Struggling with Practice Test Questions on Electromagnetism?

[AznBoi]

http://img148.imageshack.us/img148/7599/981718254748dd2.png
My work, my answers are wrong obviously.

18. $$U_{E}=\frac{kq1q2}{d}$$ -- I substituted the electric force with the work equation to get this.

25. Q=mL, Q=mcT . I combined these to get mL=mcT, stuck from there.

47. F=q(v x B), v=F/qB, F=mv^2/r
I combined these equations to get: mv/r=qB and solved for v and I then I just guessed.

48. Why is the answer D?? I know that it has something to do with lenz's law but I only know how to apply that law for magnets moving with either North or South pole. (If a north pole of a magnet moves towards the wire the induced current is counter clockwise right? If the south pole moves into the wire it is clock wise?)-- Is this right?

Thanks for your help guys! =]

 

[mukundpa]

Sorry we are not giving answers as such. You may get hints for your doubts and confusions only. Show your approach in case not getting the correct answers.

 

[AznBoi]

Oh, I just posted the picture first to see if it worked lol. Anyways, my attempts are on the board! Do you want me to post the answers too? I have the answer key btw, just not the solutions.

 

[mukundpa]

Is the diagram is given? Should be, otherwise what is Q? what about the position of the point P?

 

[AznBoi]

Is the diagram is given? Should be, otherwise what is Q? what about the position of the point P?

Yeah, I realized that lol. I forgot to include the picture because I got 17 right. So for 17, all I did was add the two electric field vectors (one to the left and one down) to get the resultant vector which is (c) right? Thanks.

 

[mukundpa]

Now it is clear. There are two charges Q both at a distance d from the third corner of the square.

Find potential V at that point due to both Q charges (Potential is a scalar quantity) then multiply q and V.

 

[mukundpa]

For the next heat is required to first increase the temperature of ice up to the melting point then latent heat to melt and then to increase temperature of water.

 

[mukundpa]

For the next you got the relation for v. Put the value of e/m for proton and probable values of B and r and will get the order of the velocity.

 

[AznBoi]

Now it is clear. There are two charges Q both at a distance d from the third corner of the square.

Find potential V at that point due to both Q charges (Potential is a scalar quantity) then multiply q and V.

So how do you find the potential v at the point? I know that $$V=\frac{U_{e}}{q} =Ed$$
I don't get what you mean by then multiply q and v.

 

[mukundpa]

The potential at that point will be KQ/d due to each charge Q so what is the total potential? and what is the potential energy?

 

[AznBoi]

The potential at that point will be KQ/d due to each charge Q so what is the total potential? and what is the potential energy?

OKay, so $$\frac{kQ}{d}+\frac{kQ}{d}=2\frac{kQ}{d} *q= 2k\frac{qQ}{d}$$? So how do you know that you need to add the potential differences rather than just the potential energies? You can do it either way right? So do you just add them because they are a scalar? Is there any time where you need to subtract them? (mabye when the charges are opposite?) So if the quantities are scalar you just add them and if they are vectors you need to use pythagorean or trig right?

 

[mukundpa]

Going in the right direction.