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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…
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 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.
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?
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
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
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
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…
- tied into value of  for a given material
 high temp? higher R; low temp? lower R
Ex: Superconductors operate at extremely low
(<10K) temperatures.
► Temperature
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?
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)
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
►
Cost to Run an Electrical Device
1.
2.
3.
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)