electricity
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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Electricity
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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?
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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 =
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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
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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
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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
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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)
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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
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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
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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 =
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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
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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 + …
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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
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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
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Generator
Magnet
Electric generator: rotate coils of wire perpendicularto magnetic field
Layers of wire coils
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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
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How do you make electricity?
Most electricity formed by steam turbine electric generators
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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 )/ DtBut
I = Dq/Dt
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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.
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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