Hello everyone, I have just started an apprenticeship we are learning basic ohms law to start with. Today at home I have been testing a lamp that I found in our shed, and I'm confused because the rating on the lamp does not seem to calculate with Ohms Law.

The lamp is clearly marked 12V 2W (its from a car I think) I have measured the resistance and it reads around 7 ohms.

So using ohms law :-

I = V/R
I = 12/7
I = 1.714 A

But when I work out power

p = I x V
p = 1.714 x 12
p = 20 W ??

I don't understand why power calculates out to 20W when the lamp is clearly marked as 12V 2W

Please can somebody explain, or am I doing this all wrong?

 

The current will be W/V that's about 2/12= 0.166amps, the bulb will alter its resistance as it heats up, to maintain the wattage,

Your bulb could be 20W, then the current will be 1.667 amps.

you cant measure the resistance of the bulb to work out the current.

Other formulas, W=I x I x R

W= V x V /R

 

You have measured the resistance with the filiment cold. It will draw 1.7 amps for a very short period of time when first switched on but when it is at it's operating temperature the resistance of the filiment will be much higher so the current will be lower.

Les.

 

The current will be W/V that's about 2/12= 0.166amps, the bulb will alter its resistance as it heats up, to maintain the wattage,

you cant measure the resistance of the bulb to work out the current.

Other formulas, W=I x I x R

W= V x V /R

Thanks Dave thats helpful, I thought I was doing something wrong.

 

Thanks for your replies, we haven't covered that part yet at college. As a mater of interest is that the same for all resistances that heat up during operation i.e. D.C. motors?

 

Have a look on this site's Education, Vol 1 direct current. All is explained there

 

Thanks for your replies, we haven't covered that part yet at college. As a mater of interest is that the same for all resistances that heat up during operation i.e. D.C. motors?

Just to complicate things.........Lights are resistive, motors are not a resistive load. The tutorials are good, and everything will make sense. Well it will until you get to RMS! That gave me a bald patch for weeks.

 

As a mater of interest is that the same for all resistances that heat up during operation i.e. D.C. motors?

Not really. All materials which exhibit electrical resistance show some temperature dependance of their resistance; for tungsten, the resistivity at 2800°K (normal incandescent bulb temperature) is about 15X its resistivity at room temperature; hence your confusing resistance reading. Some metal alloys, such as nichrome, manganin and constantan have very low temperature coefficients of resistance; see here for more on the topic.

 

...........motors are not a resistive load.

Let's not confuse things.
DC motors do look like a resistive load, but it varies with the motor mechanical load (output mechanical power), so you can't measure the non-operating resistance to determine its operating current (resistance).
The effective resistance is V/I when the motor is running.

 

Included in the DC motor load (current) is the opposing generated voltage, BEMF.
I did an empirical test with a DC 2HP T.M. motor using a Variac to bring the voltage up slowly with no load, the current was constant at around 1.4 amps through the whole rpm range from 0 to 3krpm
Max.

 

is that the same for all resistances that heat up during operation, like a motor

Copper increases resistance just a little bit with heat. There is a range of substances that have less resistance when hot. They are manufactured as NTC...Negative Temperature Coefficient devices. You merely had the bad luck to pick the worst possible thing laying around the shed to measure for your first try.

 

You merely had the bad luck to pick the worst possible thing laying around the shed to measure for your first try.

And that is called Murphy's LAW, comes into play just as much as Ohms law with electronics .

 

And that is called Murphy's LAW, comes into play just as much as Ohms law with electronics .

Actually, a lot more -- Ohm's Law only applies to some things; Murphy's Law applies to everything.

 

Hello everyone, I have just started an apprenticeship.

Where are you located?
Max.

 

Copper increases resistance just a little bit with heat.

This got me wondering so I decided to check it out. According to the table here, the TCR of copper at 20 °C is 0.404 %/°C, only slightly less than tungsten's TCR of 0.440 %/°C. The thing is, I think, that for a given wire diameter and length, copper has less than a third of the resistance of tungsten (according to this page) to begin with.

I wasn't aware of it until now, but RTD temperature sensors aren't just available in platinum (0.385 %/°C); they're available in copper, nickel (0.672 %/°C) and nickel-iron (0.518 %/°C) as well.

 

TCR of copper at 20 °C is 0.404 %/°C, only slightly less than tungsten's TCR of 0.440 %/°C.

The difference here is that we don't rate the resistance of copper at 2800F and confuse beginners trying to find out why it doesn't resemble its hot resistance at room temperature.