Help Understand Force Direction with Picture

[james11223]

Homework Statement

There is no problem, I just don't understand in which direction the force will go. Please refer to the picture below.
THANKS

Homework Equations

The Attempt at a Solution

I HAVE NO IDEA! please someone help... I'm desperate

 

[Fewmet]

Homework Statement

There is no problem, I just don't understand in which direction the force will go. Please refer to the picture below.
THANKS

Homework Equations

The Attempt at a Solution

I HAVE NO IDEA! please someone help... I'm desperate

Do you know the right and left hand rules? They apply here.

 

[james11223]

Yes I do, can you please tell me the answers that you got though?

 

[Fewmet]

Yes I do, can you please tell me the answers that you got though?

Our goal here is to help you figure out answers rather than just give them. You said you have no idea on what to do, but you know the hand rules. Just apply them.

In the the first drawing, you have a magnetic field moving into the plane of the drawing and a positive charge moving to the right. What does the right hand rule tell you about the direction of the force acting on the positive charge?

 

[james11223]

okay so would this be the answer?

 

[james11223]

Would this be the second answer?

 

[james11223]

Ohhh oops.. I got those backwards...

would the frist one be going up?
and the second one coming towards me?

 

[james11223]

Okay so my final answer is:
1) Upward
2) Out of the paper
3) Upward
4) I HAVE NO IDEA!

can someone please help me with 4 and verify my other answers? thanks

 

[Fewmet]

1) Correct
2) Correct
3) No: magnetic field lines run from the north to the south poles. Do you know that the circle with the dot in it is meant to be a head-on view of an arrow hear? It means the electric current is coming out of the page.
4) This is the tricky one. I'm not sure what that symbol is above the arrow above the V... is it an "a"? "E"?
That aside, do you know that electric current in a wire creates a magnetic field concentric about it? (It might help to think that is the explanation of an electromagnet.) What way do the field lines go?

 

[james11223]

Oh okay!

3) To right

4) That is a "q". But I still really do not understand how you would do this one. I get that your thumb follow the arrow above the V and then your four fingers go into the paper. Thus the way my palm is facing shows that the force would be upwards. But would that be reversed since the current is flowing counterclockwise? So would the answer be downwards?

 

[Fewmet]

4) That is a "q". But I still really do not understand how you would do this one. I get that your thumb follow the arrow above the V and then your four fingers go into the paper. Thus the way my palm is facing shows that the force would be upwards. But would that be reversed since the current is flowing counterclockwise? So would the answer be downwards?

OK: I now see the q in the first diagram. I should have been able to work that out.

4) The direction of the magnetic field around a current-bearing wire is given by a slightly different right hand rule (right hand assuming you are treating electricity as the flow of positive charge). If the thumb points in the direction of the current, the fingers wrap in the direction of the magnetic field. In this case that gives you the magnetic field field coming out of the page within the ring and going into the page outside of the ring.

Note that this is the opposite field direction than you had in the first diagram.

 

[james11223]

Okay, but how would you determine where the force is?

 

[Fewmet]

Okay, but how would you determine where the force is?

The same as you did with the others. The field lines come out of the page. The charge moves to the right. Twisting you right hand so your fingers points toward you and your thumb points right, your palm (which gives the direction of the force) goes down.

 

[james11223]

Dont the field lines go into the page? Because the symbol is a + inside of circle, so wouldn't that mean they go into the page?

 

[Fewmet]

Dont the field lines go into the page? Because the symbol is a + inside of circle, so wouldn't that mean they go into the page?

I see the problem now: that is a +, not an x. It is giving the charge of the particle as positive, not the direction of a field into the page. It is a subtle difference in notation that can be easily confused, especially when written by hand.

 

[james11223]

OHH! so how would you do it in this case??

would your thumb align with the positive charge??, but then what else would you do..

 

[Fewmet]

OHH! so how would you do it in this case??

would your thumb align with the positive charge??, but then what else would you do..

As always, your thumb goes in direction the charge is moving (to the right) and you fingers point in the direction of the magnetic field lines.

 

[james11223]

but since when is there a magnetic field?
I thougt there was just a current going counterclockwise?

 

[Fewmet]

but since when is there a magnetic field?
I thougt there was just a current going counterclockwise?

As I asked in post #9, "do you know that electric current in a wire creates a magnetic field concentric about it?". Then, In post #11 I detailed how to find the direction of the field.

Have another look at those and let me know if it is still not clear.

 

[gneill]

but since when is there a magnetic field?
I thougt there was just a current going counterclockwise?

A flowing current creates a magnetic field. In particular, a current in a wire produces a magnetic field that surrounds the wire with field that encircles the wire concentrically ... if the wire is straight the field is nice and symmetrical in this fashion.

The direction of the field can be found with a right-hand-rule. If you grasp the wire in the right hand with the thumb pointing in the direction of the (conventional) current flow, then the fingers will wrap around the wire in the direction that the field lines wrap around the wire.

You have a loop with a counterclockwise current flowing. If you "grab the wire" as stated, the field lines all come out of the page in the interior of the loop, and descend into the page on the outside of the loop. So the positive charge traveling to the right in the loop is proceeding through a magnetic field that is directed out of the page. What direction will the resulting force have?

 

[james11223]

I seriously do not understand, I think I'd get it for the next problems if you just gave me the answer for this one so that I could use the same method for the others. I am self-studying this course and I seriously cannot explain this one, can you please at least guide me through it but give me the answer in the process? Because I am very lost on this one..

 

[Fewmet]

I seriously do not understand, I think I'd get it for the next problems if you just gave me the answer for this one so that I could use the same method for the others. I am self-studying this course and I seriously cannot explain this one, can you please at least guide me through it but give me the answer in the process? Because I am very lost on this one..

In that fourth situation, the clockwise current causes a magnetic field out of the page. From the right hand rule, a positive charge moving right across a field coming out of the page experience a downward force.

 

[gneill]

I seriously do not understand, I think I'd get it for the next problems if you just gave me the answer for this one so that I could use the same method for the others. I am self-studying this course and I seriously cannot explain this one, can you please at least guide me through it but give me the answer in the process? Because I am very lost on this one..

What part do you not understand? Once you have the magnetic field direction, the problem becomes similar to the one in the first image where there's a positive charge moving in a magnetic field of a specified direction.

FYI, we can't just give answers on Physics Forums; we can only help you to find them

 

[gneill]

In that fourth situation, the clockwise current causes a magnetic field out of the page. From the right hand rule, a positive charge moving right across a field coming out of the page experience a downward force.

counterclockwise.

 

[james11223]

Okay... I think I'm understnading.
Is the general consensus that the force is downwards into the paper?

 

[tony.stonem23]

james, as fewmet stated, the fourth one is quite different right hand rule. physics forums strongly opposes to disclosing answers as you should be conducting your own work with intellectual integrity. the direction of the force is down.

 

[james11223]

Oh wow I LOVE SKINS <333
but is the force down into the paper or just in the downwards direction? As this is only to clarify and not giving me the answer outright

 

[gneill]

Okay... I think I'm understnading.
Is the general consensus that the force is downwards into the paper?

"Downwards" and "into the paper" are two separate directions.

The important thing is to not get answers without understanding them. Did you understand the explanation for the existence of and direction of the magnetic field in the "loop problem"?

 

[gneill]

Oh wow I LOVE SKINS <333
but is the force down into the paper or just in the downwards direction? As this is only to clarify and not giving me the answer outright

I'm not sure how giving you the answer is not giving you the answer.

 

[james11223]

HAHAHA yes I get it now, thanks so much! So to confirm... the answer is downwards correct?

 

[gneill]

Here's a bonus question for your edification. See the figure. What direction is the force on the negatively charged particle moving in the magnetic field as shown?

 

[james11223]

gneill, this is an elementary question, the answer is into the paper!

 

[gneill]

gneill, this is an elementary question, the answer is into the paper!

Well, there you are. Very good. You should have no trouble with other such questions, and you should be able to confirm for yourself your answers for the other problems that were presented.