electricity
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TRANSCRIPT
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Electric Current, Voltage and Resistance
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Electric Circuit
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Schematic Symbols
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Diagram of Electric Circuit
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How is current produced?
• When a high potential is connected by a conductive material to a low potential.
– Flow of electrons is called electric current.
– Units: C/s = Amperes (Amps)
High Potential
Low Potential Equal
PotentialEqual Potential
t
qI
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• When the potentials are equal, the current stops flowing – To increase the potential of the electrons
an electron pump must convert (do work) another form of energy into electrical potential energy.
• Electron pump examples: – Voltaic or galvanic cell (dry cell)– Several cells connected (Battery)– Photovoltaic cell (solar cell)– Generator
High Potential
Low Potential
Electron Pump
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Remember: Electric Potential Energy- Two Unlike Charges
Higher Potential Energy
Lower Potential Energy
+
-
•To cause movement of a charge, there must be a potential difference.
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Two types of current
• Direct Current – (D.C.) Flow of electrons in only one direction
– Battery
• Alternating Current – (A.C.) Flow of electrons at first in one direction and then the other direction.
– Generator– In your House– 60 Hertz
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• Direct Current
• DC• Provided by
batteries
• Alternating Current
• AC• Provided by power
companies
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Electric Current:
• The flow of electric charges.
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Electric Current, I
I = q
t
• Rate
• Unit: Coulomb / sec = Ampere (A)
• Andre Ampere (1775-1836)
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http://media-2.web.britannica.com/eb-media/36/236-004-D4AA985F.gif
Conventional current has the direction that the (+) charges
would have in the circuit.
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Conventional Current• By tradition,
direction in which “positive charges” would flow.
• Direction is opposite of electron flow.
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Ammeter• Measures electric current.
• Must be placed in series.
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What affects current flow?
• Potential Difference – Voltage
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Closing the switch establishes a potential difference (voltage) and an electric field in the circuit.
• Electrons flow in a net direction away from the (-) terminal.
High PotentialLow
Potential
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Battery (Chemical Cell):
• A device that converts chemical energy to electricity.
• A battery provides a potential energy difference (voltage source).
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A Battery Provides Energy
Electric Circuit• The battery “pumps” positive charges from
low (-) to
high (+) potential.
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Voltaic Cell• Alessandro Volta (1800’s)• Battery
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Voltmeter
• Measures the voltage between two points in an electric circuit.
• Must be connected in parallel.
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Ohm’s Law (Georg Ohm, 1787-1854)
V = IR
• The voltage , V, across a resistor is proportional to the current, I, that flows through it.
• In general, resistance does not depend on the voltage.
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Ohmic Resistor
• A device that obeys Ohm’s Law, who’s resistance does not depend on the voltage.
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What affects current flow?
• Potential Difference – Voltage
• Resistance - slowing or reduction of the flow of current
– Resistance depends on the material, length of material, and the cross sectional area of the material through which the current is moving.
A
LR R is resistance, is resistivity,
L is length, A is the cross sectional area
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ResistanceResistance is the amount that a substance or load is opposing the flow of electrons
Symbol for resistance is R
Measured in ohms
Units are
R = V I
“how hard it is for electrons to flow”
Material
FOUR FACTORS AFFECTING THE RESISTANCE OF A WIRE
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Resistance
Resistance is the amount that a substance or load is opposing the flow of electrons
Symbol for resistance is R
Measured in ohms
Units are
R = V I
“how hard it is for electrons to flow”
Material
Cross-sectional Area
Length
Temperature
Gold is a better conductor than copper
has less resistance than
The thicker the wire, the less resistance it has.
FOUR FACTORS AFFECTING THE RESISTANCE OF A WIRE
more wire = more resistance
less wire = less resistance
Electronics work better in the cold because wires have more resistance when they are hot.
A 50 metre extension cord has more resistance than a 10 metre extension cord.
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Resistance
Resistance is the amount that a substance or load is opposing the flow of electrons
Symbol for resistance is R
Measured in ohms
Units are
R = V I
“how hard it is for electrons to flow”
Material
Cross-sectional Area
Length
Temperature
Gold is a better conductor than copper
has less resistance than
The thicker the wire, the less resistance it has.
FOUR FACTORS AFFECTING THE RESISTANCE OF A WIRE
more wire = more resistance
less wire = less resistance
Electronics work better in the cold because wires have more resistance when they are hot.
A 50 metre extension cord has more resistance than a 10 metre extension cord.
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Resistance
Resistance is the amount that a substance or load is opposing the flow of electrons
Symbol for resistance is R
Measured in ohms
Units are
R = V I
“how hard it is for electrons to flow”
Material
Cross-sectional Area
Length
Temperature
Gold is a better conductor than copper
has less resistance than
The thicker the wire, the less resistance it has.
FOUR FACTORS AFFECTING THE RESISTANCE OF A WIRE
more wire = more resistance
less wire = less resistance
Electronics work better in the cold because wires have more resistance when they are hot.
A 50 metre extension cord has more resistance than a 10 metre extension cord.
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Resistance
• Resistance of an object to the flow of electrical current.
• R= V / I
• Resistance equals the ratio of voltage to current.
• Unit: Ohm (Ω)
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Resistor
• An object that has a given resistance.
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Resistors use up Energy
Electric Circuit• A resistor uses up energy.
• When the current goes through the resistor it goes to a lower potential.
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What We Know...
Definition Units Symbol
Current The rate at which electrons move from one place to another
Ampere (A) I
Voltage (Potential Difference)
Measures the amount of energy given to electrons to move them
Volt (V) V
Resistance Degree to which a substance opposes the flow of electric current through it
Ohms (Ω) R
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Ohm’s Law Practice Problems1. Find the current through a 12-ohm resistive
circuit when 24 volts is applied.
2. Find the resistance of a circuit that draws 0.06 amperes with 12 volts applied.
3. Find the applied voltage of a circuit that draws 0.2 amperes through a 4800-ohm resistance.
4. Find the applied voltage of a telephone circuit that draws 0.017amperes through a resistance of 15,000 ohms.
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Ohm’s Law Practice Problems5. A 20-volt relay has a coil resistance of
200 ohms. How much current does it draw?
6. A series circuit has 1200-ohms of total resistance with 12 V as the power supply. What is the total current of this circuit?
7. A transformer is connected to 120 volts. Find the current if the resistance is 480-ohms?
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Voltage, Current, and Resistance
In Series and Parallel Circuits:
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Pearson Investigating Science 9 11.1
Potential Difference = Voltage
At each point in a circuit where the electrons need to be pushed, they use up their voltage.
By the time the electricity gets back to the battery, the voltage is all used up and is 0 V.
Voltage is a comparison between the pushing force of electrons between two points in a circuit.
Symbol for voltage is V
Measured in volts
Units are V
V = I R
“the pushing force”
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Voltage in Series and Parallel
• Series Circuits– Voltage decreases as electrons cross each
electrical load (and lose energy)– Voltage across the source equals the sum of the
voltages across each electrical load
– Vs = V1 + V2 + V3
• Parallel Circuits– If the loads are identical, the voltage is the same
everywhere
– Vs = V1 = V2 = V3
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Pearson Investigating Science 9 11.1
Current
In a series circuit the current is the same at every point in the circuit.
Current is the measure of the amount of charge moving past a point in a circuit every second.
Symbol for current is I
Measured in amperes
Units are A
I = V R
“the number of moving electrons”
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Current in Series and Parallel
• Series Circuits– Current is the same everywhere
– Is = I1 = I2 = I3
• Parallel Circuits– Current is not the same everywhere– Current entering or leaving the source equals the
sum of the current passing through each electrical load
– Is = I1 + I2 + I3
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Series Circuits
• Only one path for current to follow• User followed by another user (ie.
Resistor followed by a light)
...321 VVVVequ
…
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Series Circuit Rules
• Current is the same for each user
• The effective resistance is the sum of all resistors in the series
–
• The sum of the voltage drop is equal to the total voltage drop
–
...321 RRRRequ
...321 VVVVequ
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Parallel Circuits
• Two or more paths for current to follow
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Parallel Circuit Rules
• Total current in the circuit is the sum of the current in all its paths (branches)
–
• The equivalent resistance decreases with more parallel resistors
–
• Voltage is the same in each path.
...321 IIIItot
...1111
321
RRRRequ
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Shopping Mall Analogy – SeriesAs a class, we are going to the mall on a field trip.
Each student (electron) starts at the ATM (source) to take out $3 (voltage – 3V).
One store (load): the whole class (current) goes to 1 store and each student spends their $3. Everyone wants to spend all of their money because it’s a field trip! Current stays the same, voltage is the same as the source
Two stores (loads): the whole class goes to each store, spending some of their money at each. Everyone wants to spend all of their money, but also wants to buy
something at each store! Current is the same, voltage of loads will add up to V of source
Everyone meets back at the ATM because they’re broke (V = 0) and have no more money to spend. This is the end of the field trip!
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Shopping Mall Analogy – ParallelAs a class, we are going to the mall on a field trip.
Each student (electron) starts at the ATM (source) to take out $3 (voltage – 3V).
One store (load): same as before! Current stays the same, voltage is the same as the source
Two stores (loads): half of the class (current) goes to one store spending their $3, while the other half goes to the other store, spending their $3. Everyone wants to spend all of their money because it’s a field
trip! Current of loads adds up to A at source, voltage is the same
Everyone meets back at the ATM because they’re broke and have no more money to spend. This is the end of the field trip!
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Resistance in Series and Parallel
• Series Circuits– Equivalent (total) resistance equals the sum of the
individual electrical loads
– RE = R1 + R2 + R3
• Parallel Circuits– Equivalent (total) resistance of the circuit is smaller
than the smallest resistance
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