Electronics > Beginners

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

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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).