Essential electricity formulas

[tncekm]

So, I've got all of these formulas, and I was wondering, which are essential and which (if any) aren't "necessary" for the MCAT.

F = KqQ/r2
F = Eq
(where E=KQ/r2)
U=Eqd
(where F=Eq and U = potential energy of a charge in an electric field)
V=Ed
Φ = kQ/r
V=ΔΦ
PE = qV = qΔΦ
(Where Φ is the electric potential.)
Welectric_field = -ΔPE = ΔKE
Note: ΔKE = -ΔPE

I've got a decent handle on most of this stuff, but it seems like there is just "too much" here. I know there are definitely a few "absolutes" in here, but I put them all up anyway

Thanks

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[tncekm]

Okay, I want to go ahead and add to this thread a few more equations I really wanna sift through

Energy With Outside Forces
E = KEi + PEi + WbyFriction = KEf + PEf
ΔE = ΔKE + ΔPE - W

Work
(K = Kinetic Energy, U = Potential Energy, E = Internal Energy)
W = ΔK + ΔU + ΔEi (no heat)
W = ΔK + ΔU (no friction, no heat)
fkdcosθ= ΔK + ΔU

And, do we need to memorize this doppler effect equation, or are the doppler approximation formulas sufficient?
fd/fs = (v±vd)/(v±vs)

 

[Kaustikos]

Okay, I want to go ahead and add to this thread a few more equations I really wanna sift through

Energy With Outside Forces
E = KEi + PEi + WbyFriction = KEf + PEf
ΔE = ΔKE + ΔPE - W

Work
(K = Kinetic Energy, U = Potential Energy, E = Internal Energy)
W = ΔK + ΔU + ΔEi (no heat)
W = ΔK + ΔU (no friction, no heat)
fkdcosθ= ΔK + ΔU

And, do we need to memorize this doppler effect equation, or are the doppler approximation formulas sufficient?
fd/fs = (v±vd)/(v±vs)

I'll say this; you want to familiarize yourself with the doppler for sure. That formula SEEMS tricky, but it isn't. As long as you understand the basics of it, the formula makes absolute sense.

And all the rest of the electrical formulas just fall into play. They're all similar and easy to memorize. The worst was memorizing the constants.

 

[tncekm]

I can apply the doppler formulas no problem, but I just didn't feel like memorizing the one I posted. However, I think its accidentally stuck in my head since I can now magically come up with it off the top of my head

The only ones I think I'm actually not very strong on are the work and energy formulas, but I guess it makes sense that losses or gains in energy of a system are the direct result of work (or heat). I think I just need some practice applying the formulas. They were just "given" and I never got to use them.

The constants? Which constants were you memorizing? I thought we didn't have to memorize any major constants, other than things like density of water, K of air (for capacitance), etc?

 

[Vihsadas]

Realistically I'd say that you could be asked to recall any of the equations that you've mentioned. The way that you make it so this isn't a problem for you is that you've got to learn the concepts behind where these equations come from. Then, just plugging one eqn into another can help you derive everything you need. An easy way to remember the emag equations is to think of them as a progression from the force eqn:

F = kQq/r^2

Since E-field only deals with one charge and not the interaction between charges, take away one charge for electric field:

E = KQ/r^2


Since electric potential doesn't follow the inverse square distance law take away a dimension of distance:

V = KQ/r

Do you see how there is a kind of progression there? Then, you can just mix and match to derive the other 'short cut' formulas:

F = Eq
V = Ed
U = Fd = qEd = qV (<== See what happened there? In your list you have these as separate eqns, but see they are the same thing if you know the basics?)

 

[tncekm]

Thanks for the suggestion! I was actually doing that earlier, because I find it does help me memorize equations better. GMTA, right? I think part of my problem was that I looked through equation lists from both TPR and EK and didn't realize right off that some of the equations were exactly the same, just in slightly different forms.

E.g. PE = Eqd = U = qV, etc.

*sigh* While I do "remember" most of these formulas, some of them I may need some practice with because my classes didn't deal with them directly. Oh well, no shortcuts on the MCAT, right?

Thanks again

 

[physics junkie]

Make sure to know all your equations. The mcat didn't provide them when I took it. You should have an intuitive grasp for these equations already...maybe not. Remember the right hand rule too. F= Qv x B. F points up and is your thumb. v points straight ahead and is your index finger. B is your middle finger and points in the direction of your palm. Also, it would be wise to know which way a positive/negative particle moves in an electric field. Do we go positive to negative or negative to positive?

 

[iA-MD2013]

Make sure to know all your equations. The mcat didn't provide them when I took it. You should have an intuitive grasp for these equations already...maybe not. Remember the right hand rule too. F= Qv x B. F points up and is your thumb. v points straight ahead and is your index finger. B is your middle finger and points in the direction of your palm. Also, it would be wise to know which way a positive/negative particle moves in an electric field. Do we go positive to negative or negative to positive?

positive to negative.
you really are a physics junkie...arent you? physics is more fun than anything.

 

[RSAgator]

while we're on the topic of the electricity I figured I'd also throw out some stuff. Aside from knowing that field lines move in the direction of positive charge, also remember that electrons move in the opposite direction of current (particularly helpful in electrochemistry) and that in magnetism field lines move north to south.

 

[Kaustikos]

while we're on the topic of the electricity I figured I'd also throw out some stuff. Aside from knowing that field lines move in the direction of positive charge, also remember that electrons move in the opposite direction of current (particularly helpful in electrochemistry) and that in magnetism field lines move north to south.

I hated learning that. It was like a complete mind-****.

 

[Kaustikos]

Also, it would be wise to know which way a positive/negative particle moves in an electric field. Do we go positive to negative or negative to positive?

Wait, what? Do you mean which way a particle travels in an electric field? Doesn't the test-charge sign matter in that regard? I'm confused.

 

[bluemonkey]

Wait, what? Do you mean which way a particle travels in an electric field? Doesn't the test-charge sign matter in that regard? I'm confused.

Positive charges always move in the direction of the electric field lines (from high to low potential) while negative charges move in the opposite direction of electric field lines (from low to high potential). Keep in mind that electric fields originate from positive charges and either go to infinity or terminate on negative charges. So it makes sense that positive charges would go in the direction of field lines (away from the positive source charges) and negative charges would go against the field lines (towards the positive source charges). Granted this a very simplified explanation, but it's enough for the MCAT.

 

[Kaustikos]

Positive charges always move in the direction of the electric field lines (from high to low potential) while negative charges move in the opposite direction of electric field lines (from low to high potential). Keep in mind that electric fields originate from positive charges and either go to infinity or terminate on negative charges. So it makes sense that positive charges would go in the direction of field lines (away from the positive source charges) and negative charges would go against the field lines (towards the positive source charges). Granted this a very simplified explanation, but it's enough for the MCAT.

D'oh. Test charges are always designated as positive, sorry. I need to get that down. I always remembered electric field lines but I thought, incorrectly, they for some magical way could originate from negative charges.

WRONG

-1

Thanks, though.

 

[physics junkie]

positive to negative.
you really are a physics junkie...arent you? physics is more fun than anything.

Yes, yes I am. My only regret about medical school is that I won't be able to pursue physics...hopefully I can get involved in some kind of biophysics research. My goal is to be somewhat competent in quantum field theory by the time I graduate medical school so I can play around a bit with string theory at some point to see what all the fuss is about. Will I get there? Probably not.

 

[tncekm]

Make sure to know all your equations. The mcat didn't provide them when I took it. You should have an intuitive grasp for these equations already...maybe not. Remember the right hand rule too. F= Qv x B. F points up and is your thumb. v points straight ahead and is your index finger. B is your middle finger and points in the direction of your palm. Also, it would be wise to know which way a positive/negative particle moves in an electric field. Do we go positive to negative or negative to positive?

Wait, would we need to memorize the cross-product form that you mention, or is F=qvBsin&#952; enough?

Thanks for all of your input, everyone.

 

[Vihsadas]

Wait, would we need to memorize the cross-product form that you mention, or is F=qvBsin&#952; enough?

Thanks for all of your input, everyone.

You don't need to know the cross-product version of the formula. The Sin&#952; version is fine.

 

[tncekm]

Thanks!

 

[iA-MD2013]

Yes, yes I am. My only regret about medical school is that I won't be able to pursue physics...hopefully I can get involved in some kind of biophysics research. My goal is to be somewhat competent in quantum field theory by the time I graduate medical school so I can play around a bit with string theory at some point to see what all the fuss is about. Will I get there? Probably not.

Wow, that's awesome. You will get there! That's a great goal. I kinda regret not getting into physics research in undergrad, it seems incredibly interesting.
I've hijacked you're thread tncekm

 

[tncekm]

That's alright. Just because you've claimed to be the "World's First Analrapist", you're on my good side for now

 

[iA-MD2013]

That's alright. Just because you've claimed to be the "World's First Analrapist", you're on my good side for now

hahahah!!! I love arrested development. that's my tribute to the show.

 

[tncekm]

I don't know anything about a show called Arrested Development

 

[unsung]

Wait, would we need to memorize the cross-product form that you mention, or is F=qvBsin&#952; enough?

Thanks for all of your input, everyone.

Remember though, A X B always = ABsin&#952; (cross product), while A*B = ABcos&#952; (dot product).

This probably isn't an MCAT essential item, but it helps me to remember when/how to use the R-hand rule. All the formulas that involve cross products, I can use the R-hand rule to determine direction.

Like F = qv X B, index in the direction of v, middle finger in the direction of B, then thumb is now pointing in direction of F. There are so many variants of how to use the R-hand rule, but I think this is probably my fav, as it's the clearest.

 

[iA-MD2013]

I don't know anything about a show called Arrested Development

that's no good. no good at all. it may seem a little weird that i have analrapist written underneath my profile name, doesnt it?

 

[tncekm]

that's no good. no good at all. it may seem a little weird that i have analrapist written underneath my profile name, doesnt it?

noooo.....

 

[iA-MD2013]

noooo.....

http://video.google.com/videoplay?docid=-7562860158049514094

 

[Vihsadas]

Remember though, A X B always = ABsin&#952; (cross product), while A*B = ABcos&#952; (dot product).

This probably isn't an MCAT essential item, but it helps me to remember when/how to use the R-hand rule. All the formulas that involve cross products, I can use the R-hand rule to determine direction.

Like F = qv X B, index in the direction of v, middle finger in the direction of B, then thumb is now pointing in direction of F. There are so many variants of how to use the R-hand rule, but I think this is probably my fav, as it's the clearest.

I will agree that knowing how the cross-products fit with the axes ties together why the right-hand rule works, and therefore, makes it trivial to remember the right hand rule. It isn't, however, necessary for the MCAT to know the cross-product formulas, or their translations to trig formulas. It's probably easier for most people just to memorize how to use the right hand rule. The down side is that memorizing how to use something doesn't mean you understand it!

 

[physics junkie]

I will agree that knowing how the cross-products fit with the axes ties together why the right-hand rule works, and therefore, makes it trivial to remember the right hand rule. It isn't, however, necessary for the MCAT to know the cross-product formulas, or their translations to trig formulas. It's probably easier for most people just to memorize how to use the right hand rule. The down side is that memorizing how to use something doesn't mean you understand it!

Oh, when I read your first post I was kind of annoyed that you didn't think that you need to know what F=Qv X B is(and thus the direction an electron will travel in a magnetic field) because you have F=qvsin(theta) to solve for the magnitude of the force. In a more rigorous treatment when it's not in vector cross-product notation you don't have enough information to solve for the direction of the new vector. I didn't even realize that you could think of the cross-product and the right-hand rule as separate topics. If anything I just think, "oh you write the two vectors as rows in a 3x3 matrix and take the determinant."

On a bizarre note, I actually have a shirt that my girlfriend made for me with a drawing of some gangsta guy throwing up the right hand rule as his gang sign.

 

[Vihsadas]

Oh, when I read your first post I was kind of annoyed that you didn't think that you need to know what F=Qv X B is(and thus the direction an electron will travel in a magnetic field) because you have F=qvsin(theta) to solve for the magnitude of the force. In a more rigorous treatment when it's not in vector cross-product notation you don't have enough information to solve for the direction of the new vector. I didn't even realize that you could think of the cross-product and the right-hand rule as separate topics. If anything I just think, "oh you write the two vectors as rows in a 3x3 matrix and take the determinant."

Yeah, I know exactly what you mean. When I first started studying for the MCAT I was thinking to myself, "Why is there no F =qv x B!!! How are you supposed to find the direction!?!" <== Perfect example of how a degree in physics can make things confusing on the MCAT.

On a bizarre note, I actually have a shirt that my girlfriend made for me with a drawing of some gangsta guy throwing up the right hand rule as his gang sign.

Lol I want one.

 

[tncekm]

God... just about to take TPR Diag 3, and this poor kid with some sort of disability was making all kinds of loud fuss, and it smells like he (or someone) just sharted themselves. Talk about concentration breaker.

 

[tncekm]

He's back again... I just can't do this. My God, this is abso-freggin-lutely ridiculous :sad: I can't do these at my house, either, because everyone just bugs the hell out of me there! arggggg

 

[Vanguard23]

He's back again... I just can't do this. My God, this is abso-freggin-lutely ridiculous :sad: I can't do these at my house, either, because everyone just bugs the hell out of me there! arggggg

I know what you mean. I could have pulled a 30 or 31 on a few of my practice tests if people weren't bugging the hell out of me at my house.

 

[tncekm]

I printed, in huge letters, signs that say "TESTING: DO NOT DISTURB UNTIL: X:XX" and then go around and tell everyone not to bug me, no matter what, so it sinks in.

Its been working for me! Give it a try!

BTW, I just got a 32 on a TPR Diag 4 (PS: 10, VR: 9, BS: 13) -- these TPR tests are freggin rough.

 

[physics junkie]

What, you've never **** yourself? Have some compassion.

 

[tncekm]

What, you've never **** yourself? Have some compassion.

I felt bad for the poor guy, it just definitely wasn't going to be a great practice test environment for me