Download Ohm`s Law Example Solutions Important Formulae

Ohm's Law
(R. Bolton - 2012)
Ohm’s Law Example Solutions
Physics, 7th Edition, Cutnell & Johnson
Chapter 20.2
Pages 605-607
(R. Bolton - 2012)
Ohm's Law
1
Important Formulae
V AB = IR
P=
I=
W
= V AB I
Δt
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity
and Magnetism
V AB
R
R=
V AB
I
P = IRI = I 2 R
Ohm's Law
P = VAB
VAB VAB
=
R
R
2
2
1
Ohm's Law
(R. Bolton - 2012)
Example 1
Phototube Circuit
(R. Bolton - 2012)
Ohm's Law
3
Example 1
Solution:

What can we say about this circuit??





I is clockwise
VAG = 10V
VR1 = 6V
VR2 = 4V
R1 = (3/2)R2
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity
and Magnetism
Ohm's Law
4
2
Ohm's Law
(R. Bolton - 2012)
Example 2
A Simple (but important) Circuit




Consider the following circuit.
circuit
What is the voltage across the resistor (VAB)?
What current through the resistor?
What is VA? VB?
(R. Bolton - 2012)
Example 2
Solution:
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity
and Magnetism
Ohm's Law
5
(12V,1.2A,???)
Ohm's Law
6
3
Ohm's Law
(R. Bolton - 2012)
Example 3
Ammeter Circuit

Consider the following circuit containing
an ammeter (used to measure current)


What is E??
What is the resistance of the ammeter??
(R. Bolton - 2012)
Ohm's Law
Example 3
Solution:

7
(12V)
The resistance of the ammeter is
assumed to be 0Ω . But is it really??
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity
and Magnetism
Ohm's Law
8
4
Ohm's Law
(R. Bolton - 2012)
Example 4
Ammeter and Voltmeter Circuit


Consider the following circuit containing a
voltmeter (used to measure voltage)
The voltmeter reads 9
9.00
00 volts and the
ammeter reads 300mA.


What is the resistance of the resistor R??
What is the resistance of the voltmeter??
(R. Bolton - 2012)
Ohm's Law
Example 4
Solution:

9
(30Ω)
The resistance of the voltmeter is
assumed to be ∞Ω. But is it really??
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity
and Magnetism
Ohm's Law
10
5
Ohm's Law
(R. Bolton - 2012)
Example 5
Electrical Energy Conversion

Consider the following circuit

At what rate is electrical energy converted
to heat by the resistor in the circuit?
(R. Bolton - 2012)
Ohm's Law
Example 5
Solution:
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity
and Magnetism
11
(2.25W)
Ohm's Law
12
6
Ohm's Law
(R. Bolton - 2012)
Example 6
Chemical Energy

Consider the following circuit with E=120 V
and R=144Ω.


At what rate does the resistor convert electrical
energy to heat?
At what rate does the battery convert chemical
energy to electrical energy?
(R. Bolton - 2012)
Ohm's Law
Example 6
Solution:
(R. Bolton - 2012)
Physics 155.3: Introduction to Electricity
and Magnetism
13
(100W)
Ohm's Law
14
7