Transcript Current-2009

It’s
Electrifying!
E-Mag
Electric Potential
Electric Potential is
measured in volts (V)
The total electric potential
energy divide by the
charge.
 1 volt is equal to 1 joule per
coulomb.
Electric potential is commonly
called voltage.
 Most outlets in your house are
110-volts
 Some use 220-volts
Electric Energy Storage
 Capacitors are used to store
energy.
 Voltage between plates of a
capacitor become equally but
oppositely charged.
Potential Difference
 The change in potential (voltage)
between two ends of a conductor
or between battery terminals.
 The flow of charges will continue
until there is no longer a potential
difference.
Potential Difference
 The change in energy between two
points in a circuit divided by the
amount of charge
 V = ∆E/q
Bird on a Power Line
SAFE: both feet are on the
same voltage line thus no
potential difference!
NOT SAFE: If one leg is on
the ground or another wire and
the other one is on the power
line, then there is a potential
difference between the bird’s
two legs.
Electric Current (I)
 The flow of electric charges.
 Measured in amperes (A) one
amp. is equal to the flow of 1 C
of charge every second
Current Direction VS. Electron Flow
 For purely HISTORICAL reasons, current is
conventionally described as going from
positive to negative!

The opposite way from electron flow!
 Current direction was decided as going from
positive to negative well before anyone had
any idea of electrons' existence, let alone their
being negative!
Voltage Sources
Generators and batteries
provide sources of voltage.
Batteries can have either
dry or wet cells
Generators
 Convert mechanical energy
into electrical energy
 ex: household generators or
car alternators
Batteries
 Energy is released during a
chemical reaction.
 Dry cells (Duracell) have a
paste electrolyte. Wet cells (car
batteries) have a liquid
electrolyte.
Electric circuits are all about
energy, not charge
 When a battery no longer
works, it is out of energy.



Chemicals inside the battery undergo
a reaction that produces energy.
This energy-producing reaction
pumps the charge through the battery
from low energy terminal to high
energy terminal and establishes the
electric potential difference across the
external circuit.
When a battery no longer works, it is
because the chemicals have been
consumed to the point that the ability
of the battery to move the charge
between terminals has been severely
diminished.
Video: Electricity & Batteries
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Simple Fruit Battery
Resistance
 Opposition to the flow of
electric charges. It is increased
if length is increased or width
of wire is decreased.
 Measured in ohms ()
Electrical Resistance
 Ohm's Law defines the relationships
between (P) power, (E) voltage, (I)
current, and (R) resistance.
 One ohm is the resistance value
through which one volt will maintain a
current of one ampere.
Formula
 Voltage (V) is equal to
Current (I) times Resistance
(R)

V = I R
Ohm’s Law sim:
http://phet.colorado.edu/simulations/sims.php?sim=Ohms_Law
Electric Current
 The flow of electrons
Electric Circuit
 Complete conducting path through which charge can
flow
 For Current (I) to flow, it must include a charge pump
and be closed
Fact or Fiction?
 When you turn on a light switch, electrons
move rapidly from the socket to the lamp to
light the bulb.
Electron Drift
 Charge moves abnormally slowly on average, about 1 meter in an
hour - through a circuit.
Direct Current (DC)
 Charges always flow in one
direction. Ex: battery
 electrons always flow from the
neg. terminal to the pos.
simulations
terminal AC/DC
http://phet.colorado.edu/simulations/si
ms.php?sim=Circuit_Construction_Kit
_ACDC
Alternating Current (AC)
 Current flows in one direction,
and then in the other
direction…back and forth.
Most US circuits operate at a
frequency of 60 Hz.
AC/DC
Electrical Relationships
Water in a
Hose
Pressure
DC in a
Electrical
Wire
Units
Voltage (V) Volts (V)
Rate of
Flow
Friction
Current (I)
Resistance
(R)
Amperes,
Amps (A)
Ohms (Ω)
Electric Power
Rate at which electric
energy is converted into
another form such as
mechanical, heat or light
energy.
Calculating Power
 Electric power (PE) is equal
to current (I) times voltage
(V)

measured in watts (W)
 PE = I V
Energy Converted
 Electricity used (energy
converted) is equal to power
(kW) times time (hours)
 Eelectric = Pt
What causes electric shock in the human
body- current or voltage?
 Current passing through the
body depends on the voltage
applied and the electrical
resistance of the body. (ranges
from 100 Ω if soaked in salt
water to about 500,000 Ω if the
skin is very dry)
Effects of Current on the body
 0.001 A – barely felt
 0.005 A – painful
 0.010 A – muscle spasm
 0.015 A – loss of muscle control
 0.1 A (100 mA) can be fatal if
current goes through the heart