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Electric Currents
Charges in motion.
Creating Potential Difference.
• Alessandro Volta (1745 1827)
– Ranked potentials created by
combing two metal discs.
– Created the first continuous
source of charge using zinc and
silver plates stacked in an acidic
solution.
Sources of Continuous Current
• 1.) Electromagnetic - By rotating a conducting loop in
a magnetic field. This forces free electrons to vibrate
in the circuit. (A generator)
• 2.) Photoelectric Effect - Electrons are emitted from a
conductor when illuminated by light with a short
enough wavelength.
– (mostly in the ultraviolet spectrum, but cesium and
potassium will emit electrons at the visible light frequency.)
• 3.) Thermoelectric - A “Thermocouple” can be
created by heating a junction between an iron and a
copper wire, and by submerging the other junction in
an ice bath.
• 4.) Piezoelectric - certain crystals under
stress will become charged.
– The crystal needle of a record player.
(quartz watch)
• 5.) Chemical - some reactions give off
electrons.
– Primary cells: chemical process that can
be “used up.”
– Storage cells: “rechargeable” reverses
reaction by introducing energy.
– Fuel Cells: primary cell, but materials are
supplied continuously.
The Electric Battery
• Metals in a battery are called
electrodes.
• The solution they are in is an
electrolyte.
• The exposed parts of the metals are
terminals.
• In diagrams, a battery is represented by:
+
-
How does it work?
• The electrolyte solution dissolves one of the
electrodes, whose ions enter the electrolyte
leaving the electrons on the terminal, which
is now negative.
• The electrolyte (with ions) becomes positive.
• The electrons from the other terminal enter
the electrolyte, leaving that terminal positive.
A Battery
Electric Current
• The amount of charge to pass a point in a
circuit per unit of time.
VQ
I
Vt
• Units of Electric Current are C/s or Amperes.
• Conventional current flows from the positive
terminal to the negative terminal at
approximately 3 x 108 m/s.
• Electrons move opposite to conventional
current at approximately 1 mm/s.
Ohm’s Law
• Current (I) is proportional to the
potential difference (V).
• V = IR,
– where R is the resistance met along the
way.
• In a diagram, a resistor is represented
by:
Resistance is measured in Ohms (Ω)
Electric Power
• The rate of energy transferred, given by:
P = IV
P = I2 R
P = V2/R
• Unit of power is the Watt. (Joules/sec)
– Usually Kilowatts are used.
• The Kilowatt-hour is the product of
power transferred for a time period.
– Unit of Energy = 3.6 x 106 J
Direct or Alternating Current
• Direct - One direction of current flow.
– Produced by constant potential difference.
• Like a battery.
• Alternating - Current flows oscillates.
– Produced by a changing electric potential.
• Like a generator.
The magnitude of voltage in A.C. circuits changes as a
function of time and frequency of oscillation such that:
V  V sin 2 ft where V0 is the peak voltage.
0
Peak Current
• Combining Ohm’s Law with the
equation for peak voltage,
(V0 sin 2 ft)
I
 I 0 sin 2 ft
R
• Thus the current changes as a function
of frequency of oscillation and time.
I0
= peak current
Combine with equation for Power,
P  I Rsin 2 ft
2
0
Some Practice
Electric Circuits
Series Circuits
• Electric current has a single path
through the circuit.
• Because there is only one path through
the circuit, the current through each
source of resistance is the same.
• The total resistance to the current is the
sum of the individual resistors in the
circuit.
• I = V/RT
(RT = R1 + R2 + …)
• If one source of resistance breaks its
connection in the circuit, power is lost to
all other devices in the circuit.
Voltage in Series
• The Voltage changes across EACH
resistor in a series circuit. Use V=IR for
each source of resistance.
• Total Voltage drop across all resistors
equals the voltage across the circuit. If
you start with 120 V, you can never lose
more than 120 V. What changes is the
current.
Example
6V
R1
R2
I = V/RT
I = 6V/(4+2) Ω
I = 6V/6 Ω
I = 1 Amp
V1=1A x 2 Ω = 2V
V2=1A x 4 Ω = 4V
2Ω
4Ω
VT= 2V + 4V = 6V
Parallel Circuits
• Electric current has more than one path
through the circuit.
In Parallel Circuits
• Each device connects to the same two points
A and B of the circuit. The voltage across
each device is the same.
• The total current divides evenly among the
branches. Current passes easier through
devices with low resistance, so the amount of
current is inversely proportional to the
resistance in the branch. I = V/R applies to
each branch separately.
Example
6V
I1 = V/R
I1 = 6V/2 Ω
I1 = 3A
R1 = 2 Ω
I2 = V/R
I2 = 6V/4 Ω
I2 = 1.5 A
IT = 4.5 A
R2 = 4 Ω
NOTICE!
• As the number of parallel branches
increases, the overall resistance
DECREASES!
• This means that the overall resistance is
less than the the resistance in any one
of the branches.
Example
6V
R1 = 2 Ω
R2 = 4 Ω
To find total resistance
in parallel circuits, you
add the inverse of each
resistor, and then take the
inverse of your sum.
1/R1 + 1/R2 = 1/RT
1/2 Ω + 1/4 Ω = 1/RT
2/4 Ω + 1/4 Ω = 3/4 Ω
RT = 4/3 Ω
BOTH!?!?!?
Upon encountering both series and parallel resistors
in a circuit, find the parallel resistance, and then
combine it with the series resistance to find the total.
1Ω
6V
2Ω
2Ω
RT= R1+ (1/R2+ 1/R3)
RT = 1Ω+(1/2Ω + 1/2Ω)
RT = 1Ω+(2/2 Ω)
RT = 1Ω+(1 Ω)
RT = 2 Ω
IR1 = ?
Try one.
R2
6Ω
R1
3Ω
IR2 = ?
IR3 = ?
RT = ?
6Ω
R3
VR1 = ?
VR2 = ?
VR3 = ?
12 V
Factors That Affect Resistance
• 1.) Temperature:
– Most metals and alloys increase resistance when
heated.
– Carbon, and semiconductors decrease resistance
• 2.) Length:
– The resistance of a uniform conductor is directly
proportional to the length of the conductor.
• 3.) Cross-Sectional Area:
– Resistance is inversely proportional to crosssection.
Superconductors
• When temperature drops toward zero, some
materials exhibit a sudden dive in resistivity.
(Transition temperature)
• Resistance in a wire can be found by
– R= l
( is resistivity)
A
• Resistivity is a property of conductors.
– Copper = 1.7 x 10-6 cm @20
– Iron = 1 x 10-5 cm @20
– Gold = 2.4 x 10-6 cm @20