Advanced Physics
Chapter 18
Electric Currents
Chapter 18Electric Currents 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 Alessandro Volta (1800’s) invented the
electric battery, the first source of a steady flow of electric charge
Parts of a simple battery: Electrodes-plates or rods of dissimilar metals
(carbon) Electrolyte-solution through which charged
material (ions) flow
18.1 The Electric BatteryElectric cell Two electrodes and an
electrolyteBattery Several cells connected
together SymbolTerminal Part of electrode that
extends outside the electrolyte
18.1 The Electric BatteryHow 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
H2SO4
Zinc Carbon
Zn2+
e-
_ +
18.1 The Electric BatteryConventional current For positive to
negative (the flow of positive charges)
Dry cell Use of an electrolyte
paste Connect batteries in
series to increase voltage
Electrolyte paste
Carbon terminal
Casing
insulation
Zinc terminal
18.2 Electric CurrentElectric circuit Continuous conducting path between terminals of a batteryElectric 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 electronsResistance How much a conductor impedes the flow of
electrons Unit: ohm ()
18.3 Ohm’s LawOhm’s Law
V = IR Only good when there
is no change in temperature due to current flow
Resistor A device of known
resistance Color codes symbol
18.4 Resistivity Resistance is greater for a thin wire and for a long
wire (why?)
R = (L/A) where: R = resistance = resistivity (p.535) and depends on material,
temperature and other factors Silver < copper < aluminum
L = length of wire A = cross sectional area of wire
18.4 Resistivity 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 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……
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 = kWhr)
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
18.7 Power in Household Circuits 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
A battery produces a direct current (DC)—current flows only in one direction (which way?)
18.8 Alternating Current
An electric generator produces an alternating current (AC)—current flows in two directions
18.8 Alternating Current
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
A graph of the current versus time produces a sinusoidal curve.
Voltage can be written as a function of time:
V = Vosin2ft where V = average voltage Vo = peak voltage f = frequency t = time
18.8 Alternating Current
V = Vosin2ft Using Ohm’s Law we can find peak current (Io)
I = Iosin2ft And average power (P)
P = Io2Rsin2ft
P = ½ Io2R = ½ (Vo
2/R)
18.8 Alternating Current
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 These rms values are called the “effective values” Io and Vo are peak current and voltage!
18.8 Alternating Current
These rms values are called the “effective values”
These values can be directly used in the power equations
P = I2rmsR = V2
rms/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
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 CurrentDrift 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 Read it and know it
Summary due at end of class
Do your homework for this Chapter!