Susan Brown

[Susan Brown]

“after running it live he got 120V for both L1 and L2 can you clarify that?”

This means that neither leg was open.

[Susan Brown]

Hi Paul – good catch! That question is not worded quite right. (This quiz is fairly new to the course.) The ignitor does continue to receive current as long as the valve does.

[Susan Brown]

1. *Current* is the flow of electrons. *Voltage* does not describe movement. Voltage is best through as being *present*. So, it is kind of a trick question – it’s making sure you understand the distinction between voltage and current.

3. The answer should be, “Think about its function in the circuit.”

[Susan Brown]

Good job!

[Susan Brown]

Hi Garry,

If you have two or more parallel branches and *each* of them has at least one load/resistance, then those are “parallel circuits” and you will have current flow in both of them. (The current in each circuit will be determined by the resistance in each circuit.)

If one of those parallel branches has NO resistance (i.e., has just a closed switch), then that is not a parallel circuit – it is a shunt. (We are talking about circuits that are designed this way, not an accidental short/failure.)

When there is a shunt in parallel with a circuit (that has resistance), there will be NO current in the circuit. Current will ONLY flow through the shunt.

Shunts are used to bypass a load when desired.

That’s just something you can memorize. But you can also think about what we taught about the equivalent resistance of parallel circuits. The Req will always be something less than the smallest resistance. Well, if one of the parallel branches has zero resistance, how can there be an Req of less than zero? Mathematically, there can’t be. That’s why a shunt is not technically a “parallel circuit.”

Does that help?

[Susan Brown]

That is correct!

[Susan Brown]

Hi Peter,

Good!

It is important so that you’ll know how to design and then interpret voltage measurements. Depending on where you place your probes and other conditions, you need to know if you are looking for/measuring voltage *potential* or *drop.* Re-read the paragraphs in Unit 8 before the videos to see if that helps.

[Susan Brown]

If you haven’t already, rewatch the second video in Unit 4. I show how to calculate the voltage drop across a load. That is what E = I x R is for. And then we discuss it even more in Unit 8.

When you have *current flowing through a load* (a load is a component that has resistance), there will be a voltage difference from one side of the load to the other. That is called voltage drop. If you have an active (current is flowing) 120v circuit with just one load in it, and you put your meter probes on either side of the load, then you would measure 120v. If current were *not* flowing through the load, you would measure 0v across it. If there were more than one load in series, then the sum of their voltage drops would equal 120v.

On the other hand, if you measure the voltage difference across an *open switch*, that would just be voltage potential, since there is no current flowing in that situation. However, when the switch closes, you would measure 0v because it would be acting just like a wire (no resistance).

Does that help?

I suggest writing this in your notebook, and even drawing some simple circuits and playing around with the calculations. Make sure you can follow what I do in the videos. Ask me more specific questions if you have them!

[Susan Brown]

Hi Dave,

You’ll find this definition in Unit 1 of this module:

“Voltage: The amount of potential difference between two points in an electrical circuit, and the driving force behind actual current (flow). Voltage is also referred to as “potential energy” or “Electromotive Force” (EMF), the force that causes electrons to move from negative to positive. Measured in “volts,” and usually referred to by “V” or “E” in equations. There is another type of voltage we will be talking a lot about called voltage drop. You’ll see it mentioned below in the definition of a load.”

Here’s another way to think about it. In order to have current, we must have a voltage difference AND a complete circuit.

The presence of a voltage difference can be called “potential” because there is the potential for current to flow if there is a complete circuit.

Voltage drop, on the other hand, is the voltage difference created when current is flowing through one or more loads.

[Susan Brown]

The correct answer is [answer hidden] But be careful about your thoughts about current.

Remember (or go back and look at) what we taught you in the first couple of units in Basic Electricity.

Current is the movement of electrons. Those electrons exist in the circuit, as part of the material that it is made of (wire, elements, etc.)

Normally those electrons are just hanging out, not moving. But when the power supply exerts electrical pressure – a voltage difference – on those electrons in a complete circuit, they move, and we call that movement “current”.

So, current does not come “from the element.”

The power supply provides the voltage difference, which results in current (assuming the circuit is not open).

There are popular terms like “current draw” which imply that loads themselves suck electrons from somewhere. It’s important to uproot those wrong notions and get a better technical understanding of what is really happening.

• This reply was modified 2 years, 11 months ago by Susan Brown.

[Susan Brown]

To make it easy, just make up numbers to play with.

Let’s say the element circuit had 1 amp of current before the failure, and the fan motor circuit had 2 amps.

What was the total current BEFORE the failure?

What was the total current AFTER?

[Susan Brown]

Remember, we are just asking if things increased, decreased, or stayed the same after the failure. In this case (Part 4 of the question), we are asking what happened to the overall current from L1 when the diode failed open.

Did it stay the same, increase, or decrease?

Based on what I told you about the total current when you have parallel circuits, you have enough info to answer.

[Susan Brown]

The *overall current from the power supply* for a circuit configuration with two or more parallel circuits is the *sum* of the currents in the parallel branches.

In other words, for this example, Itotal = Ielement + Ifan motor

Before the failure, both circuits had current. After the failure, only one did (and it did not change).

[Susan Brown]

I mean: will the failure of the diode cause the *overall* current from L1 to change from what it was before the failure.

L1 supplies both circuits.

[Susan Brown]

Exactly.

And you understand what this all means for the overall current from L1?

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