Download Ohm`s Law and Resistance

Ohm's Law and Resistance
Resistance Resistance is the property of a component which restricts the flow of electric current. Energy is used up as the voltage across the component drives the current through it and this energy appears as heat in the component.
Resistance
Resistance is measured in ohms, the symbol for ohm is an omega Ω. 1 Ω is quite small for electronics so resistance is often given in k and M. 1 k = 1 000 1 M = 1 000 000 .
Resistors used in electronics can have a resistance as low as 0.1 Ω or as high as 10 MΩ.
Factors Affecting Resistance
Length ­ Resistance varies directly with length. The longer the conductor, the larger the resistance.
Cross­sectional area ­ Resistance varies inversely with area. The larger the cross­sectional area in the conductor, the lower the resistance.
Temperature ­ In most materials, resistance varies directly with temperature. An increase in temperature will result in an increase in resistance.
Material ­ The atomic structure of the materials themselves has a marked effect on its resistance. Copper, gold, aluminum have a very low resistance, whereas mercury and nichrome have a high resistance. Glass and rubber (insulators) have a very high resistance.
The resistance per unit of a particular material is known as the RESISTIVITY (ρ). A table of known resistivity values is found on page 650 of your textbook.
If you invert the resistivity (1/ρ) you will get the object's CONDUCTIVITY (σ).
At a constant temperature, the resistance can be calculated as
ρL
R = A
animation http://phet.colorado.edu/en/simulation/resistance­in­a­wire
Resistors connected in Series
(One after the other)
R1
R2
=
R
eq
When resistors are connected in series their combined resistance is equal to the individual resistances added together. For example if resistors R1 and R2 are connected in series their combined equivalent resistance, Req, is given by:
Req = R1 + R2
This can be extended for more resistors: Req = R1 + R2 + R3 + R4 + ...
Note that the combined resistance in series will always be greater than any of the individual resistances. Resistors connected in Parallel
Joint connection
When resistors are connected in parallel their combined resistance is less than any of the individual resistances. There is a R1
special equation for the combined resistance of two resistors R1 and R2:
=
R2
Req
1 1 1 R2 R
+ 1
=
+
=
R
R1 R2
R1 R2
eq
For more than two resistors connected in parallel a more difficult equation must be used. This adds up the reciprocal ("one over") of each resistance to give the reciprocal of the combined resistance, Req:
eq
­1
Req = 1 + 1 + ... + 1
R1 R2
Rn
You just found three resistors, 1Ω, 5Ω, and 4Ω. If you connected all three together in a row (series), find the equivalent resistance to these three resistors.
Suppose you connected two of the three resistors, 1Ω & 5Ω, in parallel. Find the equivalent resistance to these two resistors.
Suppose you connected the three resistors, 1Ω, 5Ω, and 4Ω in parallel, find the equivalent resistance to these three resistors.
Probably the most important mathematical relationship between voltage, current and resistance in electricity is something called “Ohm’s Law”. Ohm's Law states:
The potential difference between any two points in a conductor varies directly as the current between the two points.
This relationship can be written as:
V = constant
I
Since the constant depends on the properties of the particular resistor being used, we give it the symbol R and call it resistance. Therefore, Ohm’s law can be written as:
V = IR
Also rearranged as:
I = V
R
V
R = I
where: V = voltage in volts (V)
I = current in amps (A) R = resistance in ohms (Ω)
animation
http://phet.colorado.edu/en/simulation/ohms­law
3 V is applied across a 6 Ω resistor, what is the current?
A 1.2 kΩ resistor passes a current of 0.2 A, what is the voltage across it?
You just found three resistors, 1Ω, 5Ω, and 4Ω. If you connected all three together in a row (series), then connect them to a 9 V battery, what is the current flowing through the circuit?
A common house hold resistor is a toaster. The toaster is plugged into the wall. Current flows from the voltage source (wall receptacle 120 V) into the toaster. The current encounters resistance in the toaster causing light and heat to be emitted ultimately toasting the bread.
Try the practice problems
Attachments
ohms­law.swf
resistance­in­a­wire.swf