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Advanced Physics
Chapter 18
Electric Currents
Chapter 18
Electric Currents
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18.1 The Electric Battery
18.2 Electric Current
18.3 Ohm’s Law
18.4 Resistivity
18-5 Superconductivity
18.6 Electric Power
18.7 Power in Household Circuits
18.8 Alternating Current
18.9 Microscopic View of Electric Current
18.10 The Nervous System and Nerve
Conduction
18.1 The Electric Battery
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Alessandro Volta (1800’s) invented the
electric battery, the first source of a steady
flow of electric charge
Parts of a simple battery:
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Electrodes-plates or rods of dissimilar metals
(carbon)
Electrolyte-solution through which charged
material (ions) flow
18.1 The Electric Battery
Electric cell
 Two electrodes and an
electrolyte
Battery
 Several cells connected
together
 Symbol
Terminal
 Part of electrode that
extends outside the
electrolyte
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18.1 The Electric Battery
How a simple cell works:
 Acid attacks Zinc
electrode
 Zinc ionizes (Zn2+) and 2
e- leaves at negative
electrode
 Zn2+ enters solution
 Zn2+ pulls e- off carbon
electrode it becomes
positive
 If terminals are not
connected then only a
small amount of zinc
reacts
 If terminals are connected
then flow of electrons
+
e-
Zinc
Carbon
Zn2+
H2SO4
18.1 The Electric Battery
Conventional current
 For positive to
negative (the flow of
positive charges)
Dry cell
 Use of an electrolyte
paste
 Connect batteries in
series to increase
voltage
Casing
Carbon
terminal
insulation
Electrolyte
paste
Zinc terminal
18.2 Electric Current
Electric circuit
 Continuous conducting path between terminals of
a battery
Electric current
 The flow of charges through a conductor (I)
 I = Q/t
 Current is the net charge (Q) that flows through
a conductor per unit time (t) at any point.
 Unit: ampere, amp, A 1A = 1C/1s
 Current is the same at any point in a conductor
between two terminals.
18.3 Ohm’s Law
To produce an electric current in a circuit a
difference in (electric) potential is required.
Simon Ohm (1787-1854)
 Experimentally determined that I  V
 Exactly how much current flows depends on
voltage and resistance to the flow of electrons
Resistance
 How much a conductor impedes the flow of
electrons
 Unit: ohm ()
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18.3 Ohm’s Law
Ohm’s Law
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V = IR
Only good when there
is no change in
temperature due to
current flow
Resistor
 A device of known
resistance
 Color codes
 symbol
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18.4 Resistivity
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Resistance is greater for a thin wire and for a long
wire (why?)
R
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R = resistance
 = resistivity (p.535) and depends on material,
temperature and other factors
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= (L/A) where:
Silver < copper < aluminum
L = length of wire
A = cross sectional area of wire
18.4 Resistivity
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Since resistivity depends on temperature
As temperature   resistance  (why?)
T = o [1 + (T – To)] where:
T = resistivity at any temperature (T)
o = resistivity at reference temperature (To)
 = temperature coefficient of resistivity
Equation holds true for “small” T’s
18-5 Superconductivity
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When a compound
(metal alloy) has a
resistivity of zero
Occurs at low
temperatures (below
transition
temperature-Tc)
Loses all resistance to
the flow of electrons
Costly and brittle
Uses: electromagnets
18.6 Electric Power
Power
 The rate at which electrical energy is
transferred or transformed into another
form of energy (thermal, kinetic, light, etc)
P
= QV/t
since I = Q/t then
P =IV
18.6 Electric Power
Power
 The rate at which electrical energy is
transferred or transformed into another
form of energy (thermal, kinetic, light, etc)
 If this energy transfer is due to resistance
then it can be calculated by……
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P=IV + V=IR  P = I2R = V2/R
18.6 Electric Power
Power
 When you purchase electricity from the
power company you are buying energy not
power.
 You purchase kilowatt-hours (energy) not
kilowatts (power)
 P = E/t so…E = Pt (or kWh = kWhr)
18.7 Power in Household
Circuits
In a household circuit the current in the
wiring can cause an increase in temperature
that can lead to fires (why?)
 Short circuits also can cause overheating
To prevent this electricians use:
 Fuses
 Circuit breaks
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18.7 Power in Household
Circuits
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Household circuits are constructed in
parallel so that…..
Each device used gets the same voltage
Total current in circuit is equal to the
current through each device
But this can lead to extreme heating of
wires (why?)—Chapter 19!
18.8 Alternating Current
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A battery produces a direct current (DC)—
current flows only in one direction (which
way?)
18.8 Alternating Current
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An electric generator produces an
alternating current (AC)—current flows in
two directions
18.8 Alternating Current
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A battery produces a direct current (DC)—current
flows only in one direction (which way?)
An electric generator produces an alternating
current (AC)—current flows in two directions
Frequency of an alternating current is number of
times the current changes direction per second (in
US 60 Hz)
18.8 Alternating Current
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A graph of the current versus time produces a
sinusoidal curve.
Voltage can be written as a function of time:
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V = Vosin2ft where
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V = average voltage
Vo = peak voltage
f = frequency
t = time
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18.8 Alternating Current
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V = Vosin2ft
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Using Ohm’s Law we can find peak current (Io)
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I = Iosin2ft
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And average power (P)
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P = Io2Rsin2ft
P = ½ Io2R = ½ (Vo2/R)
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18.8 Alternating Current
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The average value for the square of the current or
voltage is important for calculating average
power
The square root of these values (root mean
square-rms) is the average voltage/current
 Irms = 0.707Io
 Vrms = 0.707Vo
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These rms values are called the “effective values”
Io and Vo are peak current and voltage!
18.8 Alternating Current
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These rms values are called the “effective
values”
These values can be directly used in the
power equations
P = I2rmsR = V2rms/R
18.9 Microscopic View of
Electric Current
As electrons travel through a conductor
they bounce off the atoms that make up the
conductor
 This causes the electrons to speed up and
slow down and determine the speed at
which they flow through a conductor.
Drift speed-the average speed that electrons
move through a conductor
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18.9 Microscopic View of
Electric Current
Drift speed-the average speed that electrons
move through a conductor (vd)
 So current in a wire is…
 I = Q/t = neAvd where:
 n = number of free electrons
 e = charge of an electron (1.6 x 10-19 C)
 A = cross-sectional area
18.9 Microscopic View of
Electric Current
Drift speed for electrons through a wire is
very slow (0.05mm/s) but electricity
travels at close to the speed of light (3 x
108 m/s)—how can this be true???
18.10 The Nervous System and
Nerve Conduction
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Read it and know it
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Summary due at end
of class
Do your homework
for this Chapter!