Lecture 2 Resistance Ohm’s Law Electric Energy Electric Power Efficiency .
CHAPTER 8: Ohm’s law describes the Relationship of current, voltage, and resistance. UNIT 3:...
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Transcript of CHAPTER 8: Ohm’s law describes the Relationship of current, voltage, and resistance. UNIT 3:...
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CHAPTER 8: Ohm’s law describes the Relationship of
current, voltage, and resistance.
UNIT 3: Electricity
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ENERGY (E) – the ability to do workENERGY (E) – the ability to do work
There are many forms of Energy
kinetic, mechanical,
chemical, solar...
RECALL:
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KINETIC ENERGYKINETIC ENERGY POTENTIAL ENERGYPOTENTIAL ENERGY
Energy of motion stored Energyex: spring, elastic
band
Types of Energy
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When + and – charges are pulled apart there are attractive forces wanting to put them back together, this is a form of potential energy as it has the ability to do work!
Electric Potential Energy and Voltage
Section 8.1 page 250
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Stored electrical energy such as that stored in a battery
Electric Potential Energy
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Electrochemical Cell (Battery)
Converts:Chemical Energy
Electrical Energy+ and – charges are
separated into terminals located at either end of the cell, when connected – charges (electrons) travel towards the + (positive) terminal as the – charges repel one another
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Batteries have electric potential Energy because the stored e-’s have the ability to do work, i.e. run your CD player!
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An Electrochemical Cell Requires:
different electrodes (usually metals also Carbon)
And an electrolyte (electrolytic solution)
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Change in potential energy per coulomb of charge
Measured in volts (V) by a voltmeter
Electric Potential Difference (Voltage)
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Analogy!
Stairs = the voltage
Backpack = amount of charge separated
More work has been done in B, therefore it has a greater Potential Energy!
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Electric CircuitElectric Circuit
A complete pathway that allows electrons to flow
Transforms electrical energy into other forms of Energy
Interactive website
Section 8.2 Electric Current page 260
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Electric Circuit Analogy
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Parts of an Electric Circuit
Conductor – wire through which current flows
Load – converts electrical E into other forms of E
Switch – turn circuit on or off
Source – source of electrical E
Interactive Website on Switches
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Circuit Diagram Symbols
Interactive Website on Circuit Diagrams
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SampleCircuit Diagra
m
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THE CONTINUOUS FLOW OF CHARGE (ELECTRONS) IN A
COMPLETE CIRCUIT
Current Electricity
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•the amount of charge passing a given point in a conductor per second
•measured in amperes (A) by an ammeter or (mA) by a galvanometer
Electric Current
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Electrical Resistance:Electrical Resistance:Slows down the
flow of electrons and transforms electrical E into other forms of E
Electrical E
Light
Heat
Sound
Motion
etc
Resistance & Ohm’s Law Section 8.3 page 270
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• Measured in ohms (Ω)
•Equal to the ratio of the voltage to the current
R = V/I
RESISTANCE
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Current Dependent on Potential (voltage)Current Dependent on Potential (voltage)
Current Dependent on ResistanceCurrent Dependent on Resistance
Resistance and Current Analogy:
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Factors Affecting
the Amount
of Resistance
in a wire
include:
1. Length
2. Diameter (thickness)
3. Type of wire
4. Temperature
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Resistance in a Light BulbLong, Thin Tungsten
wire
Short, Thick copper wire
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The Mathematical Relationship comparing
voltage (V), current (I) and resistance (R)
V = IR
Interactive website Simulation
Ohm’s Law
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Ohm’s Law – The VIR Tree
V
I R
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Various Forms of Ohm’s Law
V=IR
I = V R
R = V I
Where:
V = voltage measured in volts (V)
I = current measured in amperes (A)
R = resistance measured in ohms (Ω)
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STEP 1: Read and reread the question. Step up variables with known information.
V = ?I= 0.5AR= 2Ω
Use a question mark for the unknown value you are trying to find. Be sure to include units (V,A,Ω)
Sample Problem: What voltage is used to run a flashlight with
a 2 Ω resistor at a current of 0.5 A?
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STEP 3: Convert units if necessary, measurements must be in V for voltage, A for current and Ω for
resistance. Fill in the information given in the question (including
units).
STEP 2: Determine the form of ohm’s law needed to solve for the unknown in the
question.
V = IR
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V =IRV = (0.5A)(2.0Ω)
V= 1.0 V
The flashlight would require a voltage of 1.0 V.
STEP 4: Multiple or divide to calculate the unknown as per the formula chosen
in step 3.Be sure to include units in your answer,
round digits if necessary.
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Used to control CURRENT or potential difference
(VOLTAGE) in a circuit
Resistor
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RESISTANCE AND OHM’S LAWPAGE 278-9
Core lab Activity: