Download Electric Current

Electric Currents
AP Physics
Chapter 18
Electric Currents
18.1 The Electric Battery
The Electric Battery
Alessandro Volta – produces the
first battery made of zinc,
silver, and cloth soaked
in salt solution
Electric Cell – two plates or rods
(electrodes) of dissimilar
metal placed a solution (like
dilute acid)
Battery – several cells connected
together
18.1
The Electric Battery
The potential difference (voltage) between the
terminals of a cell depend on
what the electrodes are
made of.
Remember half-cell reactions
from chemistry
When two cells are placed
end to end, or in series, their
voltages add up
18.1
Electric Currents
18.2 Electric Current
Electric Current
Circuit – continuous conducting path between
terminals of a battery (or other source of EMF)
Electric Current – flow of charge (electrons)
I – current (amperes)
Q
I
Q – charge (coulomb)

t
T – time
18.2
Electric Current
Ampere (for Andre’ Ampere)
1A  1
C
s
Usually called an amp
Open Circuit – break in the
circuit, no current flow
18.2
Electric Current
Short Circuit – when the load is bypassed
Current increase
Ground – allows for a
continuous path for charge
flow
18.2
Electric Current
For historical reasons, current is defined as
being in the direction that positive charge
flows
18.2
Electric Currents
18.3 Ohm’s Law
Ohm’s Law
George Simon Ohm
I V
The actual values depend on
the resistance of the conductor
IR  V
Called Ohm’s Law
R – resistance measured in Ohms (W)
18.3
Ohm’s Law
Only true for Ohmic materials
Vacuum Tubes, Transistors, Diodes are
nonohmic
18.3
Ohm’s Law
A graph of current vs. potential difference
The metallic conductor is ohmic
The diode and filament are not
18.3
Ohm’s Law
Resistor – anything that uses
electric energy
Resistor – device used to control
current
The symbol for a resistor is
18.3
Ohm’s Law
The resistance value of a resistor is indicated
by the colored bands on the resistor
18.3
Ohm’s Law
Misconceptions
1. Cells (batteries) do not put out a constant
current. They maintain a constant
potential difference.
2. Current passes through a wire and
depends on the resistance of the wire.
Voltage is across the ends of the wire.
3. Current is not a vector, it is always parallel
to the conductor. The direction is from + to
-.
18.3
Ohm’s Law
Misconceptions
4. Current or charge do not increase or
decrease. The amount of charge in one
end of the wire comes out of the other end.
18.3
Electric Currents
18.4 Resistivity
Resistivity
Resistance is found to be directly proportional
to its length and inversely proportional to
its cross sectional area.
L
R
A
ρ is called the resistivity (Wm)
Longer extension cords must
be thicker to keep
resistance low
18.4
Resistivity
Some common resistivity values
Material
Silver
Copper
Gold
Aluminum
Tungsten
Platinum
Nichrome
Resistivity Temperature
Coefficient
(Wm)
(Co-1)
1.59x10-8 0.0061
1.68x10-8 0.0068
2.44x10-8 0.0034
2.65x10-8 0.00429
5.6x10-8 0.0045
10.6x10-8 0.00651
100x10-8 0.0009
18.4
Resistivity
Best Conductor is Silver, but Copper is close
and much cheaper
Tungsten is used in filaments
Nichrome
Apparently an Anime character
18.4
Electric Currents
18.5 Electric Power
Electric Power
The rate of energy flow for an electric circuit
W qV q
P

 V
t
t
t
That is more commonly written as
P  IV
Combining with Ohm’s Law it can also be
written
2
PI R
2
V
P
R
18.5
Electric Power
The power company charges by the kilowatthour (kWh)
1kWh  (1000W )(3600s)  3,600,000 J
Just a cool picture
18.5
Electric Currents
18.6 Power in Household Circuits
Power in Household Circuits
Household circuits – wires will heat up as
current increases
In a 20A household circuit
P  IV  (20 A)(120V )  2400W
In a 15A household circuit
P  IV  (15 A)(120V )  1800W
Circuits are typically designed to run at 80% of
the rated power output
Different circuits have different gauge wires
(diameter)
18.6
Power in Household Circuits
Circuit Breakers and Fuses
Break the circuit
18.6