Electricity

Charge Defn: 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 subatomic particles that contain charge; equal amounts of + and - results in neutrality Designation of positive and negative is completely arbitrary If like charges repel and unlike attract, what holds protons and neutrons together?

Electrostatic Force The force of repulsion depends upon the size of the charges and the distance between them Force is linear in charge Force is proportional to the inverse square of the distance k Q 1 Q 2 r2r2 F =

Metals For current, need charged particles that are readily given up by atom Protons are tightly bound in nucleus Metals are atoms that have outer electrons that are not tightly bound

Current Current – time rate of flow of net charge particles past a given point In order to have a current, need a) source of charged particles b) 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 demand into fuel cell Exchange of ions across semi-permeable membrane allows 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  L A R =

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 current decreases, 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/R 1 + 1/R 2 + … Series – same current flows through each element; current drops as add new ones, as resistance increases; R = R 1 + R 2 + …

Moving Charges Oersted (1820) discovered that a current creates a magnetic field This means a current has a force on a magnet Newton’s Third Law means that a magnet has a force on a moving charge Direction of force is perpendicular to velocity 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 perpendicular to 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 =  E/  t from the definition of power The amount of energy delivered by a current is equal to the amount of charge delivered times the voltage,  E =  q V  P = (  q V )/  t But I  q/  t

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 P lost = I V = I (IR) = I 2 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