meter has a very low resistance so it does not increase dramatically the resistance of the circuit

[doctorm]

so let me see if i got this now.

the reason you dont put the ammeter in front of the resistance is because that is just measuring the current given from the battery or power source.

you have it after the resistor in series to see the "flow" that is coming out of the resistor since we are trying to determine power for it.

and you wouldnt put it in parallel because it would equal the amount going in or a high number that is being resisted from flowing and building up in the reistor.

is this correct?

[djsb]

Hi,
Sorry about the little 'IN' joke earlier.
These links will hopefully explain things better

http://www.allaboutcircuits.com/vol_6/chpt_2/7.html

http://www.allaboutcircuits.com/vol_6/chpt_2/4.html

These free books will tell you all you need to know if you want to go deeper.

http://www.ibiblio.org/kuphaldt/electricCircuits/

HTH.

David

[Time]

so let me see if i got this now.

the reason you dont put the ammeter in front of the resistance is because that is just measuring the current given from the battery or power source.

You can place the meter before or after the resistor.  The current is the same before and after the resistor.

you have it after the resistor in series to see the "flow" that is coming out of the resistor since we are trying to determine power for it.

and you wouldnt put it in parallel because it would equal the amount going in or a high number that is being resisted from flowing and building up in the reistor.

is this correct?

If you try to measure the current by placing the meter in parallel you are going to be shorting out the resistor, essentially.  This might break the fuse in your meter or damage it.  To determine the power dissipated in the resistor first measure the voltage by probing the resistor with a parallel connection.  Than place the meter in series with the resistor and measure the current.  The amount of power that is dissipated in the resistor is now the voltage you measured multiplied by the current you measured.

[KTP]

Mr. Kirchoff made this easy.  (btw, anyone ever find it strange that Billy Joe never came up with a law or unit of measurement?  I think it would be great to say "That circuit is producing 16.3 fempto Billy Joes"...but anyway I digress)

[Rhythmtech]

The reason current is most often measured by multimeters in a series connection is that current is usually measured via a current shunt. Where the voltage drop across a shunt resistor is used to find the current via Ohm's law.  So the current must flow through your resistor where you want to measure the voltage drop relative to the current passing through it.  A Shunt resistor is usually a small resistance like .001Ω,  so the voltage drop is very small (5 mV @ 5A) and is for most applications of little impact on the circuit you are measuring.

There may be a meter on the market that measures current magnetically using a coil or with hall effect sensors, or some other method internally, but these would still require the path of the electrons to be routed uncoupled through the sensor.

If you think of voltage like water pressure where water does not need to be flowing for it to be present and measured then you can see that knowing the overall water pressure tells you nothing about how much water is moving. If you stand in a stream you instantly know if water is flowing or not.  All a meter does is stand in the middle of the stream and figure out how much water is flowing relative to how much pressure the water is exerting on it.

[thedigitalprincess]

Ye it's been done already

I have that shirt





Sometimes it's a bit tricky to create a break in the circuit, especially if using a breadboard. It's easy just to take out the wire from where you need to measure + and the wire where you need to measure -, then wrap the wire around the probes. Simple
Just remember to switch the positive probe on the multimeter to the current connection point, otherwise it will not work. Set it to an appropriate range too.