1. Current Electricity Recall
Static Electricity studies excess charges making isolated jumps to move away from like charges – high potential; closer to opposite charges – low potential.
New
Current Electricity studies the continuous, constant flow of charge, again from high to low potential.
The main focus of this study is to examine circuits and how they function.
An Electrical Circuit is a man-made path of conducting materials, usually for the purpose of making an electrical device work.
And now, “The Story”…
keep this scenario in your head as we go through the important physical quantities that are measurable within a circuit…

2. 3 Measurable Physical Quantities within a Circuit
1st Current (I) – rate at which charges flow
so eq’n: I = ∆q/∆t measured in units of Amperes (A)
where 1 amp = 1 Coulomb/sec = 6.25 x 1018 e-/1 sec
(New unit for charge then… amp∙hrs…common on batteries)
Important Ideas about Current
Current does not indicate a net charge in the circuit
those e- come from the atoms in the conducting material of the various parts of the circuit,
NOT pouring in from the wall outlet or battery!
Charges/Current are not “used up” in a circuit,
the #e- entering one end = #e- leaving the other
these e- simply move from atom to atom, and typically very slowly: ≈ 1m/hr! (called drift speed)

3. 3 Measurable Physical Quantities within a Circuit
2nd Potential Difference aka Voltage (V) – a difference in potential or amount of electrical charge between 2 points units: Volts (V)
Charges always move from locations of
high potential – where there’s lots of like charges,
to locations of
low potential – where there’s opposite charges,
or at least not as many like charges
following the rule: “likes repel, opposites attract”,
& the amount of force is found using Coulomb’s law
So for the e-’s moving through a circuit, that means away from the negative – high potential
and toward the positive – low potential.

4. Potential Difference is present in both:
Please note that while current actually flows through a circuit, but voltage or potential difference, DOES NOT!
Voltage is a measurement taken across 2 points of the circuit, to determine if there is in fact a difference in the potential at those 2 locations. Voltage should never be referred to as “going through” or “flowing in” the circuit!
Potential Difference is present in both:
Static electricity – that’s why the excess stationary charges, with a high potential, decide to “jump” when there’s a low potential location somewhere close enough, for them to go to.
Ex: spark, shock, lightning
Current electricity – if we can continuously maintain the difference in potential between 2 locations, then the charges will continue to flow!
But what do we use to maintain a potential difference?

5. 2 Sources of Potential Difference in a Circuit
Batteries
provide DC or direct current – flows in only one direction, at a constant rate
since the location of the high & low potentials of the battery stay fixed
Con: runs out, once all e-’s reach the low potential
(unless rechargeable…)
Pro: portable – no plug needed

6. 2 Sources of Potential Difference in a Circuit
Generators
unless you have your own, we usually tap into a generator at an electrical power plant, by plugging into a wall outlet
Provide AC or alternating current – continually switches direction of travel by 180°
since the locations of the high &
low potentials created by the
generator alternate positions
120x’s/sec (60 Hz)
Pro: never runs out, as long as we have fuel to run the generators at the power plants…
since e-’s never get to reach the low potential
Con: needs to be connected to generator by cord/plug

7. 3 Measurable Physical Quantities within a Circuit
3rd Resistance (R) – reduces the amount of current by hindering the flow of charge. units: Ohms (Ω)
3 Sources of Resistance in a Circuit
Intentional – if we want to limit the amount of current in a particular part of a circuit, we put a resistor in
Unintentional – no matter how good of a conductor, there’s some R in everything (except superconductors) because there’s always friction if e-’s are moving… found by R = ρL/A…
Load – the device we plug into the circuit to make the current do something for us; often contributes lots of R since this is where the e-’s do work for us
Ex: light, radio, fan, hairdryer, cell phone, etc

8. Then putting them all together…
Unintentional Resistance in a Wire depends on:
Resistivity – ( (rho) in m) how tightly the material holds onto its e-’s; different for different materials
loosely? low resistivity - lower R (conductor)
tightly? high resistivity - higher R (insulator)
So R   (directly)
Cross Sectional Area (A in m2) of wire
large diameter? lower R; small diameter? higher R
So R  1/A (inversely)
Length (L in m) of wire
long wire? higher R; short wire? lower R
So R  L (directly)
Then putting them all together…
Temperature - tied into value of  for a given material
high temp? higher R; low temp? lower R
Ex: Superconductors operate at extremely low (<10K) temperatures.

9. So now what about that scenario about people that go to work to earn a paycheck?
Individual charges (q)?
Current (I)?
Conducting paths (wires)?
Intentional resistance?
Unintentional resistance?
Diameter of wire?
Length of wire?
Load?
Potential Difference/Voltage?

10. Ohm’s Law For certain resistors, called ohmic resistors,
there is a relationship between I, V & R,
called Ohm’s Law.
Think about it… (and keep in mind the story to help)
If R was constant,
how would a change in V affect the amount of I?
I α V for a constant amount of R
If V is constant,
how would a change in R affect the amount of I?
I α 1/R for a constant amount of V
So now put them together:
I α V/R where the constant of proportionality = 1,
so eq’n: I = V/R or V = IR (Ohm’s Law Eq’n)
units: Amps = Volts/ohms (A = V/Ω)

11. Electric Power Recall from Mechanics: Power is the rate at which
Work gets done, so P = W/∆t OR
Energy gets transferred, so P = ∆E/∆t
This also applies to Electricity:
Power is the rate at which electrical energy (moving charges) convert into another form of energy, such as
light, like a lamp…
heat, like a toaster…
kinetic, perhaps something spinning, like a fan…
Eq’ns: P = IV = I2R = V2/R
units: Watts = AmpVolts = Amp2 = Volts2/
but a Watt is small, so usually kW = 1000 Watts

12. Cost to Run an Electrical Device
Know amount of Current, in Amps, device requires: ____
Compute amount of Power, in kW, it uses:
P = IV what does typical V AC? = _____
what does major V AC? = _____
What about V DC? ____________
(V AC in other countries often 240 V, so we need transformers)
Compute the amount of Energy, in kWh, it used:
so 1st we need ∆t, in hrs - average time we used it for in a month, to see how much it contributed to an electric bill:
Then use P = E/∆t, where E = P ∆t
4. Compute the cost for that amt of NRG:
cost in $ = (E) ($/kWh)