Electricity Chapter 8 Joseph F. Alward, PhD  Department of Physics  University of the Pacific

   Electrons and Protons

Electrons and protons have an attribute called charge. Electrons have a negative charge. e = 1.6 x 10^-19Coulomb Protons are 1800 times more massive, and have a positive charge. (Neutrons have no charge.)

Electrostatic Attraction and Repulsion

Charging by "conduction."

Opposites attract. Likes repel.

  The Coulomb Unit

Six million trillion electrons is about - 1 C.

Six million trillion protons is about + 1 C.

  Removing Electrons from Atoms

Rubber scrapes electrons from fur atoms.

   Electric Force

Girl is touching negatively-charged sphere. Why do the strands of her hair fan out?

 Coulomb's Law

Charles Augustin de Coulomb (1736-1806)

F = k q1q2 / d^2 Compare this to the gravitational force law: F = G m1m2 / d^2

The force between two charges is 100 newtons 1. What happens if the distance d is doubled? 2. Double both charges? 3. Double d and charges?

Applications of Electrostatic Charging

Fine mist of negatively charged gold particles adhere to positively charged protein on fingerprint. (From Eugene Hecht's Physics, 2nd Edition Brooks/Cole Publishing)

Negatively charged paint adheres to positively charged metal.

Electrostatic Air Cleaner

Why are electrostatic phenomena, like shocks received from
car door handles, more common in winter than in summer?

   Hydrogen Atom is Analogous to Earth-Moon System

 Electron Cloud

Electrons surrounding nucleus may be thought of as a cloud in which the total negative charged is smeared out.

  Polarization

Unpolarized atom Put negatively charged rod on the right side... Center of electron cloud shifts to the left.

  Polarization

Polarized Atom Electrons move to other side of atom, leaving the left side positively charged.

Neutral Objects are Attracted to Charged Objects

Charged comb attracts neutral bits of paper.

Charged comb attracts neutral water molecules.

Charge Distributions

Charge on Metals  Metal Ball

Charge on Insulators Plastic Ball

Charge on Metal Points Lightning, lightning rods.

Charges Accumulate on Points

Charging by Contact

Some electrons leave rod and spread over sphere.

Charging by Induction    

Rod does not touch sphere.  It pushes electrons out of the back side of the sphere and down
the wire to ground.  The ground wire is disconnnected to prevent the return of the electrons
from ground, then the rod is removed.

 Electric Field of Dipoles

Imaginary lines with arrow heads show direction along which hypothetical positive charges would move.

  Dipole Electric Fields

Threads floating on oil bath become polarized and align themselves with the electric field.

Electric Fields Under the Sea

Elephant Gnathonemus detects nearby objects by their effects on the electric field.

Cells in shark detect weak electric fields caused by the operation of the muscles of its prey.  Fields as weak as 10^-6 N/C are detectable.

 Examples of Electric
 Field Strengths

Source	E (N/C)	Source	E (N/C)
House wires	0.01	Thunderstorm	10,000
Near stereo	100	Breakdown of air	3 million
Atmosphere	150	Cell membrane	10 million
Shower	800	Laser	10^11
Sunlight	1000	Pulsar	10^14

Compare to the field detectable by sharks, 10^-6 N/C.

 Charged Parallel Plates

   Electric Field between Plate and Ring

Electric field lines intersect the surfaces at right angles (90 degrees).

   Electric Potential Energy and Electric Potential

The kinetic energy the accelerating charge will have is equal to the electric potential energy it once had.

Gravitational Potential Energy = mgh

1 kg x 10 m/s² x 10 m = 100 Joules
--------------------------------------------------

10 meters above the surface of
the earth the gravitational potential
is 100 Joules / kilogram.

What is the potential energy of a 5 kg
mass at the same location?

   Electric Potential

If the number of coulombs q is doubled, the electric potential energy will double: Potential Energy = Charge x Potential ------------------------------------------------------ Potential  =  Potential energy / Charge       Volts = Joules / Coulomb Potential is associated with a location, not a charge.

Electric potential and voltage are the same thing.

One speaks of the potential--or voltage--at a
particular point in space

Charges do not have potential.

Electric Potential

Volts = Joules / Coulomb Joules = Volts x Coulombs The magnitude of the potential at the location of q is 50 volts. If q = 3 coulombs, what will be its kinetic energy when it's released and reaches Q?

   Electric Potential and Electic Potential Energy

The spring has more mechanical (elastic) potential energy when compressed.

The pair of charges has more electric potential energy when their separation is smaller. How will happen to this potential energy when the hand releases the charge?

   Work, Potential Energy, and Potential

Hand does work to push the small charge to the location shown.   ---------------------------------------------- The charge has potential energy. ---------------------------------------------- If the charge q is brought in from very far away, the work done per coulomb of charge  moved is the potential--or voltage at the given point.

   Work, Potential Energy, and Potential

The work done per coulomb of charge is the potential--or voltage--at the given point. ------------------------------------------------------------- Example:  1000 joules of work is done to move the charge q from far away to the place indicated. If q = 10 C, what is the voltage (the potential) at the new location? Voltage = Work / Coulomb

   Potential versus Potential Energy

Which one of the two smaller charges has the greater potential energy? Electric potential is a property of space, not charge.   Is the potential at the site of the (smaller) red charge different from the the potential at the site of the (larger) blue charge?

  How Dangerous is 5000 Volts?

As one million electrons are added to a neutral balloon, its potential rises from zero to 5000 volts. This is like the 5000 C spark from a sparkler. Temperature is the measure of the kinetic energy per molecule, and voltage a measure of the potential energy per coulomb. If there aren't many molecules at the high temperature, or coulombs at high voltage, there is too little energy to do any harm.

 Voltage is Analogous to Water Pressure Difference

The electrons on the ballon were at a high "electrical presssure", or voltage, but that
pressure quickly falls once most of the electrons have flown off the balloon.  If they
were constantly replaced as the electrons passed into a person, then the balloon
would give a serious shock.

  The Electric Eel

Eel uses chemical activity to maintain a potential difference between head and tail of about 600 volts. Potential difference causes charge in water to flow from head to tail.

   Electric Current

Electrons in metal wire and lightbulb filament are pushed around the circuit. One ampere = One coulomb                           per second This battery pushes electrons in one direction only:

  Alternating Voltage Sources        

 Alternating Current     

--------------------------------------------------------------------------------------

Current surges first one way, then the other through the toaster
heating filament, changing direction 120 times per second.

   Inside the Dry Cell Battery                                        

The source of the voltage  is  chemical energy.

A battery ripped open.

  Batteries Set up Electric Fields
  between their Terminals

If a wire is connected between the terminals, in which direction do the electrons move?

  Analogy between Water Pump and a Battery

Water pumps are rated according to the pressure difference they can maintain across their ends. Batteries main- tain an elect- rical pressure difference-- a "voltage" across their ends.

   Resistors

Resistance is measured in ohms. Resistors reduce the flow of electrons in an electrical circuit.

Current

The number of coulombs per second which travel around a circuit each second is called the current, symbolized by I. Current  I is measured in amperes. One ampere = One coulomb                            per second.

Andre M. Ampere 1775-1836

    Ohm's Law:  I = V/R

V = 10 volts   R = 4 ohms   I = 10 / 4 = 2.5 amperes

Filament provides  resistance to the flow of  electrons.

Georg Simon Ohm (1787-1854)

   Calculating V from Ohm's Law

Ohm's Law:  V = I R V = (6 amperes)(3 ohms)     = 18 volts

   Calculating I from Ohm's Law

Ohm's Law:  I = V / R I = (12 volts) / 3 ohms   = 4 amperes

 Calculating R from Ohm's Law

Ohm's Law: R = V / I R = (36 volts) / (6 amperes)     = 6 ohms

Will Either Bird be Shocked?

Connecting wire has no insulation and negligible
resistance.

 Resistance, Ohm's Law, and Short Circuits

Why isn't this bird shocked? What if the right foot of the bird were moved to the back wire?

  Safety in Electricity

Other end of ground plug is connected to the appliance cover. The wall jack which receives the ground plug is connected to the ground, so any charge leaking onto the appliance will drain to ground.

Appliance without Short

Impropery Grounded Appliance with Short

One milliampere:    tingling sensation
Ten milliamperes:   nerves and muscles overloaded
200 milliamperes:   potentially fatal; heart fibrillation
500 -1000 milliamperes:  not necessarily fatal; heat will restart
One ampere or more:  burn alive

Grounded Appliance

   Electrical Circuits

The electrons inside the filament surge back and forth when the terminals are connected to an AC voltage source.

   Series Circuits

Current through one bulb travels through the others.

  Parallel Wiring

Two pipes in parallel can  be replaced by one.    The single pipe will have  less resistance than the  pipe with the least  resistance.

   Parallel Circuits

Voltage same across each bulb. Current same because resistance of each bulb is the same.

  House Wiring is Parallel 

Which connecting wire, A, B, C,  D, or E, will be the first to  become dangerously hot  if too  many appliances are turned on? How can overheating be prevented, even if all the appliances in the house are turned on?

House Wiring and Fuses

Appliances are connected in parallel.

When ribbon carries too much current,
it melts, interrupting the current.

   Power

Power = current x voltage Units: Watts = amperes x volts ------------------------------------------ Example: Voltage = 120 volts What is the planned current? Current = Power / voltage              = 100 / 120              = 0.825 amperes

Power Dissipated in a Resistor:  Three Ways to Calculate It         

P = I^2R             = (0.5)^2 100             = 25 watts

P = (V / R)^2 R             = V^2/ R             = (50)^2 /100             = 2500/100             = 25 watts

P = I^2 (V / I)            =  IV            = (0.50)50            = 25 watts