lab21--5-14 (Faraday'sLaw and Lenz's Law)

The shape of the B vs. t graph is shown below for two complete cycles of magnetic field oscillations.  apply Faraday's law to explain it.

Activity: The magnetic field in a  loop

Turn on the current to measure the magnetic field change due to one

loop.  Next measure the magnitude of the field in milli-teslas [mT] as you coil the 

insulated wire into more loops.  Use the ammeter to measure the current through 

the wire.  Record the measurements in logger pro

Conclusion: The magnetic filed quantity is proportional to the number of loops. The more loops, the larger the magnetic field.

B is proportional to NI

Next, we explore the magnetic flux in two situations;GIVEN surface area=ab

a: Magnetic flux through a surface between two parallel fields 

b:Magnetic flux if a number of magnetic field lines that pass perpendicularly through a surface.

a: Flux=B.A=B.A.cos@=B.ab.cos(90)(normal vector of surface perpendicular to field)=0

b: Flux=B.ab.cos(0)=Bab

 

Prof Mason is demonstrating the magnetic field due to a certain amount of copper loops, as we can see, the magnitude of magnetic field is directly proportional to the number of loops, when the current and voltage are constant

Since we know B is proportional to NI, we can max current by 4 ways;

Next, we are introduced to Lenz's Law in the next two experiments;

a.)The bulb is lighten. as prof Mason place metal close

b.)Professor Mason added an aluminum coil, the silver coil levitated higher because the density was lower than the copper coil.The next one was a steel ring with a gap in between, which means that current can't flow, so nothing will happen.

c.)Place two metal inside the tube, suppose they will drop through the tube at the same time. when we use plastics, they drop at the same time; when we use alum they don't. 

Question: Why the other one drop slowly?

Answer: The induced current in a  loop causes additional magnetic field and flux in the area bounded by the loop.The direction is in the opposite of that of the original mag field so it tends to cancel the effects of that field.As the magnet was sliding down it generated a current moving counterclockwise in the aluminum that slowed the magnet down since there was a magnetic field going up as a consequence.

Whenever the plane of a coil of area A is perpendicular to the magnetic field vector, then the dot product can be  dropped and the equation can be simplified to emf=-NA(dB/dt)

Consider a pickup coil consisting of a coil of wire with N loops of radius R.  Suppose it is placed perpendicular to a uniform magnetic field B that varies sinusoidally with time with an angular frequency w so that 

B = B0 sin w t

where B0 is a constant representing the maximum magnetic field at the site of the pickup coil.

Thus, we derive the flux and emf.

 

Given the graph for magnetic field B, draw emf on whiteboard:

1.

2.

lab20--5.12(Motors and Magnetic Fields)

Prof Mason is heating up a magnetic metal. Will the magnetism disappear?

Yes.

Question:What is the difference between an ordinary pin and a magnetic pin?

Answer:A magnetic pin has straight line domain. While ordinary pin has random domain

Comp up two ways to destroy magnet:

Heat;  Physical destroy like thor(?)

Consider a rectangular current loop as shown below. 

According to our work in the previous lesson, the net force on a current loop is 0

Net Torque is shown below

A current loop in a magnetic field can experience forces on its sides which can generate a torque.  Consider a rectangular loop with the field in the plane of the loop.

Finally, we use this problem to derive the torque t and magnetic dipole moment u(=NIA) below.

Use these equations to solve the problem

What is the torque on a 50-loop coil of radius 1.00-m in the problem below?

Two-pole DC electric motor.

WHat is the most likely things to break in a motor?

This is a model of  motor:

We are going to build out own motor!

Material:

coil, (we build a coil loop and use knife to eliminate the edges at two ends.)

wire,

magnetic metal plates,

batteries,(3)

Initially the motor built by ourselves does not work. The problem is on the brushes.

The coil loop will spin around

THE MAGNETIC FIELD NEAR A CURRENT-CARRYING WIRE

Repeat some of Oersted’s observations and study the pattern of magnetic field lines in a plane perpendicular to a long straight conductor that is carrying current.

Yes, the compasses pointing directions make a circle pattern. which proves the straight current makes up a circular magnetic field

Next, we know that we can magnetic field superposition(can add up) if they are in same direction. They can cancel out if in different direction

Derive the magnetic field formula in terms of u,q V,r,GIVEN Idl=qV

lab14--4/16(Potential of continuous charges distribution)

Find electrical potential due to a finite -length line charge

P(10cm,15cm)

length=16cm

density=2.7mc/cm

cut into 16 segments

distance r=sqrt(x^2+y^2)

E=kq/(r)

calculate each segment in spreadsheet and calculate the total

We increment x by 0.01, starting point is the middle of the last segment of the line.

x=(0.1-0.09)/2+0.09=0.095

x=(0.09=0.08)/2+0.08=0.085

x=(0.08-0.07)/2+0.07=0.075

......

Then we plug in the equation,E=k*dQ/sqrt(x^2+y^2) by increment x

total E=2.46E6

We can also use integral to evaluate the finite length charged E field:

Activity: Calculation of the Potential from a Charged Ring

Then, we derive the electrical field at point p from the charged ring E=kQx/(x^2+a^2)^1.5

Activity:  Sketches of Electric Field Lines and Equipotentials

1.Suppose that you are a test charge and you start moving at some distance from the charge below (such as 4 cm).What path could you move along without doing any work—that is,Eds• is always zero?  What is the shape of the equipotential surface?  Remember that in general you can move in three dimensions.(left top corner on white board）

2.Find some equipotential surfaces for the charge configuration shown below, which consists of two charged metal plates placed parallel to each other.  What is the shape of the equipotential surfaces?（right top corner)

3. Find some equipotential surfaces for the electric dipole charge configuration shown below.(bottom left)

Electric Potential Lab/Activity

Questions:

How much work would you have to do to move 1 coulomb of positive charge from

a) x = 0 cm to x =3 cm? _____4.2E-19____

b) x = 4 cm to x =6 cm? ______0.44E-20___

c) x = 5 cm to x =2 cm?? ______2.7E-19___

How much work would you have to do to move 32 mC of charge from

a) x = 0 cm to x =3 cm?? _________

W=q(V2-V1)=32*10E-6*(0.23+1.55)=5.60E-5 J

b) x = 4 cm to x =6 cm? _________

W=q(V2-V1)=32*10E-6*(0.37+0.22)=1.88E-5 J

c) x = 5 cm to x =2 cm? _________

W=q(V2-V1)=32*10E-6*(0.37+0.44+0.9)=1.44E-3 J

How much work would you have to do to move –25 millicoulombs of charge from

a) x = 0 cm to x =3 cm?? _________

W=q(V2-V1)=-25*10E-3*(0.23+1.55)=-0.0445 J

b) x = 4 cm to x =6 cm? _________

W=q(V2-V1)=-25*10E-3*(0.37+0.22)=-0.01475 J

c) x = 5 cm to x = 2 cm? _________

W=q(V2-V1)=-25*10E-3*(0.23+1.55)=-0.04275 J

--

Take your Position and Potential columns copy them to LoggerPro.

Make a graph of Potential (y-axis) vs. Position (x-axis, in meters).

lab18--5/5(Electronics and Oscilloscope)

This is the square mode on oscilloscope

This is the CRT, which is an important part of the oscilloscope.

It is possible to prove  that the deflection of an electron passing between charged metal plates is proportional to the voltage across the plates.  It is this proportional relationship between voltage and deflection that allows us to use an oscilloscope to display voltage changes graphically.

Inside the CRT there is an electron gun. The electrons go every direction, and a vertical deflation plate, horizontal deflation plate.

We predict the line on oscilloscope will jump vertically by some time if we apply 1.2 V battery on vertical deflation

a pair of parallel plates with voltage V across them will have a uniform electric field between them of magnitude E=V/d

1.An electron,q is placed in an E field E. what is the expression for F on the electron in terms of charge q, electrical field E?

F=qE

2.IF the mass is m, what is the acceleration?

ma=qE=qV/d

a=qV/(md)

3. Assume that the uniform electric field between the plates is in the y-direction.  Show that if the initial velocity, v0x, is in the x-direction, it remains constant as the electron moves perpendicular to the electric field so that vx = v0x.

4.d. If the plates have a length L, what is the time it takes an electron moving at an initial velocity v0x to pass between them?

t=L/V

v=at=qVL/(mdVx)

What is the distance electron travels during this time?

The work is shown below

Producing voltage change for input to a scope:

1 function generator

1 small speaker

1 BNC to clip lead cable

 Turn function generator on to DC offset

Activity:sounds from function generator

1,Describe the sound you hear when you have the wave output set at 96HZ

It sounds like deep horn

2. Describe what happens to the sound s when you use triangle and square wave output instead

Square sounds deeper and louder than triangle

3. What will happen if you change the frequency of the function generator?

Sounds change to higher if we set it smaller than 96hz

4. How do change in Amplitude affect the sounds?

the larger the amplitude, the louder sound

Activity: the oscilloscope controls

1.Play with the intensity control and the power/illumination controls. What do they do?

intensity larger makes line brighter

voltage change causes line to shift upward

2. Use the intensity control to adjust the brightness of the spot on the oscilloscope screen so that it is just comfortably visible. Play with the focus control what do they do?

Focus makes graph blurred or sharper

3. play with the time base control. what does it do?

turn Time base control will make the line shift upward(?)

 Activity :Measuring changing voltage

1. With the function generator set to 96 hz, Use your oscilloscope to determine the period of the sin wave form

5.5ms*2=11*10^-3 s

2.How does this period compare with he period calculated on the basis of the frequency reading on the dial?

f=1/t=1/(11*10^-3)=90HZ

it is 95HZ on dial

3.Experiment with DC offset control on the function generator and the AC DC  button on the oscilloscope. Explain how these controls affect the oscilloscope display

The display becomes a straight horizontal line switching to DC mode

4.switch over to the sin wave and square wave outputs of the function generator . Take a picture of each form for  your blog

Square form:

Triangle form:

5.Experiment with the freq dial and multiplier on the function generator and the time base control on the oscilloscope. How do changes in these settings affect the wave form?

When we decrease the freq dial, line becomes longer

Mystery box:

 8.5V

 5V

8V