Susan Brown

[Susan Brown]

Hi Mike,

Good question! If you re-listen to that part of the video, Samurai mentions that the power company is trying to deliver a certain amount of power along the lines. And we know that Power = Current x Voltage (P=IxE). So if you have a certain goal for the amount of power you want to deliver, the higher your voltage, the lower the current needs to be.

But you are correct that if we are looking at a circuit, and it’s got a fixed resistance, if you changed the voltage supply from a 120vac to a 240vac, then the current would increase according to I = E/R.

But, say you need to generate 1000 watts of power. Play around with the numbers, using P=IxE. What would the current need to be if you used a 120 vac supply and then a 240vac supply? You’d need half as much current if you use 240vac.

Make sense? There are various “moving parts” to creating the electrical situation that an engineer wants in a circuit: power, voltage, current, and resistance. And they all act according to Ohm’s Law equations!

[Susan Brown]

That will work ONLY if you have 2 resistances in parallel.

If you take 1/(1/R1 + 1/R2) you can rearrange it to equal (R1xR2)/(R1+R2)

This does NOT work for 3 or more loads in parallel.

[Susan Brown]

Thanks, Gaspare! I was just getting on here to link you guys to the previous threads on Equivalent Resistance.

Jonas – be sure to click and read that thread above. And in that thread, you’ll see a link to another one where we break down the calculation for you.

Let me know if you have any other questions.

[Susan Brown]

Hi Raymond,

Those two statements are actually saying the same things. We just changed the wording slightly for the sake of being a little challenging.

The main point is the direction of electron movement. Both cases indicate moving from the cathode end to the anode end and not vice versa. This makes sense, given that the voltage is more positive at the anode end than the cathode end, and electrons move towards more positive charge.

Hope that helps!

[Susan Brown]

“Shunt” is correct in that scenario.

Or does shunt lead to another load, and that’s what makes a shunt (vs. a short)?

That’s more like it. A shunt will be deliberately used to bypass one or more loads, but there will still be a load in the circuit somewhere.

A short creates a condition where there is no longer a load in a path from line to neutral/ground, so there’s nothing to hinder the flow of electrons, leading to a failure of some type (blowing a fuse, burnt wire, etc.).

[Susan Brown]

Hi Jonas,

Thanks for posting a question!

Yes, the * symbol is the same thing as x . Both are “times”, or multiplication.

You might have just made this up as an example, but I want to make sure you know that V=W*R is not a valid Ohm’s Law equation! It would be V= I * R (voltage equals current times resistance)

[Susan Brown]

Hi Abbey,

Thanks for posting a question! We show this in several examples in the Basic Electricity module, but just to be clear: when you have loads in series in a circuit, the total circuit resistance is simply the sum of the resistances of the loads.

Again, this is a series circuit only. Loads in parallel are different. We can calculate an equivalent resistance with those, which we describe more in Unit 5, and also in this Forum thread:
https://my.mastersamuraitech.com/appliance-repair-course-support/student-forums/topic/equivalent-resistance-in-parallel-circuits/

[Susan Brown]

Hi Abbey,

It all depends on what information you are given.

If you look on the Ohm’s Law “Pie chart”, there are 3 equations that can be used to calculate current (I). You choose the one that best fits the scenario you have.

What info about the circuit do you have?

[Susan Brown]

Hi Desert,

The first step on this question is to figure out how the loads are laid out in relation to the others. Are they in parallel? Series? Are all loads receiving current? (that’s an important one!)

The “Zen trick” is a great help in figuring this out.

let me know what you think.

[Susan Brown]

LOL – Some threads can confuse rather than clarify, depending on the number of people chiming in – you can always start your own if there’s something you’d like to try to clear up!

[Susan Brown]

Hi Robert,

The Danfoss app can be set to either gauge or absolute.

[Susan Brown]

Hopefully you just misheard something we said.

I’m very glad to hear about the new job and that your being in our training helped! It doesn’t surprise me – we do have a good reputation! It’s too bad his other techs won’t take him up on the offer.

[Susan Brown]

but I had found a statement from the training that current was the same in parallel circuits

Can you tell me where this is in the course?

“current varies in each branch of a parallel circuit depending on the total resistance of loads in each corresponding branch of the parallel circuit. However, the current will be the same outside of the branches.”

Yep, that’s it. Also, the total current will equal the sum of the currents in the individual parallel circuits.

If you look over our quiz questions for Units 4 and 5, it’s pretty clear how current behaves in series and parallel circuits.

[Susan Brown]

Hi Sean,
Have you watched the video at the end of Unit 3 where we show how to do the calculation, using slightly difference numbers for resistance? Try imitating our calculation yourself on paper. If there’s a step you don’t quite follow, tell me what it is and I’m happy to help you further.

[Susan Brown]

You figured it out! And so glad to hear you’re enjoying the course so far – thanks for letting us know!

[Author]

Posts