Electricity 6.notebook
February 26, 2013
What is Resistance?
Resistance & Ohm's Law
Resistance is the property of any material that slows down the flow of electrons and converts electrical energy into other forms of energy.
Example: Light Bulb Filament (heat & light)
(Think about cars on a highway that meet road construction.)
Feb 24­8:56 PM
Feb 24­8:59 PM
Ohm's Law
Ohm
Georg Ohm was a german physicist who studied the relationship between voltage and current, discovering that there was another factor involved.
Through measuring the amount of current that a given voltage produces, Ohm was able to calculate the circuit's resistance. Electrical resistance is the ratio of the voltage to the current. The unit of measure for electrical resistance is the ohm.
The mathematical relationship comparing voltage (V), current (I), and resistance (R) is known as Ohm's law.
R= V/I
or
V= IR
Thus Ohm's law can be used to calculate resistance and voltage.
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Feb 24­8:59 PM
Examples
Current ­ I
Voltage­ V
Resistance­ R
What is the resistance of a bulb if there is a current of 0.75 A connected to a 3 volt battery?
V
I
R
Note: A vertical line means multiplication and a horizontal line means division.
Feb 28­4:53 PM
Calculate current of a circuit connected to a 5 ohm resistor running off a 5 volt battery
Mar 2­7:54 AM
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Electricity 6.notebook
February 26, 2013
The Resistor
Now lets try some questions!
Page 273: Practice problems 1,2,3
Page 274: Practice problems 1,2,3
As we know, any electrical component that has electrical resistance slows down current and transforms electrical energy into other forms of energy.
Resistor­ an electrical component that has a specific resistance.
Resistors can be used to control current or voltage in a circuit to provide the correct voltage and current to specific components.
The symbol for a resistor.
Feb 24­8:59 PM
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resistors are marked with colored bands
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these indicate the resistance of the resistor
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see page 277 for color coding
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The Four Main Factors Affecting Resistance Levels:
(1) Length: the longer the wire the more resistance
(2) Diameter: narrower the wire the more resistance
(3) Type:the material determines the resistance (Ex. copper has a low resistance)
(4) Temperature: the higher the temp of the wire the less resistance (more voltage = less resistance
Mar 2­8:00 AM
Feb 24­8:59 PM
Series Circuits
Series & Parallel Circuits
When a light bulb in your home burns out or removed, the other lights in your home stay on.
Why?
When a light is taken out from a xmas light stand the rest may go out, why?
What kind of circuits are in each?
Feb 28­2:16 PM
In a series circuit there is only one path for electric current to travel.
The current is the same in each part of a series circuit (see pg 289).
Each load in a series circuit uses a portion of the same source voltage (see pg 289).
The more resistors added to the circuit, the greater the resistance.
Thus if you have light bulbs in series and one goes out they all go out
Feb 24­9:00 PM
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Electricity 6.notebook
February 26, 2013
Parallel Circuits
In a parallel circuit there is more then one path for electric current to travel.
Draw a series circuit with a 2 cell battery, a switch, a resistor and a voltmeter
The voltage traveling over each load is the same.
Current entering a parallel circuit must divide among all possible paths (see pg 292).
The current in each path depends on the resistance of that path (see pg 292).
When you connect resistors in parallel, the total resistance decreases.
Light bulbs in parallel mean if one goes out the others do not
Mar 2­8:23 AM
Draw a parallel circuit with one 3 cell battery, two bulbs and two switches in parallel
Mar 2­8:24 AM
Feb 24­9:00 PM
Differences between series and parallel circuits
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see table 9­1 on page 294
Mar 2­11:08 AM
Readings:
Pages 260 ­ 277
Questions:
Page 277 #'s 1,2,3,4,5
Page 290 #'s 1,2,3,4
Page 293 #'s 1,2,3,6
Mar 2­7:58 AM
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