Ohm's Law (R. Bolton - 2012)

Physics 155.3: Introduction to Electricity and Magnetism 1

Ohm’s Law Example Solutions

Physics, 7th Edition, Cutnell & JohnsonChapter 20.2

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Chapter 20.2Pages 605-607

Important Formulae

IRV VI AB VR ABIRVAB =R

VI AB=I

VR AB=

IVt

WP AB=Δ

= RIIRIP 2==R

VR

VVP ABABAB

2

==

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Ohm's Law (R. Bolton - 2012)

Physics 155.3: Introduction to Electricity and Magnetism 2

Example 1Phototube Circuit

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Example 1Solution:

What can we say about this circuit?? What can we say about this circuit?? I is clockwise VAG = 10V VR1 = 6V VR2 = 4V

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R2

R1 = (3/2)R2

Ohm's Law (R. Bolton - 2012)

Physics 155.3: Introduction to Electricity and Magnetism 3

Example 2A Simple (but important) Circuit

Consider the following circuit Consider the following circuit.

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What is the voltage across the resistor (VAB)? What current through the resistor? What is VA? VB?

Example 2Solution: (12V,1.2A,???)

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Ohm's Law (R. Bolton - 2012)

Physics 155.3: Introduction to Electricity and Magnetism 4

Example 3Ammeter Circuit

Consider the following circuit containing Consider the following circuit containing an ammeter (used to measure current)

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What is E?? What is the resistance of the ammeter??

Example 3Solution: (12V)

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The resistance of the ammeter is assumed to be 0Ω . But is it really??

Ohm's Law (R. Bolton - 2012)

Physics 155.3: Introduction to Electricity and Magnetism 5

Example 4Ammeter and Voltmeter Circuit

Consider the following circuit containing a Consider the following circuit containing a voltmeter (used to measure voltage)

The voltmeter reads 9 00 volts and the

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The voltmeter reads 9.00 volts and the ammeter reads 300mA. What is the resistance of the resistor R?? What is the resistance of the voltmeter??

Example 4Solution: (30Ω)

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The resistance of the voltmeter is assumed to be ∞Ω. But is it really??

Ohm's Law (R. Bolton - 2012)

Physics 155.3: Introduction to Electricity and Magnetism 6

Example 5Electrical Energy Conversion

Consider the following circuit Consider the following circuit

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At what rate is electrical energy converted to heat by the resistor in the circuit?

Example 5Solution: (2.25W)

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Ohm's Law (R. Bolton - 2012)

Physics 155.3: Introduction to Electricity and Magnetism 7

Example 6Chemical Energy

Consider the following circuit with E=120 V Consider the following circuit with E=120 V and R=144Ω.

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At what rate does the resistor convert electrical energy to heat?

At what rate does the battery convert chemical energy to electrical energy?

Example 6Solution: (100W)

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