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Electricity

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.

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Electricity

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  1. Electricity

  2. 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 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?

  3. 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 F = r2

  4. Metals • For current, need charged particles that are readily given up by atom • Protons are tightly bound in nucleus • Metals are atomsthat have outerelectrons that are not tightlybound

  5. 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

  6. 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

  7. 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)

  8. Battery • Two dissimilar metals in an acid or base • Negative and positive ionsare 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

  9. 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

  10. 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 L R= A

  11. 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

  12. 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 + …

  13. Moving Charges Oersted (1820) discovered that a currentcreates a magnetic fieldThis means a current has a force on a magnet Newton’s Third Law means that amagnet has a force on a moving chargeDirection of force is perpendicular tovelocity and magnetic field

  14. 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

  15. Generator Electric generator: rotate coils of wire perpendicularto magnetic field Magnet Layers of wire coils

  16. Options Water behind a dam Chemical energy in biomass or fossil fuels 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 Front view Convert the potential energy in something to kinetic energy to turn magnet

  17. How do you make electricity? Most electricity formed by steam turbine electric generators

  18. Electrical Power 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 )/ DtBut I = Dq/Dt P = I V

  19. 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 byPlost= I V = I (IR) =I2 R • This loss accounts for about 10% of all electricity in the U.S.

  20. 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

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