Electricity

ChargeDefn: Property of matter that determines how it will interact with other matter via the electrostatic force; opposite charges attract, like repel

All atoms are composed of subatomicparticles that contain charge; equal amounts of + and - results in neutrality

Designation of positive and negative iscompletely arbitrary

If like charges repel and unlike attract, what holds protons andneutrons together?

Electrostatic Force

• The force of repulsion dependsupon the size of the chargesand the distance between them

• Force is linear in charge• Force is proportional to the inverse square of

the distance

k Q1 Q2

r2F =

Metals

• For current, need charged particles that are readily given up by atom

• Protons are tightly bound in nucleus• Metals are atoms

that have outerelectrons that are not tightlybound

Current

• Current – time rate of flow of net charge particles past a given point

• In order to have a current, needa) source of charged particlesb) a conducting path

Electric Potential

• An electric field is created by any separation of charges

• Needed to accelerate/move electrons• Electrical potential energy related to electric

field• E.P.E. = Q E d (Note: G.P.E. = mgh)• Electric potential is V = E d• E.P.E. = V d

Potential Sources

• Batteries – chemical storage unit; metals in acid or base

• Fuel Cell – similar to battery; chemicals react with no metals necessary

• Generators – electromagnetic induction; kinetic energy to electricity

• Photovoltaic Cells – direct creation of electricity from sunlight (later chapter)

Battery

• Two dissimilar metals in an acid or base• Negative and positive ions

are able to migrate in solution• With a completed circuit,

negative ion goes to negative terminal, releasing election

• At positive terminal, electron combines with positive ion

Fuel Cell

• Similar to battery in that chemicals are reacting to produce current

• Reactants are fed on demandinto fuel cell

• Exchange of ions across semi-permeable membraneallows electrons to complete circuit

• Weight is still major issue

Conducting Path

• For current, need path that will conduct electricity; metals best for this in most cases

• Except for superconductors, some resistance to current will occur

• Amount of resistance depends on a) length,b) cross sectional area, and c) resistivity

r LAR =

Ohm’s Law

• The amount of current flowing through a device depends on the potential and resistance

• V = I R• As resistance increases,

the amount of currentdecreases, and vice versa

Circuits

• Two ways to wire a circuit if more than one element

• Parallel – each element directly wired to source; more current needed as add new elements; 1/R = 1/R1 + 1/R2 + …

• Series – same current flows through each element; current drops as add new ones, as resistance increases; R = R1 + R2 + …

Moving Charges

Oersted (1820) discovered that a currentcreates a magnetic field

This means a current has a force on a magnet

Newton’s Third Law means that amagnet has a force on a moving charge

Direction of force is perpendicular tovelocity and magnetic field

Faraday’s Induction Law

• Faraday (1831) noticed that electric potential could by induced by changing magnetic field

• If potential induced in closed loop, get a current

• Size of potential depends upon rate at which magnetic field changes

Generator

Magnet

Electric generator: rotate coils of wire perpendicularto magnetic field

Layers of wire coils

Front view

How do you make electricity?

Need energy source to do the work of turning the magnet, causing a flow of current (electricity) in

the wire coils

Convert the potential energy in something to kinetic energy to turn

magnet

Water behind a dam

Options

Chemical energy in biomass or fossil fuels

How do you make electricity?

Most electricity formed by steam turbine electric generators

Electrical Power

P = I V

We know that P = DE/Dt from the definition of power

The amount of energy delivered by a current is equal to the amount of charge delivered times the voltage,

DE = Dq V P = (Dq V )/ Dt

But

I = Dq/Dt

Power Lost

• Resistance of wire between source and appliance causes loss

• Since the voltage in a resistor is given by Ohm’s Law (V = IR), the power lost is given by

Plost = I V = I (IR) = I2 R

• This loss accounts for about 10% of all electricity in the U.S.

Transformers

• Energy loss in lines is decreased by sending electricity at high voltage/low current over long distances

• Near homes, the voltage must be stepped down to prevent lethal accidents

• Transformer use electromagnetic induction to do this