Transcript 4.3 Electrical Resistance Notes

Chapter 4.3
Notes
Resistance in
Electricity
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Charges can easily flow through
conductors because they contain
many free electrons.
Even conductors have resistance
though.
Some conductors can be cooled to
very low temperatures to reduce
the friction. These are called
superconductors.
In
insulators, electrons are
tightly bound and cannot
move freely and so they do
not allow charges to flow
easily.
Examples of insulators
include wood, plastic,
glass, and rubber.
Some
substances are
intermediate in their ability
to conduct charge and are
called semiconductors.
Insulators are used for
computer chips and
microprocessors.
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In wire, free electrons move
throughout the wire, but not in straight
lines.
The electrons continuously bump into
other electrons and atoms.
Each collision causes the electron to
change direction.
However, Electricity allows flows from
the negative terminal to the load (light
bulbs) to the positive terminal.
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When fast moving electrons bump
into atoms, they transfer energy.
Energy is transferred from the
electron to the atom which increases
the wire’s temperature.
These collisions between electrons
and atoms is what slows down the
free flow of electrons and is the
cause of electrical resistance.
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Resistance = voltage /
current
R=V/I
Unit for resistance is the
ohm.
Symbol for resistance is Ω
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If we have a current of 2 A
when the power supply is
producing a voltage of 110 V,
what is the resistance?
R=V/I
R = 110 V / 2 A
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R = 55 Ω
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Physicist George Simon Ohm
discovered that the ratio of
voltage to current is constant
for most conductors.
Ohm’s Law says if its
resistance is constant, than
change in Voltage will equal
current times resistance.
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The amount of resistance in a
wire depends on 3 things:
a. Length of the wire – longer
increases resistance
b. The radius of the wire – the
smaller the wire, the less
resistance
c. The material of which the
wire is made
Resistivity is a
measure of the capacity
of a material to resist
electric charge flow.
 In a series circuit their is
only 1 path for electricity
to flow.
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3 important rules for Series circuits:
Since there is only path for charges to
flow, the current is the same
everywhere.
The current is slowed down by the first
light bulb and then slowed down more by
a second light bulb. Therefore, the
resistance is the sum of the individual
resistances of each light bulb.
The sum of the voltage drop across each
light bulb = the voltage of the battery.
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In a series circuit, when there is a
break in the circuit, the current
everywhere is stopped.
In a series circuit, light bulb one has a
resistance of 90 ohms, light bulb 2 has
a resistance of 70 ohms, and light bulb
3 has a resistance of 120 ohms. What
is the total resistance in the circuit?
Total resistance in series equals sum
of each resistance: 90 + 70 + 120 =
280
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In a parallel circuit, the current
from the battery flows through
one lamp and part of the
current flows through the other
lamp equally.
If one bulb is removed, the
current can still flow through
the second path and the
other bulb will remain lit.
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3 rules for Parallel:
Add the current through the bulbs to
get the total current of the circuit.
To calculate resistance, use this
equation:
_1_ _1_
1_
Rtotal = R1 + R2
The voltage drop across the light
bulbs is equal to the voltage of the
battery.
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A circuit in parallel has two light
bulbs. Bulb 1 has a resistance of
90 and bulb 2 has a resistance of
70. What is the total resistance?
_1_ _1_
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Rtotal = R1 + R2
1 / Rtotal = 1/90 +1/70 (Time out!)
1 / Rtotal = .0111 +.0143
1 / Rtotal = .0254
Rtotal = 1/.0254 = 39.37
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Sometimes, we want
resistance in a circuit. A
resistor is an electrical
device that has a
specific resistance we
can add to a circuit.
How to Read Resistors
•First find the tolerance band, it will
typically be gold or silver.
•Starting from the other end, identify the
first band - write down the number
associated with that color
•Now 'read' the next color, here it is red
Now read 'multiplier' band and write
down that number of zeros.
Read Resistors
What is our resistor?
•Blue = 6
•Red = 2
•Multiplier = 2 (Number of
zeros is 2)
•6200 - Resistor