Shock Value B

[tuftedtitmouse12]

oh..had regionals 2 days ago....a few questions stumped us...i mean, we still got 5th ,but we were like guessing, logical guessses, but we made it to state so were going to HAVE to learn...but like therew as one and it had a electrical plugin heater and it gave us the watts, 1500, and it was like "find the resistance"....

[andrewwski]

Not very difficult at all. Assuming it's an electrical resistance heater and supplied 120V (standard outlet voltage), just use the simple equation . P=1500 W and V=120 V. Solve that and you get R=9.6 ohms.

[sean9keenan]

Andrewwski is right, although I think he meant R = 9.6 ohms.

You can also easily rederive that specific Power equation with a combination of P=VI (What I normally consider to be the simplest power equation) and ohms law, V=IR

Therefore

It's also important to note that for any problem you get involving AC voltage (Ie voltage from the wall) you can just treat it as a 120V DC source, and that will work for most problems (infact it should work for any problem you're expected to do)

Some questions I might ask involving household voltage might include: A question like the one you posed, or questions like, "Are all of the components in your house hooked up in series or parallel? Why?"

[JSGandora]

How much are we to know about inductors? Does anyone have examples of inductor questions they ask?

[space scientist]

Please may someone explain the various systems of measurement and equations involved calculating and converting information about magnetic fields. In addition, pleas may someone explain how you calculate the quantity of a magnetic field in an electromagnet when you have information about the voltage, current, etc.? How do you find the polarity of an electromagnet?

[sean9keenan]

I do believe that the rules say you dont need to know much about inductors as they apply in circuits, but you do need to know about electromagnets. For instance you probably won't be asked to solve for the current through an inductor as a function of time. They can definitely ask you about the strength of the magnetic field or to draw the field lines.

I would recommend reading this: http://hyperphysics.phy-astr.gsu.edu/hb ... lemag.html It covers a lot of the material you'd need! You might also want to do some reading up about how toroids are used as cores of electromagnets

[andrewwski]

Andrewwski is right, although I think he meant R = 9.6 ohms.

Whoops!

[JSGandora]

Thanks for the response sean9keeman! I have another question, how would you tackle trying to find resistances across the two upper and lower nodes of a bridge circuit as found here? What if the "null" was another resistor? I am more interested in the response to the latter question.

[sean9keenan]

That looks like a wheatstone bridge (I think I've seen that come up in previous posts a few times). In the case where Null is just a regular null detector there is no current through it which means that it's just two halves of resistors, or two sets of resistors in parallel (Ie Ra and Rb are in series and in parallel with R1 and R2 which are in series).

Now, trying to find the equivalent resistance when Null is a resistor is tricky. In the general case you have to convert Ra R1 and null into a Y, from it's current Delta formation:
ie what this wikipedia here explains rather well : http://en.wikipedia.org/wiki/Y-%CE%94_transform

Once you apply that transformation you have a circuit that is much easier to solve, if you have problems with this step just tell me.

Now, for a very specific case it becomes much much easier to solve for the equivalent resistance, when Ra/Rb = R1/R2 you find that once again the current through null is zero, because the potential across it is zero which means that you can ignore it just like we did in the first problem! If you short it, or make it infinite resistance it won't change the circuit at all and therefore it can't change the equivalent resistance. This circuit is much easier to solve and doesn't involve any Wye-Delta transformations!

[tuftedtitmouse12]

Not very difficult at all. Assuming it's an electrical resistance heater and supplied 120V (standard outlet voltage), just use the simple equation . P=1500 W and V=120 V. Solve that and you get R=9.6 ohms.

but it didn't tell us the voltage that was supplied....