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!