Advanced Physics Chapter 18 Electric Currents

Chapter 18 Electric 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 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 Battery Electric cell Two electrodes and an electrolyte Battery Several cells connected together Symbol Terminal Part of electrode that extends outside the electrolyte

18.1 The Electric Battery How a simple cell works: Acid attacks Zinc electrode Zinc ionizes (Zn 2+) and 2 e - leaves at negative electrode Zn 2+ enters solution Zn 2+ 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 H 2 SO 4 ZincCarbon Zn 2+ e-e- _ +

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 Electrolyte paste Carbon terminal Casing insulation 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 ( ) 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 (  )

18.3 Ohm’s Law Ohm’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 – T o )] where:  T = resistivity at any temperature (T)  o = resistivity at reference temperature (T o )  = 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-T c ) 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 = I 2 R = V 2 /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

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 = V o sin2  ft where V = average voltage V o = peak voltage f = frequency t = time

18.8 Alternating Current V = V o sin2  ft Using Ohm’s Law we can find peak current (I o ) I = I o sin2  ft And average power (P) P = I o 2 Rsin2  ft P = ½ I o 2 R = ½ (V o 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 I rms = 0.707I o V rms = 0.707V o These rms values are called the “effective values” I o and V o 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 = I 2 rms R = V 2 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 (v d ) So current in a wire is… I =  Q/  t = neAv d where: n = number of free electrons e = charge of an electron (1.6 x 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 10 8 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!