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Chapter 34 Notes
Electric Circuits
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
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
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
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.
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
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.
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?
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
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.
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
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
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
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
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)
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.
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
Power in appliances
61” Flat screen
2 hr 30 min/day
Power in appliances
Blu Ray
2 hr 23 min/day
(length of
Skyfall)
Power in appliances
Receiver for DTS
surround sound
2 hr 23 min/day
(length of Skyfall)
Power in appliances Nintendo Wii
45 min/ day
Power in appliances Microwave
1 min 30 sec/ day