Download Ohm`s Law Preamble Resistor Construction

Ohm's Law
(R. Bolton - 2012)
Ohm's Law
(R. Bolton - 2012)
Preamble

Ohm’ss Law
Ohm

Physics, 8th Edition Custom Edition
Cutnell & Johnson
Chapter 20.2
20 2
Pages 601-602
(R. Bolton - 2012)
Ohm's Law
(R. Bolton - 2012)
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Physics 155.3: Introduction to Electricity and Magnetism
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Ohm's Law
In this section of my lectures we will be
developing the relationship between
voltage, current and resistance.
This relationship will have to become
second nature to you in the subsequent
work.
work
(R. Bolton - 2012)
Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
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Ohm's Law
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Resistor Construction

Old Style
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Why is carbon used for resistors?
Why did they change the shape?
Why use resistors at all?
New Style
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity and Magnetism
Resistance
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Physics 155.3: Introduction to Electricity and Magnetism
Ohm's Law
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Ohm's Law
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Ohm's Law
Resistor Symbol

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Wires
Note the resistor symbol and notation
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Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
(R. Bolton - 2012)
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Ohm's Law
Consider a battery with two wires
(conductors with 0 Ω resistance)
attached to it.
The energy contours might look like
(R. Bolton - 2012)
Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
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Ohm's Law
(R. Bolton - 2012)
Example 1
Phototube Circuit
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Wires with 0 Ω resistance must be an
energy contour.
Why?
Do wires with 0 Ω resistance really
exist??
C
Conclusion?
l
?
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity and Magnetism
Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
Ohm's Law
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Ohm's Law
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Ohm's Law
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Example 1
Solution:

What can we say about this circuit??
(R. Bolton - 2012)
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Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
(R. Bolton - 2012)
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Ohm's Law
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

Clearly moving a wire into an electric
field will change the field.
The field will do work on the electrons
in the wire
wire, so the field will get weaker
unless the field is created by a battery
that can separate more charge
charge.
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity and Magnetism
Ohm's Law
Consider a wire in an electric field
Ohm's Law
Physics 155.3: Introduction to Electricity and Magnetism
(R. Bolton - 2012)
Consider a resistor connected to a battery
using wires.
15V

R1
VAB is
i 15 V both
b th before
b f
and
d after
ft the
th
switches are closed.
(R. Bolton - 2012)
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Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
Ohm's Law
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Ohm's Law
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Ohm's Law
Current through a resistor

(R. Bolton - 2012)
Ohm’s Law
Recall that current is charge that is
moving.

Ohm’s Law states
V AB = IR

or
I=

or
R=
(R. Bolton - 2012)
Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
V AB
I
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Ohm's Law
V AB
R
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Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
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Ohm's Law
(R. Bolton - 2012)
Example
p 2
A Simple (but important) Circuit

where
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Consider the following circuit.
V is in Volts (V)
I is in Amperes (A)
R is in Ohms (Ω)
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Physics 155.3: Introduction to Electricity and Magnetism
Ohm's Law
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What is the voltage across the resistor (VAB)?
What current through the resistor?
What is VA? VB?
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Physics 155.3: Introduction to Electricity and Magnetism
Ohm's Law
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Ohm's Law
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Example 2
Solution:
Ohm's Law
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Example 3
Ammeter Circuit
(12V,1.2A,??)

Consider the following circuit containing
an ammeter ((used to measure current))
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Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
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Ohm's Law
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Example 3
Solution:
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Ohm's Law
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
Consider the following circuit containing a
voltmeter (used to measure voltage)
The voltmeter reads 9.00 volts and the
ammeter reads 300mA
300mA.
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Physics 155.3: Introduction to Electricity and Magnetism
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Example
p 4
Ammeter and Voltmeter Circuit
(12V)
Ohm's Law
Ohm's Law
Physics 155.3: Introduction to Electricity and Magnetism
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(R. Bolton - 2012)
What is E??
What is the resistance of the ammeter??
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What is the resistance of the resistor R??
What is the resistance of the voltmeter??
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Physics 155.3: Introduction to Electricity and Magnetism
Ohm's Law
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Ohm's Law
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Example 4
Solution:
Ohm's Law
(30Ω)
(R. Bolton - 2012)
Power (revisited)


Recall that power is the rate at which
energy is transferred or converted from
one form to another.
We saw previously that
P=
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Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
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Using Ohm’s Law we have two (2) other
equations for power
V AB = IR

∴P =
W
= V AB I
Δt
Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
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Ohm's Law
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Example 5
Electrical Energy Conversion
Other Power Equations

Q = IΔt
Units are watts (W)
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Ohm's Law
W
− WBA
VAB =
Q
Δt

Consider the following circuit
∴ P = IRI = I 2 R
and
I=
V AB
R
∴ P = V AB
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity and Magnetism
V AB V AB
=
R
R
Ohm's Law

2
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At what rate is electrical energy converted
to heat by the resistor in the circuit?
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Physics 155.3: Introduction to Electricity and Magnetism
Ohm's Law
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Ohm's Law
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Example 5
Solution:
Ohm's Law
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Example 6
Chemical Energy
(2.25W)

Consider the following circuit with E=120 V
and R=144Ω.
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Ohm's Law
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Physics 155.3: Introduction to Electricity and Magnetism
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Ohm's Law
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Example 6
Solution:
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity and Magnetism
(100W)
Ohm's Law
At what rate does the resistor convert electrical
energy to heat?
At what rate does the battery convert chemical
energy to electrical energy?
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Physics 155.3: Introduction to Electricity and Magnetism
Ohm's Law
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