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Physical Science Chapter 20

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					Physical Science: Chapter 20. Electricity
GA Performance Standards
SPS10. Students will investigate the properties of electricity and magnetism.
   a. Investigate static electricity in terms of
             Friction
             Induction
             conduction
   b. Explain the flow of electrons in terms of
             Alternating and direct current
             The relationship among voltage, resistance, and current
             Simple series and parallel circuits

20.1 Electric Charge and Static Electricity
A. Electric Charge
     Electrical energy is the energy associated with electric charges
     Electric charge is a property that causes subatomic particles such as protons and electrons to attract or
        repel each other
     Protons have a positive charge and electrons have a negative charge and electrons have a negative charge
     Electric charges attract one another in clothes taken from the dryer
     An atom has a cloud of negatively charged electrons surrounding the positively charged nucleus.
     The atom is neutral sense it has equal numbers of protons and neutrons
     If an atom loose an electron it becomes positively charged
     An excess or shortage of electrons produces a net electric charge
     SI unit of electric charge is the coulomb [C] and it takes about 6.24 x 1018 electrons to produce a single
        coulomb
B. Electric Forces
     Like charges repel, and opposite charges attract.




    
      Electric force is the force of attraction or repulsion between electrically charged objects
      Charles-Augustin de Coulomb discovered that electric force between two objects is proportional to the net
       charge on each object, and inversely proportional to the square of the distance between them
    Example: doubling the charge on one object doubles the electric force
    Example: doubling the distance between the objects the electric force is one fourth as strong
    Inside an atom electric forces are stronger then gravitational force, but on large scale matter is mostly
       neutral and electric forces are close to zero.
C. Electric Fields
    Electric field is a field in a region of space that exerts electric forces on charged particles
    the electric field is produce by electric charges or by changes magnetic fields
       the strength of an electric field depends on the amount of charge that produces the field and on the
        distance from the field.
     The more net charge an object has the greater is the force on it
     The electric field around a positive charge points outward
     The electric field around a negative charge points inward
D. Static Electricity and Charging
     Two types of electricity: Static Electricity and Electric Circuit
     Static electricity = sock sticking on your pants.
     Electric circuit= wall outlet.
     Static electricity is the study of the behavior of electric charges, including how charg is transferred
        between objects.
     Charge can be transferred by friction, by contact, and by induction
     The law of conservation of charge states that the total charge in an isolated system is constant
     When a charge is transferred the total charge is the same before and after the transfer occurs
        1. Charging by friction
             o Rubbing a balloon on your hair is an example of charging by friction
             o Electrons move from your hair to the balloon because atoms in rubber have a greater attraction
                 for elections
             o The balloon picks up a net negative charge and your hair becomes positively charged
        2. Charging by Contact
             o A Van de Graff generator has charged a metal sphere,
                 when you touch the sphere you can acquire enough
                 charge to make your hair stand on end
             o The sphere is still charged but its net charge is reduced.
        3. Charging by induction
             o Induction is the transfer of charge without contact
                 between materials
             o Example: when a positive charged object is placed near a
                 neutral object the electrons move closer to the charged
                 object. The net charge is still neutral, but charge has moved within it




E. Static Discharge
    Static discharge occurs when a pathway through which charges can move forms suddenly
    Air can become charged suddenly when the gap between your fingers and a doorknob becomes very small
    Lighting is an example of discharge
    Charge can build up in storm clouds from friction between moving air masses
    Negative charge in lower part of the cloud induces a positive charge in the ground.
    Eventually the air becomes charged forming a pathway for electrons to travel from the cloud to the
        ground
20.2 Electric Current and Ohm’s Law
A. Electric Current
     Electric current is the continuous flow of electric charge
     The SI unit of electric current is the ampere (A). 1 A = 1 C/s
     two types of current are direct current and alternating current
     direct current (DC) is where the charge flows only in one direction
     Alternating current (AC) is where the flow of electric charge regularly reverses its direction.
     Scientists define current as the direction in which positive charges would flow
B. Conductors and Insulators
     Electrical conductor is a material though which charge can flow easily
     Electrical insulator is a mater through which charge cannot flow easily
     Metals such as copper and silver are good electrical conductors. Wood, plastic, rubbers are good
        electrical insulators.
C. Resistance
     Resistance is opposition to the flow of charges in a material
     The SI unit of resistance is the ohm
     A material’s thickness, length, and temperature affect its resistance
     Resistance is greater in a longer wire because the charges have to travel farther
     As temperature increases the resistance increases because electrons collide more often
     As the thickness increases the resistance decrease because the electrons can flow more easily
     Superconductor is a material that has almost zero resistance when it is cooled to low temperatures
D. Voltage
     In order for charge to flow in a conducting wire, the wire must be connected in a complete loop that
        includes a source of electrical energy
             1. Potential difference
                 Charges flow spontaneously from a higher to lower potential energy
                 Potential energy of a charge depends on its position in an electric field
                 Potential difference is the difference in electrical potential energy between two places in an
                     electric field
                 Potential difference is measured in joules/coulomb and equals 1 volt
                 Potential difference is commonly called voltage
             2. Voltage sources
                 Three common voltage sources are batteries, solar cells and generators
                 Battery is a device that converts chemical energy to electrical energy
                 Voltage sources have terminals that connect to wires in a circuit
                 One thermal is positive and the other is negative
                 The voltage drop (potential difference) is maintained across the terminals
                 Example 9-volt battery has potential difference of 9 volts
E. Ohm’s Law
     The unit of resistance it the ohm
     The voltage is not the same everywhere in a circuit.
     George Ohm hypothesized that resistance reduces the voltage
     Ohm’s Law is that the voltage (V) in a circuit equals the product of the current(I) and the resistance (R)
     V=I x R
     Current is measured in amperes, resistance in ohms, and voltage in volts
       Increasing the voltage increases the current. Keeping the same voltage and increasing the
        resistance decreases the current

20.3 Electric Circuits
A. Circuit Diagrams
     Electric circuit is a complete path through which charge can flow.
     Electrician uses circuit diagrams to keep track of how elements in a circuit are connected
     Circuit diagrams use symbols to represent part of a circuit, including a source of electrical energy
        and devices that are run by the electrical energy
     A circuit diagram shows on or more complete paths in which charge can flow
     Switches show places where the circuit can be opened
     If a switch is open the circuit is not complete loop and the current stops
     If the switch is closed the circuit is complete and charge can flow (called a closed circuit)




   
    The + and – on the battery symbol indicate the positive and negative terminals
    Arrows show the direction of current, from positive to negative
    Direction of current is defined as the direction in which positive charges would flow
B. Series Circuits
    Series circuit is where charge has only one path through which it can flow
    If one elements stops functioning in a series circuit, non of the elements can operate




    
      Adding bulbs to a series circuit increases the resistance and decreases current, resulting in each bulb
       shining less brightly
C. Parallel Circuits
    Parallel circuit is an electric circuit with two or more paths through which charges can flow
       If one element stops functioning in a parallel circuit, the rest of the elements still can operate.




    
D. Power and Energy Calculations
     Power is the rate of doing work
     Electric power is the rate at which electrical energy is converted to another from of energy
     Uinit of electric power is Joule/second or watt (W)
     Electric power can be calculated by multiplying voltage by current
     Power (in watts) = I (in amps) x V (in volts)
     To find electrical energy used by an appliance multiply power by time
     Energy = Power x Time
E. Electrical Safety
     Correct wiring, fuses, circuit breakers, insulation, and grounded plugs help make electrical energy
        safe to use.’
        1. Home Safety
             o Fuse prevents current overload in a circuit
             o A wire in the center of the fuse melts if too much current passes through it
             o The fuse must be replaced before the circuit can carry a current again
             o Circuit breaker is a switch that opens when current in a circuit is too high
             o The circuit breaker must be reset before the circuit can be used again
        2. Personal Safety
             o Insulation protect people and if it is damaged you may accidentally touch the bare wire and get a
                 shock
             o Insulation also prevent short circuits
             o Grounding is the transfer of excess charge through a conductor to Earth
             o

				
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posted:11/7/2011
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