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Physics of Technology PHYS 1800. Lecture 31 Electric Circuits. PHYSICS OF TECHNOLOGY Spring 2009 Assignment Sheet. *Homework Handout. Physics of Technology PHYS 1800. Lecture 31 Electric Circuits. Review. Water flowing in a pipe is similar to electric current flowing in a circuit. - PowerPoint PPT Presentation
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Electric Circuits
Introduction Section 0 Lecture 1 Slide 1
Lecture 31 Slide 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Physics of Technology
PHYS 1800
Lecture 31
Electric Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 2
Lecture 31 Slide 2
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
PHYSICS OF TECHNOLOGY Spring 2009 Assignment Sheet
*Homework Handout
PHYSICS OF TECHNOLOGY - PHYS 1800 ASSIGNMENT SHEET
Spring 2009 Date Day Lecture Chapter Homework Due Feb 16 17 18 19 20
M Tu W H F*
Presidents Day Angular Momentum (Virtual Monday) Review Test 2 Static Fluids, Pressure
No Class 8 5-8 5-8 9
-
Feb 23 25 27
M W F*
Flotation Fluids in Motion Temperature and Heat
9 9 10
6
Mar 2 4 6
M W F*
First Law of Thermodynamics Heat flow and Greenhouse Effect Climate Change
10 10 -
7
Mar 9-13 M-F Spring Break No Classes Mar 16 18 20
M W F*
Heat Engines Power and Refrigeration Electric Charge
11 11 12
8
Mar 23 25 26 27
M W H F*
Electric Fields and Electric Potential Review Test 3 Electric Circuits
12 13 9-12 13
-
Mar 30 Apr 1 3
M W F
Magnetic Force Review Electromagnets Motors and Generators
14 9-12 14
9
Apr 6 8 10
M W F*
Making Waves Sound Waves E-M Waves, Light and Color
15 15 16
10
Apr 13 15 17
M W F*
Mirrors and Reflections Refraction and Lenses Telescopes and Microscopes
17 17 17
11
Apr 20 22 24
M W F
Review Seeing Atoms The really BIG & the really small
1-17 18 (not on test) 21 (not on test)
No test week 12
May 1 F Final Exam: 09:30-11:20am * = Homework Handout
Electric Circuits
Introduction Section 0 Lecture 1 Slide 3
Lecture 31 Slide 3
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Physics of Technology
PHYS 1800
Review
Lecture 31
Electric Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 4
Lecture 31 Slide 4
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
+q, -q charge mass m
wire pipe
switch valve
R resistor narrow pipe
I=(q/V)vA Charge flow rate (current)
Mass flow rate
(m/V)vA
q
PEV electric
Electric Circuits
Introduction Section 0 Lecture 1 Slide 5
Lecture 31 Slide 5
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• Water flowing in a pipe is similar to electric current flowing in a circuit.– The battery is like the pump.– The electric charge is like the water.– The connecting wires are like the thick pipe.– The filament is like the nozzle or narrow pipe.– The switch is like the valve.
Electric Circuits
Introduction Section 0 Lecture 1 Slide 6
Lecture 31 Slide 6
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• In a water-flow system, a high pressure difference will produce a large rate of water flow or current.– High pressure can be produced by raising the storage tank: this
pressure is related to the gravitational potential energy.– Likewise, a large difference in potential energy between the
charges at the two ends of a battery is associated with a high voltage and a greater tendency for charge to flow.
Electric Circuits
Introduction Section 0 Lecture 1 Slide 7
Lecture 31 Slide 7
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Ohm’s Law and Resistance
• The electric current flowing through a given portion of a circuit is directly proportional to the voltage difference across that portion and inversely proportional to the resistance:
– Resistance R is the ratio of the voltage difference to the current for a given portion of a circuit, and is in units of ohms:
1 ohm = 1 = 1 V / A.– The resistance of a wire is proportional to the length of the wire,
inversely proportional to the cross-sectional area of the wire, and inversely proportional to the conductivity of the material.
R = L / (A σ)– It also depends on the temperature of the material.
Electric Circuits
Introduction Section 0 Lecture 1 Slide 8
Lecture 31 Slide 8
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Series and Parallel Circuits
• In a series circuit, there are no points in the circuit where the current can branch into secondary loops.– All the elements line up on a single loop.– The current that passes through one element must also
pass through the others.
Electric Circuits
Introduction Section 0 Lecture 1 Slide 9
Lecture 31 Slide 9
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Physics of Technology
PHYS 1800
Series Circuits
Lecture 31
Electric Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 10
Lecture 31 Slide 10
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• In a series combination of resistances, each resistance contributes to restricting the flow of current around the loop.– The total series resistance of the combination Rseries is the sum
of the individual resistances:
– A common mistake is to think the current gets used up in
passing through the resistances in a series circuit.– The same current must pass through each component much
like the continuous flow of water in a pipe.
Series Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 11
Lecture 31 Slide 11
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• It is the voltage that changes as the current flows through the circuit.– Voltage decreases by Ohm’s Law: V = I R
as the current passes through each resistor.• The total voltage difference across the combination is the
sum of these individual changes.– If two light bulbs are connected in series with a battery, the
current will be less than with a single bulb, because the total series resistance is larger.
– The bulbs will glow less brightly.
Series Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 12
Lecture 31 Slide 12
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Two resistors are connected in series with a battery as shown. R1 is less than R2. Which of the two resistors
has the greater current flowing through it?
The current is the same in each, since it is a series circuit.
a) R1
b) R2
c) Bothd) Neither
Series Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 13
Lecture 31 Slide 13
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Two resistors are connected in series with a battery as shown. R1 is less than R2. Which of the two resistors has the
greatest voltage difference across it?
The voltage difference is greater across R2. According to Ohm's Law, V = IR, so for the same current, the larger the resistance the greater the potential difference.
a) R1
b) R2
c) Bothd) Neither
Series Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 14
Lecture 31 Slide 14
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
In the circuit shown, the 1- resistance is the internal resistance of the battery and can be considered to be in series with the
battery and the 9- load. What is the current flowing through the 9- resistor?
a) 0.1 Ab) 0.3 Ac) 0.9 Ad) 3 Ae) 10 A
A 3.0 10
V 3
10 1 9
series
totalseries
series
R
VI
R
Series Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 15
Lecture 31 Slide 15
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
In the circuit shown, the 1- resistance is the internal resistance of the battery and can be considered to be in
series with the battery and the 9- load. What is the voltage across the 9- resistor?
a) 0.1 Vb) 0.3 Vc) 1.0 Vd) 2.7 Ve) 3.0 V
since I9 Iseries :
V9 IseriesR9 0.3 A 9 2.7 V
Series Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 16
Lecture 31 Slide 16
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Three resistors are connected to a 6-V battery as shown. The internal resistance of the battery is negligible. What is the current through the 15-
resistance?
a) 0.1 Ab) 0.15 Ac) 0.4 Ad) 1.5 Ae) 4.0 A
Rseries 15 20 25 60
I15 Iseries Vtotal
Rseries
6 V
60 0.1 A
Series Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 17
Lecture 31 Slide 17
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Does this same current flow through the 25- resistance?
a) Yes.b) No.c) It depends on
various things.
Yes. Since all the resistors are in series, the same current must flow through all three. There is no other path for the current through the 15- resistor to follow, except to go through the 20- resistor and then the 25- resistor.
Series Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 18
Lecture 31 Slide 18
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
What is the voltage difference across the 25- resistance?
a) 0.1 Vb) 2.5 Vc) 6 Vd) 25 Ve) 60 V
since I25 Iseries :
V25 IseriesR25 0.1 A 25 2.5 V
Series Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 19
Lecture 31 Slide 19
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Physics of Technology
PHYS 1800
Parallel Circuits
Lecture 31
Electric Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 20
Lecture 31 Slide 20
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• In a parallel circuit, there are points at which the current can branch or split up into different paths.– The flow divides and later rejoins.– The total cross-sectional area the current (or water) flows
through is increased, therefore decreasing the resistance to flow:
Parallel Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 21
Lecture 31 Slide 21
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Parallel Circuits• In a parallel combination of resistances, the voltage
difference across each resistance is the same, since they are all connected between the same two points.– The currents can be different, since they divide: they add to give
the total current through the combination.– A portion of the total current flows through each branch.
Electric Circuits
Introduction Section 0 Lecture 1 Slide 22
Lecture 31 Slide 22
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Two 10- light bulbs are connected in parallel to one another, and this combination is connected to a 6-V battery. What is
the total current flowing around the loop?
a) 0.6 A b) 1.2 A c) 6 A d) 12 A e) 60 A
Parallel Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 23
Lecture 31 Slide 23
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
How much current passes through each light bulb?
a) 0.6 A b) 1.2 A c) 6 A d) 12 A e) 60 A
Parallel Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 24
Lecture 31 Slide 24
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Three identical resistors, each 24 , are connected in parallel with one another as shown. The combination is connected to
a 12-V battery whose internal resistance is negligible.What is the equivalent resistance of this parallel
combination?
a) 0.0417 b) 0.125 c) 8d) 24 e) 72
1
Rparallel
1
R1
1
R2
1
R3
1
24
1
24
1
24
3
24
1
8
Rparallel 8
Parallel Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 25
Lecture 31 Slide 25
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
What is the total current through the combination?
a) 0.5 Vb) 1.0 Vc) 1.2 Vd) 1.5 Ve) 12 V
Itotal Vtotal
Rparallel
12 V
8 1.5 V
Parallel Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 26
Lecture 31 Slide 26
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
How much current flows through each resistor in the combination?
a) 0.5 Vb) 1.0 Vc) 1.2 Vd) 1.5 Ve) 12 V
The resistors are identical and are in parallel,
so the same current must flow through each resistor.
The total current is plit into three identical parts:
Ione 24 - resistor 1
3Itotal
1
31.5 V 0.5 V
Parallel Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 27
Lecture 31 Slide 27
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
In the circuit shown, R3 is greater than R2, and R2 is greater
than R1. is the electromotive force of the battery whose internal resistance is negligible. Which of the three resistors
has the greatest current flowing through it?
R3 has the greatest current since the current in it is the sum of the currents in R1 and R2.
a) R1
b) R2
c) R3
d) R1 and R2 are equal, and greater than R3
e) They are all equal
Parallel Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 28
Lecture 31 Slide 28
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Physics of Technology
PHYS 1800
Power in Electric Circuits
Lecture 31
Electric Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 29
Lecture 31 Slide 29
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Energy and Power
• Energy is supplied to a water-flow system by the pump, which increases the gravitational potential energy of the water by lifting it up to a higher tank.
• As the water flows down through pipes to a lower tank, gravitational potential energy is transformed into kinetic energy of the moving water.• Once the water comes to rest in the lower tank, the kinetic energy is dissipated by frictional or viscous forces which generate heat.
Electric Circuits
Introduction Section 0 Lecture 1 Slide 30
Lecture 31 Slide 30
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• Similarly, in an electric circuit energy is supplied by a battery, which draws its energy from the potential energy stored in its chemical reactants.– The battery increases the potential energy of electric charges
as it moves positive charges toward the positive terminal and negative charges toward the negative terminal.
– When we provide an external conducting path from the positive to the negative terminal, charge flows from points of higher potential energy to points of lower potential energy.
• As potential energy is lost, kinetic energy is gained by the electrons.• This kinetic energy is converted to heat by collisions with other electrons and atoms.
Electric Energy and Power
Electric Circuits
Introduction Section 0 Lecture 1 Slide 31
Lecture 31 Slide 31
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• Since voltage is potential energy per unit charge, multiplying a voltage difference by charge yields energy.
• Since current is the rate of flow of charge, multiplying a voltage difference by current yields power, the rate of energy use.
• The power supplied by a source must equal the power dissipated in the resistances.
Energy source potential energy kinetic energy heat
RII
RIRIRP
IRVVI
IP
2
2)(
;
Electric Energy and Power
Electric Circuits
Introduction Section 0 Lecture 1 Slide 32
Lecture 31 Slide 32
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
What is the power dissipated in a 20- light bulb powered by two 1.5-V batteries in series?
a) 0.15 Wb) 0.45 Wc) 3.0 Wd) 6.67 We) 60 W
1 2 3 V
R 20
IR
I R
3 V
20 0.15 A
P I I2R (0.15 A) 2(20 ) 0.45 W
check :
P I (3 V)(0.15 A) 0.45 W
Electric Energy and Power
Electric Circuits
Introduction Section 0 Lecture 1 Slide 33
Lecture 31 Slide 33
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• The ease with which electric power can be transmitted over considerable distances is one of its main advantages over other forms of energy.– The source of the energy might be gravitational potential
energy of water, chemical potential energy stored in fossil fuels, or nuclear potential energy stored in uranium.
– Power plants all use electric generators that convert mechanical kinetic energy produced by turbines to electric energy.
– These generators are the source of the electromotive force.
Electric Energy and Power
Electric Circuits
Introduction Section 0 Lecture 1 Slide 34
Lecture 31 Slide 34
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• The unit of energy commonly used to discuss electric energy is the kilowatt-hour, which is a unit of power (the kilowatt) multiplied by a unit of time (an hour).– 1 kilowatt equals 1000 watts– 1 hour = 3600 seconds– 1 kilowatt-hour equals 3.6 million joules
• The kilowatt-hour is a much larger unit of energy than the joule, but it is a convenient size for the amounts of electrical energy typically used in a home.
Electric Energy and Power
Electric Circuits
Introduction Section 0 Lecture 1 Slide 35
Lecture 31 Slide 35
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
How much does it cost to light a 100-watt light bulb for one day? Assume an average rate of cost of 10 cents per
kilowatt-hour.
a) 0.24 cents b) 2.4 cents c) 24 cents d) $2.40 e) $24
Energy used = power x time Cost = energy used x rate of cost = (100 W)(24 hr) = (2.4 kWh)(10 cents / kWh) = 2400 Wh = (2.4 kWh)(10 cents / kWh)
= 2.4 kWh = 24 cents
Electric Energy and Power
Electric Circuits
Introduction Section 0 Lecture 1 Slide 36
Lecture 31 Slide 36
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Physics of Technology
PHYS 1800
Alternating Current and Household Wiring
Lecture 31
Electric Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 37
Lecture 31 Slide 37
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Alternating Current and Household Circuits
• The current we draw from a wall outlet is alternating current (ac) rather than direct current (dc).– Direct current implies that the current flows in a single
direction from the positive terminal of a battery or power supply to the negative terminal
– Alternating current continually reverses its direction -- it flows first in one direction, then in the other, then back again.
– In North America the ac goes through 60 cycles each second (60 Hz).
Electric Circuits
Introduction Section 0 Lecture 1 Slide 38
Lecture 31 Slide 38
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• The plot of electric current as a function of time for an alternating current is a sinusoidal curve.– The average value of an ordinary alternating current is zero.– The power dissipated in a resistance is proportional to the
square of the current.– The effective current or rms current is obtained by squaring
the current, averaging this value over time, and taking the square root of the result.
– The effective current Ieff is 0.707 times the peak current Ipeak.
Alternating Current and Household Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 39
Lecture 31 Slide 39
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• If we plot the voltage across an electrical outlet as a function of time, we get another sinusoidal curve.– The effective value of this voltage is typically between 110 and
120 volts in North America.– The standard household power supplied in this country is 115
volts, 60 hertz ac.– Household circuits are wired in parallel so that different
appliances can be added to or removed from the circuit without affecting the voltage available.
Alternating Current and Household Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 40
Lecture 31 Slide 40
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
A 60-W light bulb is designed to operate on 120 V ac. What is the effective current drawn by the bulb?
a) 0.2 Ab) 0. 5 Ac) 2.0 Ad) 72 Ae) 7200 A
P 60 W
Veffective 120 V
P IV
I P
V
60 W
120 V
0.5 A
Alternating Current and Household Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 41
Lecture 31 Slide 41
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
• Household circuits are wired in parallel so that different appliances can be added to or removed from the circuit without affecting the voltage available.– As you add more appliances, the total current drawn
increases, because the total effective resistance of the circuit decreases when resistances are added in parallel.
– Since too large a current could cause the wires to overheat, a fuse or circuit breaker in series with one leg of the circuit will disrupt the circuit if the current gets too large.
– Appliances with larger power requirements (stoves, clothes dryers, etc) are usually connected to a separate 220-V line.
Alternating Current and Household Circuits
Electric Circuits
Introduction Section 0 Lecture 1 Slide 42
Lecture 31 Slide 42
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Physics of Technology
Next Lab/Demo: Electric Circuits
Magnetism Thursday 1:30-2:45
ESLC 46 Ch 13 and 14
Next Class: Friday 10:30-11:20
BUS 318 roomRead Ch 14