My idiot's guide tells me I should measure amps in series with a component (in this case an LED.)
I did so, but the thing is this:
I first measured the LED with the multimeter needles either side of it [is this parallel ?]and the reading accorded accurately with Ohm's Law. When I put them is series (before the LED) the reading was way off my calculations.
Also how can I have such wild fluctuations in amps depending on where I measure ? The circuit had an LED, a switch and a 100 Ohm resistor. (3v DC)
Thanks in anticipation.
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Comments
I write as a complete novice and would welcome an experts correction.
But it got me exactly the reading I'd calculated should be there using Ohm's law. They reckon it's OK to measure voltage in parallel and this doesn't screw up readings.
Secondly, an ammeter has (ideally) zero resistance. A real ammeter is a good enough approximation to this.
So if you short-circuit the LED with an ammeter, the current you read will be pretty close to what you calculated ... because you calculated it wrong!
I guess what you did is to take V = 3 volts and R = 100 ohms, so I = V/R (Ohm's Law) = 3/100 = 0.03 amps = 30 milliamps. And that's what you got, because the circuit comprised a 3V battery and a 100 ohm resistor; the ammeter resistance being negligible and the LED being shorted out by it.
Now if you put the ammeter in series with the LED and the resistor (the order doesn't matter), what you have is a voltage source of 3 volts due to the battery, and a voltage drop of 2 volts due to the LED, leaving 1 volt across the resistor and ammeter. (Technically this is an application of Kirchhoff's Voltage Law or KVL: the voltages around a circuit loop add to zero. 3 - 2 - 1 = 0. Or, as we've used it, 3 - 2 = 1. There's also a Kirchhoff Current Law: the currents entering a node add to zero. In other words, what flows in must flow out. But we don't need KCL in this simple circuit, which is just a loop.)
We'll neglect the series ammeter, assuming that its resistance is much less than 100 ohms.
Now we know the voltage across the resistor, 1 volt, we can apply Ohm's Law to it, because Ohm's Law is for resistors: I = V/R = 1/100 amps = 10 milliamps. This is what the ammeter should read in practice.
Circuits make perfect sense if you understand what's going on. But my experience of teaching undergrads is that most people find them very unintuitive and difficult to get to grips with. (As I did myself when just starting out.) And a.c. circuit theory adds a whole extra layer of complexity!
As ESBlonde says, you can use a clamp-on Hall-effect probe to measure d.c. currents; and they do in fact go down to milliamps if you can afford an expensive bit of gear.
I doubt I'll ever visualise what is going on in circuitry but it's fun to try.
Once again. Thanks.
Not completely out of the ball park. I assume there has been some extra resistance in the somewhere. I'll do it again tomorrow just to be sure.