Exam II Questions

Hootenanny said:
Or you could use calculus.
i havent taken Calc yet :)

Hootenanny
Staff Emeritus
Gold Member
Alt+F4 said:
i havent taken Calc yet :)
In that case, your only option is to graph the function. If you can download a graphing program or use an online application I would do that (much easier than by hand).

Hootenanny said:
In that case, your only option is to graph the function. If you can download a graphing program or use an online application I would do that (much easier than by hand).
I can use a Ti-84 i just cant seem to get the Answer they have

I did 5Sin(2Pi60X + 25 (Phasor Angle or should a number go in there) * 120 Sin 20pi60X) = i got 561.79 instead of their answer

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Hootenanny
Staff Emeritus
Gold Member
Alt+F4 said:
I can use a Ti-84 i just cant seem to get the Answer they have

I did 5Sin(2Pi60X + 25 (Phasor Angle or should a number go in there) * 120 Sin 20pi60X) = i got 561.79 instead of their answer
Do I read it correctly? Is that what you typed into your calc?

$$P = V\cdot I = 120\sin(2\pi60t)\cdot\left(5\sin(2\pi60t+\Phi)\right)$$

Hootenanny said:
Do I read it correctly? Is that what you typed into your calc?

$$P = V\cdot I = 120\sin(2\pi60t)\cdot\left(5\sin(2\pi60t+\Phi)\right)$$
Exactly, except for the Phasor i put in the Angle

http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/fa02 [Broken]

Why is Question 11 C??

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All right last question and i think i will be done
http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/fa02 [Broken]

Question 5

Force = VB sin theta

but there is all this other stuff,

so i did
Force = A*V*B*R / (T) = .001 so why am i off by a factor of 10^-2. Thanks for everyones help

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OlderDan
Homework Helper
Alt+F4 said:
Exactly, except for the Phasor i put in the Angle

OlderDan said:
Nope.i get 1.7E-9, 2.2E-8 unless i have to convert it somehow

OlderDan
Homework Helper
Alt+F4 said:
http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/sp06 [Broken]

Question 11

I wanted to use this forumla B = UoI/ 2 pi R but i have no idea how to use it cause i keep getting a big number
That is not the formula you need. That is for finding the field produced by a current in a wire.

The induced emf is the negative of the rate of change of flux through the loop. Flux is related to field strength and area. The field is constant. The area is changing at a constant rate. The current is related to the induced emf and resistance of the loop.

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OlderDan said:
That is not the formula you need. That is for finding the field produced by a current in a wire.

The induced emf is the negative of the rate of change of flux through the loop. Flux is related to field strength and area. The field is constant. The area is changing at a constant rate. The current is related to the induced emf and resistance of the loop.
o yaaaaa thanks alot

Alt+F4 said:
one more,

http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/sp06 [Broken]

question 24.

i did this like 20 times in other exams, i dont know why it doesnt work here but

F= ILBSintheta = MG

(X)(.1)(.5) = (9.8)(1)
X = 196Amps, but the answer is 98 Amps what gives?
all right so why am i only counting current for one side icne 196/2 = 98

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OlderDan
Homework Helper
Alt+F4 said:
Nope.i get 1.7E-9, 2.2E-8 unless i have to convert it somehow
You are doing something wrong in your calculator. If the current and voltge were in phase you would have a maximum power of 120V*5A=600W. The phase difference reduces this somewhat, but not by a huge amount.

OlderDan said:
You are doing something wrong in your calculator. If the current and voltge were in phase you would have a maximum power of 120V*5A=600W. The phase difference reduces this somewhat, but not by a huge amount.
hmm well i think there is something wrong with their calculation, i have no idea how they got 25 Degrees to .436 radians, i got .4663 radians so maybe that is why i cant get their answer

http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/fa05 [Broken]

So i tried doing it on Calculator but since sin180 = 0 i get 0

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OlderDan
Homework Helper
Alt+F4 said:
hmm well i think there is something wrong with their calculation, i have no idea how they got 25 Degrees to .436 radians, i got .4663 radians so maybe that is why i cant get their answer
25/180*π = .43633 not .4663

OlderDan said:
25/180*π = .43633 not .4663
ya i cant get the answer at all, i tried on diffrent calculators. i am postiive im typing it correctly

OlderDan
Homework Helper
Alt+F4 said:
ya i cant get the answer at all, i tried on diffrent calculators. i am postiive im typing it correctly
Are you using the calculator to find the maximum of the product IV? How are you doing this?

i went to Y = and type in that whole equation, then hit Graph, then did 2nd Graph and found the maximum value

Alt+F4 said:
http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/fa02 [Broken]

Why is Question 11 C??
is there an explanation for this one or is this one of the thing you memorize

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OlderDan
Homework Helper
Alt+F4 said:
i went to Y = and type in that whole equation, then hit Graph, then did 2nd Graph and found the maximum value
When I do that I get 571.9W using their value of 0.436 for phase in radians, which is correct. What do you get?

OlderDan
Homework Helper
Alt+F4 said:
http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/fa05 [Broken]

So i tried doing it on Calculator but since sin180 = 0 i get 0
It is not sin180. It is sin180t with t constantly changing. You need to find the maximum rate of change of the flux and use the number of turns to find the maximum emf.

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OlderDan
Homework Helper
Alt+F4 said:
http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/fa02 [Broken]

Why is Question 11 C??
Did you find the current in the circuit? Did you compute the voltages for each of the elements from that current?

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OlderDan
Homework Helper
Alt+F4 said:
Last Question of the day i promise :), thanks for everyones help

http://online.physics.uiuc.edu/cgi/courses/shell/phys102/fall06/prep2a.pl?practice/exam2/fa05 [Broken]

Question 3

Why is it just (3)(.7*.2*2) = .084 Why not take into account the left side. Thanks
This is from the left side. It is the only side in the magnetic field. The right side contibutes nothing.

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OlderDan said:
When I do that I get 571.9W using their value of 0.436 for phase in radians, which is correct. What do you get?
same but the answer is 543 that is why im so confused