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5 – 19 Do Now Problems1. Define electric current and give the associated units
2. What is the formula associated with Ohm’s law?
3. Differentiate between the terms voltage source and potential difference
Homework Read & outline 34.5 – 34.11
Water vs. Electric Current
There are many analogous properties between water and electric current
Characteristics such as
•Pressure•Volume
•Flow
PressureIn an electrical system pressure is measured in
A higher voltage pushes the charges through the wire with greater force
In a water system we
measure water
pressure infoot lbs or PSI (pounds per square in)
Volts
Pressure
How a water tower works:1. Pump station2. Reservoir3. Water user
Voltage• Voltage is a measure of electric
potential energy, just like height is a measure of gravitational potential energy.
• Voltage is measured in volts (V). Using a voltmeter
• A voltage difference of 1 volt means 1 amp of current does 1 joule of work in 1 second.
Voltage
• Since 1 joule per second is a watt (power), you can interpret voltage as
• measuring the available electrical power per amp of current that flows.
Voltage Source
A pump is a source of fluid pressure
difference
A battery is a source of electrical
potential difference
Voltage & Batteries• The positive end of a 1.5 volt
battery is 1.5 volts higher than the negative end.
• If you connect batteries positive-to-negative, each battery adds 1.5 volts to the total.
• Three batteries make 4.5 volts.
• Each unit of current coming out of the positive end of the three-battery stack has 4.5 joules of energy.
Voltage source
Charges do NOT flow unless there is potential difference
The battery or source is represented by an escalator which raises charges to a higher level of energy.
A voltage source is needed to provide a sustained potential difference i.e. batteries or generators
VOLTAGE causes current
VOLTAGE causes current
Current and voltage• A battery uses chemical energy to create a
voltage difference between its two terminals.
• In a battery, chemical reactions provide the energy to pump the current from low voltage to high voltage.
• A fully charged battery adds energy proportional to its voltage.
Volume
In our water system volume is in
The Electrical equivalent is a
Coulomb is a unit of charge, consists of
6.3 x 1018 electrons (a big number indeed)
cubic feet (ft3) or liters Coulomb
Electric Charge
• A building up at the negative terminal of a cell/battery which then flows from this terminal as negative charges repel one another.
Flow• Fluid flowing through a system of pipes can
behave in many respects like electricity flowing in a circuit
• The main difference is that water will fill any space whereas electricity will 'fill' only a conductor
Flow• Charge can only flow when there is a
potential difference or a difference in voltage across the ends of a conductor
Current is a flow of charge
Current• Electric current is measured in units called
amperes, or amps (A) for short.
• One amp is a flow of a certain quantity of electricity in one second.
• The amount of electric current entering a circuit always equals the amount exiting the circuit.
Electric Current
What is need for current to flow?
• Electrons have a charge of –1.610-19 Coulombs
– one amp = 61018 electrons per second
By convention, electrical current flows from high voltage to low voltage
In other words, a positive current flows from higher to lower electrical potential
NO POTENTIAL DIFFERENCE = NO FLOW OF CURRENT
Recap• Relate water pressure and electric pressure.
• Units for electric pressure?
• Relate water volume to electric volume.
• Units for electric volume?
• Relate water flow and electric flow.
• Units for electric flow?
• Condition needed for electric flow.
5 – Do Now Problems
1. What condition is necessary for the sustained flow of charge in a conductor?
2. Differentiate between AC and DC.
3. What is voltage?
HW. Pg. 545 - 546 Review questions
How does current move through a circuit?
Conventional current
• Conventional current was proposed by Ben Franklin in the 1700’s.
• Scientists later discovered that the particles that carry electricity in a wire actually travel from negative to positive.
• Today, we still use Franklin’s definition.
Current Getting it Started
A potential difference is established between two points and some charges are released
The positive terminal of a battery will attract electrons charges
They will be acted on by the electrical force and start to move
Voltage source
Charges do NOT flow unless there is potential difference
The battery or source is represented by an escalator which raises charges to a higher level of energy.
A voltage source is needed to provide a sustained potential difference i.e. batteries or generators
VOLTAGE causes current
VOLTAGE causes current
Current & flow of chargesElectrical circuits, consist of current in the wire moving from higher to lower electrical potential
Current Electricity Example• The continuous flow of charge
in a complete circuit.
What does a battery do?
• A battery uses chemical energy to move charges.
• If you connect a circuit with a battery the charges flow out of the battery carrying energy.
Effects of Electric Current on the Body
Shock examples
Electrical Resistance and Ohm’s LawKey Question:
How are voltage, current, and resistance related?
Electrical resistance
• Resistance measures how difficult it is for current to flow.
Electrical Resistance• The total amount of electrical resistance in a circuit
determines the amount of current that in the circuit for a given voltage.
• The more resistance the circuit has, the less current that flows.
The ohm• Resistance is measured in ().
• One ohm is the resistance when a voltage of 1 volt is applied with a current of 1 amp.
Ohm’s Law The formulae
The resistance of electrical devices
• The resistance of electrical devices ranges from very small (0.001 Ω) to very large (10×106 Ω).
• Each device is designed with a resistance that allows the right amount of current to flow when connected to the voltage the device was designed for.
Ohm Quiz
Ohm plate
Ohm on the range
Broken Ohm
Ice cream Ohm
Ohm sweet Ohm Ohm Alone
Practice Quiz
1. If the resistance of your body were 100,000 ohms, what would be the current in your body when you touched the terminals of a 12 volt battery?
12 V / 100000 = = 0.00012 A
2. If your skin were very moist so that your resistance was only 1000 ohms, and you touched the terminals of a 24 volt battery, how much current would you draw?
= 24 V / 1000 = 0.024 ASee
slide 18
More Practice ProblemsI = V/R
I = 110 V / 2200 Ώ
I = .05 A
R = V/I
R = 110 V/ .5A
R = 220 Ώ
V = IR
V = 1.2 A (100 Ώ)
V = 120 V
Electric Resistance1. Length
as the electrons will have more distance to move and will therefore be bumping around more.
resistance varies with the length of a conductor
Ex: The length of a conductor is similar to the length of a hallway. A shorter hallway would allow people to move through at a higher rate than a longer one.
2. Cross-sectional area
resistance varies inversely with the cross sectional area
If the area doubles then the resistance is halved.
The wider the pipe, the easier it is for water to flow through…………………………………………………
Type of material3. Type of material
good conductors have low resistance and many free electrons
electrical conductance is measured in siemens (S). the inverse of the resistance
Temperature4. Temperature
most materials experience an increase in resistance as their temperature increases
experience a decrease in resistance as their temperature decreases
due to the kinetic theory of matter…………………………………………………
…………………………………………………
Electric Power
Power = Volts x Amps
The work done by an electric current moving through a circuit is given by
1 watt = 1 volt x 1 amp
W = V I t
Power EquationsPower (P)
Rate at which work is performedMeasured in watts (W)
•Power• P = V I P = (I R) I = I 2 R
• Decided to power homes with DC power– DC means direct current: just like what batteries
deliver
Getting Power to Our HomesThomas Edison's GE company held the patents on the DC generators being used in the early days
• Do we want power plants close to our home?
• Probably NOT…so we need to have– an ability to “ship” electricity across states
• So power lines are long– Therefore resistance no longer negligible
Getting Power to Our Homes
Power Dissipated in an Electricity Distribution System
120 WattLight bulb
12 Volt Connection Box
Power Plant on Colorado River
150 miles
• Estimate resistance of power lines: say 0.001 Ohms per meter, times 200 km = 0.001 W/m 2105 m
= 20 Ohms• We can figure out the current required by a single bulb
using P = VI so I = P/V
=120 Watts/12 Volts
= 10 Amps
Power Dissipated in an Electricity Distribution System
120 WattLight bulb
12 Volt Connection Box
Power Plant on Colorado River
150 miles
• Power in transmission line is P = I 2R = 102 20 = 2,000 Watts!!
• “Efficiency” is έ = 120 Watts/4120 Watts = 0.3%
The Tradeoff• The big problem is the high current
through the (fixed resistance) transmission lines
• Need less current• The square in I 2R that has the most effect
• Our appliances needs a certain amount of power
• I = P /V less current means higher voltage
The Solution• The Solution is high voltage transmission• I = 120 Watts/12 kV = 0.01 Amps for one
bulb, giving P = I 2R = (0.01)220 = 2010-4 Watts, so
P = 0.002 Watts dissipated in transmission line• Efficiency in this case is • έ = 120 Watts/120.004 = 99.996%
The Tradeoff
• But having high voltage in each household is a recipe for disaster– sparks every time you plug something in– risk of fire– not cat-friendly
• We need a way to step-up/step-down voltage at will– We can’t do this with DC, so go to AC
Basic Terminology• Direct Current (DC) – charge
flows in one direction.– Batteries– Solar Panels
Basic Terminology• Alternating Current (AC)
– Magnitude & direction of current flow periodically change
– Each sequence called a cycle– Frequency is cycles per second
(Hz)
AC Circuits
• An AC voltage source reverses the positive and negative terminals many times per second.
• The current flows one direction and then the other then the other…..
• This changing of polarity or cycling is called frequency, measured in cycles per second (hertz).
Why AC?• AC is like a battery where the terminals
exchange sign periodically!
• approx. 60 cycles per sec in household current
• AC sloshes back and forth in the wires
• Although net electron flow over one cycle is zero, useful work can still be done!
• Imagine sawing (back & forth), or rubbing hands together to generate heat
DC (direct current) and AC (alternating current)
I
Batteries produce DC Household/power plant: AC
DC vs AC
DC•Electrons flow constantly•Electrons flow in only one direction•Batteries
AC•Electrons flow in short burst•Electrons switch directions (60 times a second)•House current
AC
Nikola Tesla invented & George Westinghouse introduced his system based on high-voltage alternating current (AC), which could carry electricity hundreds of miles with little loss of power
RESISTOR COLOR CODES
The resistor code is a color code. Around each resistor are colored stripes. Each color stands for a number. You can tell how much resistance by looking at the numbers.
•Gold or silver tells you how accurate the resistor is
•gold is (+) Or (–) 5% silver is (+) Or (–) 10%
Black = 0 Yellow = 4 Gray = 8
Brown = 1 Green = 5 White = 9
Red = 2 Blue = 6
Orange = 3 Violet = 7
RESISTOR COLOR CODESThis is how to crack the code.
Write the numbers for the first two colors. The third color tells you how many zeros to add.
Resistance is measured in ohms.
This resistor has a resistance of…..
1000 ohms
Resistors
• Electrical components called resistors can be used to control current.
• Resistors have striped color codes to record their "values" (writing on them is difficult).
Resistors
• Used to control current or potential
difference in
a circuit.
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