# 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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 views: 15 posted: 11/7/2011 language: English pages: 5