For those with the Princeton Review "Cracking the MCAT CBT 2nd Ed"

[Byrne]

This has been driving me crazy for a little while now and I *think* there might be an error in the book, but I guess I could be looking at it wrong.

 

On pages 180-181 they talk about the right-hand rule that determines the direction of force on a charged particle under the influence of a magnetic field.

 

So, here's what the text says to do:

- Fingers in direction of the magnetic field

- Thumb in direction of current (flow of positive charge)

 

The direction in which the palm of the right hand faces is the direction of force on a positively charged particle.

 

Sounds easy enough. However, if you read on to the example on page 180 and then the problem on page 181, when I use the above rules, I end up getting the opposite of what the book suggests the direction of the force is.

 

I suppose anyone can answer this, but here is a summary of the problems--sorry I can't actually draw the diagrams they have, hopefully words will suffice:

 

1.

 

- The magnetic field ( is orientated upward

- Electron movement is left to right, perpendicular in direction to the magnetic field

 

What is the direction of the force on an electron?

 

2.

 

- Electron movement is upward

- Electron passes threw a magnetic field, causing the electron to be deflected into the page (or screen)

 

What is the direction of the magnetic field?

 

Any insight on these problems is appreciated as I am getting the exact opposite to what the correct answers supposedly are.

[OneDay]

For positive charge (current) use right hand rule

For negative charge (electrons) use left hand rule

 

That's from the TPR science book

[spicandspan]

For positive charge (current) use right hand rule

For negative charge (electrons) use left hand rule

 

OneDay is correct. Also, you sort of answered the question yourself:

 

...On pages 180-181 they talk about the right-hand rule that determines the direction of force on a charged particle under the influence of a magnetic field.

 

So, here's what the text says to do:

- Fingers in direction of the magnetic field

- Thumb in direction of current (flow of positive charge)

 

The direction in which the palm of the right hand faces is the direction of force on a positively charged particle...

[Byrne]

Weird there's no mention of a left hand rule in the book I use.

 

It says if the particle is negatively charged, the direction of the force is opposite the palm.

 

Whatever the case, what do you guys say the answers would be to those two problems I posted? Like I said, I think the book might be wrong.

[Avinash]

I suppose anyone can answer this, but here is a summary of the problems--sorry I can't actually draw the diagrams they have, hopefully words will suffice:

 

1.

 

- The magnetic field ( is orientated upward

- Electron movement is left to right, perpendicular in direction to the magnetic field

 

What is the direction of the force on an electron?

 

2.

 

- Electron movement is upward

- Electron passes threw a magnetic field, causing the electron to be deflected into the page (or screen)

 

What is the direction of the magnetic field?

 

Any insight on these problems is appreciated as I am getting the exact opposite to what the correct answers supposedly are.

 

As was previously mentioned, the right hand rule is opposite for negatively charged particles (such as the electron). The easiest way to do the problem for negatively charged particles is to point your thumb in the direction opposite to the electron's velocity, then point your fingers in the direction of the magnetic field, and the force exerted on the electron will come out of your palm.

 

Here are my answers:

1. into the page

2. right to left

[Byrne]

As was previously mentioned, the right hand rule is opposite for negatively charged particles (such as the electron). The easiest way to do the problem for negatively charged particles is to point your thumb in the direction opposite to the electron's velocity, then point your fingers in the direction of the magnetic field, and the force exerted on the electron will come out of your palm.

 

Here are my answers:

1. into the page

2. right to left

 

 

Which is exactly what I came up with.

 

I'm hoping someone here is using the same book I am because maybe I'm reading the question wrong. If not, TPR screwed up.

[always_panicked]

Hey hey....so I'm using the same book....are you talking about Figure 10.19 on page 180? I did it and it works for me....the electron is deflected out of the page because it shows you the electron moving from right to left, and you have to have to reverse what you would get because its a negative charge (i.e. your palm would be facing down if you do the right hand rule with the electron, but you reverse it to face up for a positive charge).

[Byrne]

Hey hey....so I'm using the same book....are you talking about Figure 10.19 on page 180? I did it and it works for me....the electron is deflected out of the page because it shows you the electron moving from right to left, and you have to have to reverse what you would get because its a negative charge (i.e. your palm would be facing down if you do the right hand rule with the electron, but you reverse it to face up for a positive charge).

 

I think this is where I may be missing something -- electron movement is right to left. Why is it that you point your thumb in the direction of electron movement?

 

I've been pointing my thumb in the direction opposite of electron movement as the text states to point your thumb in the direction of current (opposite the direciton of electron flow). When pointing your thumb to the right (direction opposite of electron flow), you get your palm facing up. Book states that the direction of the force on a positvely charged particle is where the palm faces -- so I reverse this direction since the problem deals with an electron.

 

... where am I going wrong here?

[always_panicked]

If you point your thumb towards the right and extend your fingers so that they are pointing to the top of the page, your palm should be facing up...are you doing that, and your palm is facing down (I can't even bend my hand that way! hehe)

 

edit: wait, I misread what you said! Yes, so your palm is facing up, but because you already pointed your thumb to the right because you were thinking of the current (positive) flow, your palm is already facing in the correct direction. If you had pointed your thumb to the left, which would be in the direction of the electron, THEN you would reverse whatever you get (which is what the book is saying)...I know, it can get confusing!

[Byrne]

If you point your thumb towards the right and extend your fingers so that they are pointing to the top of the page, your palm should be facing up...are you doing that, and your palm is facing down (I can't even bend my hand that way! hehe)

 

edit: wait, I misread what you said! Yes, so your palm is facing up, but because you already pointed your thumb to the right because you were thinking of the current (positive) flow, your palm is already facing in the correct direction. If you had pointed your thumb to the left, which would be in the direction of the electron, THEN you would reverse whatever you get (which is what the book is saying)...I know, it can get confusing!

 

So I sort of took care of the negative charge by pointing my thumb to the right and then reversed it back by taking the direction opposite to what my palm was facing?

 

If so, TPR did a poor job explaining the RHR in the text!

 

The book instructs:

- Point thumb in direction of current*

- Palm faces direction of positive charge (opposite of palm if negative charge)

 

If you follow those instructions, you get your palm facing up, but direction of the force would be opposite the palm since you're dealing with an electron.

 

*I don't understand why the word "current" is used. Usually you're dealing with a charged particle that travels through a magnetic field. If the magnetic field vector is given, there is no need to know which direction current flows in--you're dealing with the force exerted on a single charged particle in space, there is no current. I think the authors are confusing the RHR for determining the magnetic field associated with current flowing through a wire with the RHR for determining the direction of the force exerted on a charged particle exposed to a magnetic field.

 

All that needed to be said was something along the lines of: "Point thumb in the direction of the charged particle's movement and fingers in the direction of the magnetic field. If the particle is positively charged, palm represents the force vector. If the particle is negatively charged, opposite of palm represents the force vector." OR "Point thumb in the direction of the charged particle's movement if the particle is positive. If the particle has a negative charge, point thumb opposite the direction of the particle's movement. In either case, if fingers are oriented with the magnetic field, the palm represents the force vector."

[always_panicked]

Yes, you've got it!

 

Yeah, the TPR book as some mistakes in it that I've picked up, as well as a lot of bad grammar! Have you looked at the aldol reaction yet? I think there's a lot of mistakes in that as well...

[Byrne]

Alright, thanks for the help