Electric Circuits Introduction Section 0 Lecture 1 Slide 1 Lecture 30 Slide 1 INTRODUCTION TO Modern...

Electric Circuits

Introduction Section 0 Lecture 1 Slide 1

Lecture 30 Slide 1

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of Technology

PHYS 1800

Lecture 30

Electric Circuits

Electric Circuits


Lecture 30 Slide 2


Fall 2004


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


Lecture 30 Slide 3


Fall 2004


Spring 2009


PHYS 1800

Elements of Circuits

Lecture 30

Electric Circuits

Electric Circuits


Lecture 30 Slide 4


Fall 2004


Spring 2009

Can you get a flashlight bulb to light, with a battery and a single wire?

Electric Circuits


Lecture 30 Slide 5


Fall 2004


Spring 2009

Electric Circuits and Electric Current

• Dennison’s Law of Circuit Analysis—Follow the electrons with your finger Dummy!

•For the flashlight bulb to light, there must be a closed or complete path from the bulb to both ends of the battery.•Such a path is called a circuit.

Electric Circuits


Lecture 30 Slide 6


Fall 2004


Spring 2009


In this circuit, the battery is the energy source, using energy from chemical reactions to separate positive and negative charges.•This leads to a voltage difference, with an excess of positive charges at one end of the battery and an excess of negative charges at the other.These charges will tend to flow from one terminal to the other if we provide an external conducting path (the circuit).

Electric Circuits


Lecture 30 Slide 7


Fall 2004


Spring 2009

• A flow of electric charge is an electric current:

where I is electric current, q is charge, and t is time.

• The standard unit for electric current is the ampere:1 A = 1 C / s

I q

t

•For example, if 3 C of charge flow through a wire in 2 s, then

the electric current I is 3 C / 2 s = 1.5 A.•Positive charges moving to the right have the same effect as negative charges moving to the left.


Electric Circuits


Lecture 30 Slide 8


Fall 2004


Spring 2009

• A flow of electric charge is an electric current:

where I is electric current, q is charge, and t is time.

• The standard unit for electric current is the ampere:1 A = 1 C / s

I q

t


•The direction of current is defined as the direction that positive charges would flow.

•In reality, the charge carriers in a metal wire are negatively charged electrons.

Electric Circuits


Lecture 30 Slide 9


Fall 2004


Spring 2009


PHYS 1800

Resistance

Lecture 30

Electric Circuits

Electric Circuits


Lecture 30 Slide 10


Fall 2004


Spring 2009

• In addition to an energy source and a conducting path, a circuit also includes some resistance to the current.

•In the flashlight bulb, a very thin wire filament restricts the current because of its very small cross-sectional area.

•The wire filament gets hot as charges are forced through this constriction.

•Its high temperature makes it glow, and we have light.

Electric Circuits and Resistance

Electric Circuits


Lecture 30 Slide 11


Fall 2004


Spring 2009

Two arrangements of a battery, bulb, and wire are shown below. Which of the two arrangements will light the bulb?

a) Arrangement (a)b) Arrangement (b)c) Bothd) Neither

The bulb will light in arrangement A in which the filament of the bulb is connected to the two sides of the battery for a closed circuit. In B there is no voltage across the filament and thus no current in the filament.

Electric Circuits


Lecture 30 Slide 12


Fall 2004


Spring 2009


PHYS 1800

Analogy with Water Flow

Lecture 30

Electric Circuits

Electric Circuits


Lecture 30 Slide 13


Fall 2004


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


Lecture 30 Slide 14


Fall 2004


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


Lecture 30 Slide 15


Fall 2004


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


Lecture 30 Slide 16


Fall 2004


Spring 2009

In the circuit shown, the wires are connected to either side of a wooden block as well as to the light bulb. Will the light bulb light in

this arrangement?

a) Yesb) Noc) Maybed) Impossible to tell

from the picture

The bulb will not light since (dry) wood is a very poor conductor. The resistance will be so high that virtually no current is in the lamp circuit.

Electric Circuits


Lecture 30 Slide 17


Fall 2004


Spring 2009

In the circuit shown, could we increase the brightness of the bulb by connecting a wire between points A and B?


from the picture

No. Connecting A and B will provide a short circuit for the battery that will damage it while allowing virtually no current in the bulb.

Electric Circuits


Lecture 30 Slide 18


Fall 2004


Spring 2009

Which of the two circuits shown will cause the light bulb to light?

Diagram B will allow the light bulb to light since there is a closed circuit providing current from the battery through the bulb. Whether the switch is open or closed is immaterial here since it is in parallel with another conductor. In diagram A no potential difference is in the closed circuit.

a) Arrangement (a)b) Arrangement (b)c) Bothd) Neither

Electric Circuits


Lecture 30 Slide 19


Fall 2004


Spring 2009

Suppose we use an uncoated metal clamp to hold the wires in place in the battery-and-bulb circuit shown. Will this be

effective in keeping the bulb burning brightly?


from the picture

No. The metal clamp will provide a conducting path across the battery causing the battery to discharge. If we want to use a clamp we can put insulating tape between one of its jaws and the electrical connection.

Electric Circuits


Lecture 30 Slide 20


Fall 2004


Spring 2009


PHYS 1800

Ohm’s Law

Lecture 30

Electric Circuits

Electric Circuits


Lecture 30 Slide 21


Fall 2004


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


Lecture 30 Slide 22


Fall 2004


Spring 2009

• If we know the resistance of a given portion of a circuit and the applied voltage, we can calculate the current through that portion of the circuit.

• For example, consider a 1.5-V battery connected to a light bulb with a resistance of 20 ohms.• If the resistance of the battery itself is negligible, the current can be found by applying Ohm’s Law:

I = 1.5 V / 20 = 0.075 A = 75 mA

Ohm’s Law and Resistance

Electric Circuits


Lecture 30 Slide 23


Fall 2004


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


Lecture 30 Slide 24


Fall 2004


Spring 2009


Next Lab/Demo: Electric Charge

Electric CircuitsThursday 1:30-2:45

ESLC 46 Ch 12 and 13

Next Class: Wednesday 10:30-11:20

BUS 318 roomRead Ch 13

Electric Circuits Introduction Section 0 Lecture 1 Slide 1 Lecture 30 Slide 1 INTRODUCTION TO Modern...

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Transcript of Electric Circuits Introduction Section 0 Lecture 1 Slide 1 Lecture 30 Slide 1 INTRODUCTION TO Modern...