Done with Batteries, On to Direct Current

Battery topic finished today - exams graded, and good scores.

Now on to Direct Current -

Ohm's Law will be a centerpiece of this -

And use of basic Algebra -

We discussed balancing equations, and substitution today.

Example -

A 3600W Heater on a 240VAC circuit isn't working.... When you measure the resistance of the heater elements, what resistance reading do you expect for a good element?

Well - It seems we know the voltage, and we know the power - How to get at resistance with just plain old Ohm's Law?

Well -

Let's look at what we want to get -

R = E/I  - Resistance = Electromotive Force (Voltage) over Current (I)

Power, which we know, = E x I.

Since we know the power and voltage, but not the current, we can re-write this equation in terms of what we don't know:

So if we divide both sides by E, we get:   P/E = E/E x I   ----> P/E = I  ----> I = P/E

Now for the real POWERFUL tool!  - SUBSTITUTION

So if we want to know R, and we know R = E/I, and we ALSO know that I = P/E,  we can SUBSTITUTE P/E (which we DO know) for I (which we DON'T know) in the first equation:

So now we have:

R = E/(P/E) ---->  R = ExE/P  ------->  R = 240 x 240 / 3600  = 16 Ohms.

So - the resistance reading we expect if the elements are good, is 16 Ohms.

That's all for now - Cheers.