chapter 34 notes - the fiberglass bed coverssciencewithhoffman.weebly.com/uploads/5/9/7/6/... ·...
TRANSCRIPT
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Chapter 34 Notes
Electric Circuits
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Electric Current Intro• Voltage is an “electric pressure”
• That can produce a flow of charge, or current,
within a conductor
• The flow is restrained by the resistance it
encounters
• When the flow takes place along one direction,
it’s called direct current (DC)
• When it flows to and fro, it is called alternating
current (AC)
• The rate at which energy is transferred by electric
current is power
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Flow of Charge and Electric Current
• Charge flows through a conductor until there is a common potential, i.e. a common voltage.
• Voltage provides the “electric pressure” to move electrons between the 2 ends of a wire.
• Charges will only flow through a circuit if there is a voltage acrossthe circuit.
• Sources of continuous voltage
• A battery
• Electromagnetic induction part 1 part 2
• Chemical cells
• Solar cells
• Thermocouple
• Piezoelectric cell
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Current and Voltage Sources
• To maintain a continuous current there must be a voltage source (an “electric pump”) to provide a sustained potential difference.
• Current is the flow of electric charge.
• In solids, only electrons are free to move.
• In fluids, + and - ions may move as well (like the electrolyte in a car battery).
• Examples of energy suppliers (providing a steady flow) were discussed on the previous slide
• Electric current is measured in amperes (A)
• the flow of 1 coulomb of charge per second
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Current (I) and Voltage (V)
sources• An alternating potential difference of 120 volts is supplied to
each outlet in your home.
• Remember: Electric Potential (Volts) = Electric Potential Energy
(Joules)/Amount of Charge (Coulombs)
• 120 Joules of energy is supplied to each Coulomb of charge that is
made to flow in the circuit.
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Electric Resistance (R)• The amount of current depends on both voltage and
resistance. (Ohm’s Law)
• Resistance is a measure of how “easily” the electrons can flow
through a conductor.
• The resistance of a wire depends on:• The conductivity of the material its made from (how well it
conducts)
• Its thickness (thick have less resistance than thin)
• Its length (long wires have more resistance than short (optical vs coaxial
audio cables))
• And, its temperature.• The higher the temperature, the more resistance (why there’s fans in
our electronics or our phones get hot when playing games).
• Resistance can become zero at very low temp, near absolute zero
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Measuring Resistance
• Electric resistance is measured in Ohms (Ω ).
• Current in a circuit is directly proportional to the voltage impressed across the circuit, and is inversely proportional to the resistance of the circuit
• Ohm’s Law:
Current = Voltage/Resistance• Current is measured in Amperes (A).
• An older symbol for ampere is the “amp.”
• 1 ampere = 1 volt/ohm
• The greater the resistance, the less the current
• Resistors are used to regulate current inside electric devices, such as radios.
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Ohm’s Law Practice
• What is the resistance of an electric griddle that draws 12
amps of current when connected to a 120 V circuit?
• How much current is drawn by a lamp that has a resistance of
100 ohms when a voltage of 50 V is impressed across it?
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Fill in the blankVoltage (V) Current (A) Resistance (Ώ)
1.5
0.30
0.05
30
1.5
15
3.0
0.10
3.0
0.15
0.2
30
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Electrical Transmission
• DC, direct current, current in one direction (a battery)
• AC, alternating current, electrons move in both directions
• Electrical transmission using AC is more efficient than DC.
• High voltage electricity is more efficiently transmitted than low
voltage electricity.
• Easier to increase/decrease AC voltage using transformers than
DC voltage.
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Power
• The rate at which electric energy is converted into another
form is called electric power.
• Power = current x voltage
• P = I x V
• Power measured in Watts (W)
• A 60 watt device uses 60 joules of energy each second
• Easy to do calculations
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Calculating Electrical Energy
usage• The electric power rating (in watts) of an appliance is usually
listed on the appliance
• Use the power rating to calculate how much energy it uses for
the time that it is on
• Example
• A toaster has a power rating of 1200W. If it takes 1 minute to
toast a slice of bread, how much energy does the toaster use?
• Energy = power (watts) x time (seconds)
• 1200W x 60s
• 72,000 watt-seconds or joules
• This energy could run a 60W light bulb for more than 20 minutes
• 72,000J is a huge number and can scare consumers so electric
companies use something different
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Calculating Electrical Energy
Usage (cont)• Electric companies use the kilo-watt hour (kWh)
• Still calculated by multiplying power by time
• But power rating is expressed in kWh and time of usage in hours
• You can convert a power rating in watts to kilowatts by
dividing by 1000W/kilowatt
• The 1200W toaster uses 1.2 kW
• Measure time in hours
• The toaster takes 1 minute/60min = 0.017 hours
• Calculate electrical energy used as follows
• Energy = Power x time
• 1.2 kW x 0.017 h
• = 0.02 kWh
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Power examples
• How much power is used by a calculator that operates on 8V
and 0.1A? If it is used for one hour, how much energy does it
use?
• Will a 1200W hair dryer operate on a 120V line if the current
is limited to 15A by a fuse? Can two hair dryers operate on
this line?
• Assignment
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Electricity Formulas
Formulas: V = I R I = V/R R= V/I P = IV E = Pt
Variable Unit (abbreviation)
• Voltage (V) Volts (V)
• Current (I) Amps (A)
• Resistance (R) Ohms (Ω)
• Power (P) Watts (W)
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Electric Bills
• The electric company charges you for the energy you use each
month.
• If they used the standard unit, Joules, they would have to
charge you for millions of them. This would scare people.
• So they invented a larger unit of energy, the kilowatt-hour,
that is 3,600,000 times larger than a Joule.
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Kilowatt-hour
• Joules are watts x seconds. If you use a 100-W bulb for 30
hours (108,000 s) you use 10,800,000 Joules of energy.
• E = Pt = 100 x 108,000 = 10,800,000 J
• Kilowatt-hours are kilowatts x hours. If you use a 100-W bulb
(0.100 kW) for 30 hours you use 3 kilowatt-hours of energy.
• E = Pt = 0.1 x 30 = 3 kWh
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Power in appliances
61” Flat screen
2 hr 30 min/day
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Power in appliances
Blu Ray
2 hr 23 min/day
(length of
Skyfall)
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Power in appliances
Receiver for DTS
surround sound
2 hr 23 min/day
(length of Skyfall)
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Power in appliances Nintendo Wii
45 min/ day
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Power in appliances Microwave
1 min 30 sec/ day