OBJECTIVE:  

1) I will be able to relate electrical potential to electrical resistance after today's class.

2) I will be able to accurately define how electrons actually move through a wire in an electric circuit after today's class.

3) I will be able to relate Electromotive Force to electrical current after today's class

4) I will be able to compare and contrast EMF with electric potential after today's class

WORDS/FORMULAE FOR TODAY

TERMS:

• Capacitor - two charged surfaces that can store electrical energy
• ElectroMotive Force = EMF= ε=Voltage
• Resistor - an object in an electric circuit which interferes with the flow of electrons through that circuit.

CONSTANTS:

 

UNITS:

• EMF = ε = V
• Power = watts = I²R
• Ohms = resistance = Ω
  • (SI Units = ohms)
  • breakdown units = volts/camper
• Capacitance = C
  • (SI Units "farads" = F)

FORMULAE:

• I = ∆Q/∆t
• I = dq/dt
• ε=IR
• P = I²R
• ∆V = IR

WORK O' THE DAY: 

I'm calling a wee bit of an audible today... we're going to skip chapter 27. Here's why:

1) We skipped it last time I taught the course, with no impact

2) I searched through all the text of the EM tests that I have access to, and I found 1 m/c question and 1 FR dealing with resistivity. I found no, none, zilch, ixnay references to conductivity and drift or drift velocity

3) Resistivity and Conductivity get technical and mathy VERY quickly, and I don't thing the buck is worth the bang... so to speak

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Let's take a look at one of the most amazing electrical systems known in our solar system!

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Now back to our regularly scheduled programming:

Let's review how electrons actually "move" through a wire (we spoke about this on our first day or two of E & M)

As we just learned (hopefully!) electricity is defined as the flow of charged particles, although most of our work will involve the 'flow' of electrons.

When we think about 'electrical current', however, we will confine our conversations to the 'flow' of electrons.

I put "flow" in quotes because in the micro-universe, electrons moving inside a copper wire from the wall socket to the toaster rarely move very far.

A common misconception is that electrons "flow" from the wall socket, through the wire to the toaster... much in the way that water flows through a pipe.

A better analogy is marbles inside a hollow tube. The marbles just sit their minding their own marble business until something makes them move. In this analogy, let's consider adding a single marble to the front of the tube.... almost immediately a marble exits the tube. If we supply a steady flow of marbles then we have a continuous 'current' as marbles exit and leave the tube.

Much in the same way, electrons are constantly in motion in the metal of a copper wire. For those of you who have had chem, please recollect that the outermost electrons in the atoms bound together in a copper wire are pretty much shared among those atoms and are in constant motion.

By adding an electric potential (voltage), those outermost electrons orient themselves very quickly, like a whole company of soldiers called to attention.

That orientation carries the charge down the wire....

This is where it might get a bit confusing, that orientation can be viewed AND measured as a "flow" of electrons. We give that "flow" a name: current

We measure the electric current by the amount of electric charge that passes a fixed point in a certain amount of time:

I = ∆Q/∆t

I = dq/dt

Since charge is measured coulombs and time is measure in seconds, the breakdown units of charge are coulombs/sec. The unit measure of charge in SI units is an ampere (often called an 'amp' for short)

Here's a question for you:

If the charge on an electron is 1.60 x 10^-19 coulombs, how many electrons "flow" past a fixed point in your household if the current flowing through your wires there is 20.0 amperes?

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Resistance

• Resistance is a specific measure for how a particular member of an electric circuit resists the flow of electrons through it.
• Resistance is measure in Ohms (Ω) and the break down units for ohms are volts/ampere
• There is a VERY important relationship between electric potential, current and resistance:

Resistance is DEFINED to be the electric potential (volts) divided by the current (in amperes)

R≡∆V/I

Although we usually remember it using "Ohm's Law": V = IR

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Let's learn how to read a circuit diagram:

This symbol represents the EMF (most often a battery)

• The large plate indicates positive charge and signals the direction the current will flow (the +/- sign are often omitted so get ready for that

This symbol represents a resistor present in the circuit:

A *very* basic electric circuit is shown below.

• Think of the battery as the pump (not the supply) of electrons

1. Current is pumped from the battery into the wire
2. Current flows through the wire to the first resistor
3. Current then flows through the first resistor (in physics we use the term 'across the resistor')
4. Current continues through the wire across the second resistor
5. Current flows through the wire back to the battery

Now consider the analogy of water flowing through a pipe (it isn't a perfect analogy and it does fall flat sometimes but let's mush on nonetheless)

1) The pump increases the water pressure in the pipe

2) Water flows into the pipe

3) The water encounters some sort of obstacle...

What happens to the water pressure in the pipe as a result of that encounter?

 

 

A: It drops

which leads us to our first rule of the day...

1) Voltage drops across a resistor

Let's take a gander:

Notice how the drop in voltage between the terminals is equal to the voltage drop across the resistors

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A note on convention -- we will follow the 'standard' physics convention for electron flow (as our book and most books do), which is to say we will always say that current flows from positive to negative.

For those of you purists out there (read: chemists) the ACTUAL flow of current goes from negative (where the electrons are) to the positive (where the electrons ain't)

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A note on EMF vs Potential Difference:

EMF is the TOTAL difference across the terminals of a power supply as measure in the power source only and is independent of any potential lost due to internal resistance in the battery.

Potential Difference is the ACTUAL amount of oommphhh the battery can supply to the circuit.

Both EMF and potential difference are measured in volts.

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Try not to get too confused with this one.... if an internal resistance is given for a battery, use that in your calculations.... if it isn't then don't.

To continue our water pump analogy:

EMF would be the RATING given by manufacturer of the maximum pressure the pump can provide and Potential Difference is the ACTUAL USABLE pressure the pump provides.

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POWER:

Lean wayyyyy back into last semester and recollect that power is measured in joules/second.... otherwise known as watts

In an electrical current, power is found by the following:

P = I²R

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COURSEWORK: