Ohms Law Quiz

[ban-all-sheds]

B****red if I can remember....

I do remember though thinking how interesting it was, and how it wasn't as simple as it appeared at first glance..

 

[Spark123]

I imagine as soon as you start calculating the wattage for the given voltage, the current will change hence so will the voltage. As you say, not as easy as at first glance!!

 

[ban-all-sheds]

I'll go and look for the calculations I did

 

[blondini]

I get 11.429 volts (rounded) across the last lamp.

I've taken the resistance of 2.5 copper as being 0.009 ohms/metre.

 

[ericmark]

Yes as I considered it I realised as the voltage drops so it uses less current so step one was to work out ohms. So I worked out ohms working from last lamp back to supply.

From this I could work out current to first lamp and hence voltage across first lamp then current to second lamp and hence volt drop and so on.

However I did not take into account internal resistance of the transformer or that the current taken by a quartz halogen bulb is not linear and takes more current when cold so in real terms the voltage would be less than calculated.

And only 6 lamps over 8 meters I know that looking at all the spec than ELV is supposed to be better but that's only when supplied with correct voltage and with these figures one wonders if the GU10 is not a better option?

Since BAS does not have answers I have stuck all the figures into Electronic workbench to see the answer that gives.

Untitled

• ericmark
• 23 Feb 2009
• 1

I see no takers for the problem I gave! Not that I really expected any. But seeing all the different answers for BAS's problem I think maybe better no one does challenge my results.

Seems I was 0.032 volts out on my answer!!!! I reckon Spark 123 and Me got it right and the programs out. My excuse and I'm sticking to it!

 

[Stoday]

However I did not take into account internal resistance of the transformer or that the current taken by a quartz halogen bulb is not linear and takes more current when cold so in real terms the voltage would be less than calculated.

You've skipped the part that makes BAS's problem interesting.

These days "transformers" in these applications are better described as switch-mode voltage convertors. The electronics stabilises the output so transformer losses are not likely to produce a significant voltage drop.

A good estimate of a filiament lamp's deviation from Ohm's Law can be made from the application of Stephan's Law to obtain the lamp power variation. This, along with the temperature coefficient of resistance for tungsten, shows that 10V instead of 12V across the lamp, a 17% drop, results in the lamp resistance falling by only 7%. So for practical purposes, ignoring these interesting aspects does not introduce significant errors and ericmark can award himself full marks for his answer.

On the other hand I never cease to be amazed that some contributors to threads like this embarrass themselves by not making a simple "fag packet" check that their answers are in the right ballpark.

 

[tim west]

well plenty of comments but not one person dared to answer and that was an easy one im shocked!

Well here's the answer

View media item 8849
Do i make it even easier next time?

 

[blondini]

I have stuck all the figures into Electronic workbench to see the answer that gives.

What resistance value are you using per metre of cable? It looks like 9.0 ohms from the numbers in the pic?

 

[blondini]

...and that was an easy one im shocked!

Well here's the answer

View media item 8849

Correct on both counts

 

[tim west]

Ok bit harder this time with an AC circuit


solve for
θI1 in degrees
θU1 in degrees

U1 rms volts

View media item 8864

 

[blondini]

Ow! I'm out of the game on that one Tim. C a n ' t r e m e m b e r...

 

[Spark123]

I'll have a go at it when I get in later (after 11pm prb), looks like a case of calculating capacitive and inductive reactances and working it out from there.

 

[LiamPope]

RE: the parallel lamps question...

Why's nobody considered the resistance, and hence voltage drop, of the return? BAS never said the 0V cable was made of superconductor!

So ericmark - isn't your model missing a few resistors?

Liam

 

[ericmark]

I have taken 18mV/A/m to work out resistance of cable and V/A = Ohms so it will be 18mOhm for both cables I did not see point of using two 9mohm resistors so used 1 x 18mohm resistor.

As to new problem vital information missing. Until all the information is given it can't be done.

Untitled

• ericmark
• 23 Feb 2009
• 1

At 50Hz I get
current at 0.1464 Amp
voltage 0.00115 Volts
At 60Hz I get
current at 0.122019 Amp
voltage at 0.00115 Volts

 

[ericmark]

Sorry missed a set of brackets I got it wrong.
At 50 Hz Current = 142.1031304ma and voltage = 4.523283127 volts
At 60Hz Current = 119.4963906ma and voltage = 3.169740207 volts
Since frequency was not given so here is a chart to show frez compared with both milliamp and voltage.

Untitled

• ericmark
• 23 Feb 2009
• 1