10k resistor across a 5v supply.
The voltage would be 5v with a little current, stick something into that circuit and now the voltage will drop...
by decreasing the resistor's vale eg 10k to a lower value, as the current rises so to will the voltage
so what causes the voltage to change? how come we don't see a constant 5v just with varying current to the thing we power?
Is it a switched power supply ?
Try adding a large capacitor and try again.
It might be a noisy 5V, that makes the measurements not fully valid.
Or it might be a overcompensated regulator (but again with a switched power supply).
cjdelphi:
10k resistor across a 5v supply.The voltage would be 5v with a little current, stick something into that circuit and now the voltage will drop...
by decreasing the resistor's vale eg 10k to a lower value, as the current rises so to will the voltage
so what causes the voltage to change? how come we don't see a constant 5v just with varying current to the thing we power?
Some of your statements don't ring true, but it might be a language thing. But first to answer anything is this matter one needs to know more about the +5vdc voltage source. Is it a regulated source that keeps the voltage constant with changing load current demand up to some maximum current rating? Or is it some form of unregulated DC voltage?
Ohm's law is very clear on the true relationship between voltage, current, and resistance and any apparent deviation from it's basic calculations is the result on some kind of real world deviation on assumptions not being accounted for.
Lefty
I heard Ohm's Law was going to be repealed....
I = E / R
E = I x R
R = E / I
Power = E x I
stick something into that circuit and now the voltage will drop...
how come we don't see a constant 5v just with varying current to the thing we power?
May I ask what power supply are you using?
Inadequate (or bad) PSU's may drop their output voltage if the load is too high (i.e. lower resistance, more current draw).
If a PSU is rated only a certain wattage maximum, and can only supply that much power and no more, then the law Power = E x I will hold true.
In this case E = Power / I (as current increases, your voltage will drop)
May I ask about the metering device (multimeter) that you use? Almost multimeter had error reading (usually described on user manual). It also have input impedance, usually analog voltmeter have 20kohm input impedance. With your 10k resistor its about 50% of input impedance. Almost digital voltmeter comes with 100kohm or greater. Theoretically, greater input impedance voltmeter give you better measurement. Assume you use digital voltmeter with 1Mohm, its about 1% but still affect your result.
Other analysis, may your resistor relative value about 5% (with gold color sign), More precision and accurate resistor will give you better result. Use resistor with 1% relative value or smaller.
Ok i'm going to investigate and see and get back 
JimboZA:
I heard Ohm's Law was going to be repealed....
Faster than light electrons..........Then again, electrons might not be fundamental particles in 20 years
JimboZA:
I heard Ohm's Law was going to be repealed....
Ohm's Law isn't a suggestion, like speed limits.
Many power sources are not ideal - but can be thought of as like an ideal voltage source in series with an "internal resistance" - thus
taking current from them causes the voltage to fall. Big well regulated power supplies will have a very low internal resistance (well
below an ohm), cheap watch battery may have 100's of ohms of internal resistance.
And the suggestions are getting better and better, e.g. SH45.
JimboZA:
I heard Ohm's Law was going to be repealed....
What a lot of people don't realize is, Ohm's Law is an "empirical" law, based upon empirical
measurements, and not an absolute law of nature - like the speed of light in a vacuum.
"Ohm's law is an empirical law, a generalization from many experiments that have shown
that current is approximately proportional to electric field for most materials. It is less
fundamental than Maxwell's equations and is not always obeyed".
This is how it was actually done - I know because that was one of the first laboratory
experiments we did in school,
And despite myths to the contrary, analog electronics is still more of an art, or soft
science, than a hard science. This is obvious, since just about every guy has a different
solution to the same problem.
Faster than light electrons
Actually electrons travel at about 30 miles per hour.
What a lot of people don't realize is, Ohm's Law is an "empirical" law, based upon empirical
measurements, and not an absolute law of nature
Well it is more of a definition than a law. It is how the units are defined and therefore is immutable for linear materials.
It always surprises beginners that the units of capacitance and units of resistance when multiplied produce the units of time. It is all part of how the quantities are defined.
To answer the OPs question
so what causes the voltage to change
The answer is the impedance of the power supply. To get those results it must be poor, that is high in this case.