Multiple Choice (1 Viewer)

[tomdoml]

multiple choice with explanation.
1- C (obvious)
2- B (obvious)
3- B (obvious)
4- C (obvious)
5- B (obvious)
6- C (back emf is induced through changing flux in lens's law)
7- C (calculation)
8- A (initially 1 due to reaction force, gradually increasing due to increased acceleration in loss of fuel mass, 0 one stopped)
9- B (obvious)
10- D (obvious)
11- D (obvious)
12- A (right hand rule)
13- A (group V dopant increases conductivity and electrons in conduction band)
14- A (measures EMF across x and y, which does exists as the flux is changing as it enters the field, but when in the field no flux is changing, when it leaves there is another change in flux. The emf is a potential and can exist without the flow of current.)
15- C (calculation)
16- B (v=sqrt(GM/r))
17- B (E=hf - W, f=c/y, E=hc/y - W, hyperbolic graph, y = wavelength)
18- B (calculation)
19- D (obvious)
20- C (since gpe decreases towards the center of mass, you decrease GPE as you move closer to the centre. Thus r=0.5r, thus 4 times the regular gravitational force.)

 

[cheezcake]

Thank god, i thought i was insane when i was getting a bazillion C's and B's at the start

 

[zhuang281]

multiple choice with explanation.
1- C (obvious)
2- B (obvious)
3- B (obvious)
4- C (obvious)
5- B (obvious)
6- C (back emf is induced through changing flux in lens's law)
7- C (calculation)
8- A (initially 1 due to reaction force, gradually increasing due to increased acceleration in loss of fuel mass, 0 one stopped)
9- B (obvious)
10- D (obvious)
11- D (obvious)
12- A (right hand rule)
13- A (group V dopant increases conductivity and electrons in conduction band)
14- A (measures EMF across x and y, which does exists as the flux is changing as it enters the field, but when in the field no flux is changing, when it leaves there is another change in flux. The emf is a potential and can exist without the flow of current.)
15- C (calculation)
16- B (v=sqrt(GM/r))
17- B (E=hf - W, f=c/y, E=hc/y - W, hyperbolic graph, y = wavelength)
18- B (calculation)
19- D (obvious)
20- C (since gpe decreases towards the center of mass, you decrease GPE as you move closer to the centre. Thus r=0.5r, thus 4 times the regular gravitational force.)

20 should be D, i almost got tricked as well. Look at what it says, GPE is halved. Say it was originally -1, now its -0.5....Now if you think about it this signifies it has moved closer to infinity, ie the radius is now 2r not r/2

 

[IamBread]

1.C
2.B
3.B
4.C
5.B
6.C
7.C
8.A
9.B
10.B (wasn't too sure about this, I couldn't remember what the independent variable was..)
11.D
12.D
13.A
14.C
15.C
16.D (Both B and D seemed to makes sense for this question so I'm not sure..)
17.E
18.B
19.D
20.D

 

[kangy93]

1.c
2.b
3.b
4.c
5.b
6.c
7.c
8.a
9.b
10.d
11.d
12.a
13.a
14.a
15.c
16.b
17.b
18.b
19.d
20.d

This

 

[brent012]

99.9% sure these answers are right, i went up to a phys teacher to confirm my answers were correct lol.
1.C
2.B
3.B
4.C
5.B
6.C
7.C
8.A
9.B
10.D
11.D
12.A
13.A
14.A
15.C
16.B
17.B
18.B
19.D
20.D
Im more then happy to explain reasoning if you wish

I'm not disputing your answers, but since you offered can you explain 14 and 15 for me?

Lol i did get a couple of others wrong (instantly knew when answering = P) but 15 i dont understand how you got C and 14 i dont understand A.

 

[zhuang281]

Ok, when the Ring moves into the field, the field lines passing through the ring is increasing correct? So according to lenz's law, this will induce an EMF which will produce another magnetic field which opposed this increase, ie creates a magnetic field out of the page to counteract this increase. When it moves out of the field, the field lines passing through the ring is decreasing. Again according to lenz's law, a magnetic field is created which opposes this decrease, ie creates a field into the page. So to recap, as it moves into the field, a magnetic field is produced which is out of the page. When it moves out of the field, a magnetic field is produced which is into the page. What causes this change in direction of magnetic field produced? A change in direction of EMF. Ie A. Rather long explanation i know, hope you understand lol.

Now for 15. Since this asks for time to hit floor, the only thing that matters is vertical velocity which is 0 at the start. Using S= UT +.5at^2, where U = 0 therefore S= 4.9t^2, where S = 1. therefore T = 0.45 = c

 

[utanobeiiby]

99.9% sure these answers are right, i went up to a phys teacher to confirm my answers were correct lol.
1.C
2.B
3.B
4.C
5.B
6.C
7.C
8.A
9.B
10.D
11.D
12.A
13.A
14.A
15.C
16.B
17.B
18.B
19.D
20.D
Im more then happy to explain reasoning if you wish

yes i believe you are all correct ! hope you are cause that means i am too xD

 

[utanobeiiby]

ok since i got the same as him i'll might be able to help
for 15 this is a simple projectile question use y=uyt+1/2at^2 with uy=0 since the table is horizontal and delta-y is -1 since the marble dropped off the tabke to the ground.... then you will end up with 4.9t^2 =1 and so t= squareroot of 1/4.9 which gives C

14: well this one can be done quickly if you realise that the emf would go through a change in polarity as it enters and leave the field. when the ring enters the field the emf will be induced in one direction and when it leaves the field the emf MUST be in the opposite direction to its initial direction. also when the ring is within the field there is no change in magnetic flux as the field is uniform so emf=0 which makes A the obvious answer

 

[khfreakau]

WTF HOW THE FK DID I HEADER Q18... LOL. 19/20 for me.

 

[taeyang]

If grav Ep is halved, then radius is doubled... So according to F = Gm1m2/d^2 , Grav force is quartered... Hence answer is D .... i said this earlier

I don't think this is right; gravitational potential energy is the work done to move an object from a very large distance away to a point in a gravitational field; this being said, that at an infinite distance, the GPE = 0, at any distance lower than infinity, the GPE < 0. Therefore as you get closer to a planet, or mass for that matter, your gravitation potential energy is less, therefore if you increase the radius, you are increasing the gravitational potential energy. Ergo it doesn't decrease with alititude, it increases.

EDIT: I don't really know what the question was asking, I forgot already, it might be something ENTIRELY different haha, sorry if it is. I may also be COMPLETELY wrong in which I just made a total fucking idiot out of myself.

 

[COLDBOY]

i think 14 should be D, as i was trying to figure out the answer, i kept alternating between A and D, however then i realised that, the split ring is attached to a rope, which does not conduct electricity and further assuming that nothing was changed in the setup, currents will be produced, however no emf, because there is no complete circuit, this is stated in many textbooks, im not saying it is D, but im suggesting that it could be.

 

[jamesfirst]

Shouldn't 20 be C? The gravitational potential energy is half its initial value, ie it's moving closer to earth. Twice as close, therefore 4 times the gravitational force, right?

How can F increase when the radius is increasing ?

 

[jamesfirst]

How is 14 A when the pendulum only swings across the magnetic field once ???....

 

[jamesfirst]

What? As u said the Repulsive force is dependant on the current? Sooo te current is the independant? If im mistaken, the distance between them will change unless the top rod is fixed.

Um, the question even says "distance is changed" or something. So it HAS to be the distance...

 

[jamesfirst]

I got A as well. Once the pendulum is in the magnetic field there is no change in flux, so the induced (whatever it was) must be zero?

How is it 0 ??...

 

[taeyang]

How is it 0 ??...

As the pendulum enters the magnetic field there is a change in magnetic flux which makes it slow down, but when it is inside the loop, the pendulum is going through the same magnetic flux/flux density and hence their is no change when it swings through the magnetic field. However, when it leaves the magnetic field, there is again a change in magnetic flux which makes it want to come back into the magnetic field, lenz's law of course. So, the eddy currents in the pendulum appear, leave and appear again, but in different directions.

 

[_deloso]

I don't think this is right; gravitational potential energy is the work done to move an object from a very large distance away to a point in a gravitational field; this being said, that at an infinite distance, the GPE = 0, at any distance lower than infinity, the GPE < 0. Therefore as you get closer to a planet, or mass for that matter, your gravitation potential energy is less, therefore if you increase the radius, you are increasing the gravitational potential energy. Ergo it doesn't decrease with alititude, it increases.

EDIT: I don't really know what the question was asking, I forgot already, it might be something ENTIRELY different haha, sorry if it is. I may also be COMPLETELY wrong in which I just made a total fucking idiot out of myself.

you have a point but it says half the original gpe. Gpe us inversely proportional to radius so if gpe is halfed then radius would be doubled. that's why we use 2r

 

[o_0]

uhh question 16 can only be B

A is wrong; satellite undergoes centripetal acceleration in orbit
C is wrong; there is NO thrust when a satellite is in orbit. The only force acting on it is the gravitational force (same as centripetal)
D is wrong; centripetal force IS the gravitational force

 

[wretched]

Guys PLEASE PLEASE PLEASE PLEASE can some one just take a moment to upload their physics multiple choice section. PLEASE I will love you forever!!!!!