# electrostatics and electromagnetism

#### olygt

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I just went over the Berkeley review section on electrostatics and electromagnetism and I can't seem to understand it enough to do the 10 passages at the end of the section. Does anybody else find these hard or is it just me? Also, I'm having a problem figuring out how to apply the right hand rule; what fingers are B, v, F and I, E, how do you find counterclockwise and clockwise direction? ahh...so frustrating

#### BloodySurgeon

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there are many version of the right hand rule and any of them will work fine. My version is a very uncommon version but i'll say it anyway. Take your right hand and point all your fingers into the direction the ion/election is moving. Then curl your fingers into the direction of your magnetic field which will leave you with a closed fist and a pointing thumb. Which ever direction your thumb is pointing is your magnetic force. But remember it is the opposite direction if its an election.

Also, wires exert their own magnetic field by elections moving through them. These magnetic field do not effect the wire itself but everything around it. All you have to do to find this magnetic field is point your thumb in the direction of the current (opposite of election velocity) and the direction your other fingers curl are the direction the magnetic field is going.. either clockwise or counter clockwise.

#### RSAgator

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they're both very important concepts so make sure that you know them. They are perhaps slightly more difficult concepts to grasp because they aren't entirely intuitive. Post some specific questions in the study questions sub forum, you should get a lot of help in there.

#### Chris127

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Which right hand rule are you referring to?

While I have not yet started studying for the MCAT, from my Physics class, we were taught three.

RHR - 1 --> Refers to a charge within a magnetic field. For the charge to experience a magnetic force, it must A) Be moving, and B) Have a component of its velocity perpendicular to the direction of the magnetic field. For an electron in said field, take your right hand, and point the tips of your fingers in the direction of the field. The direction of the force points outward from your palm, and the electron travels in the direction that your thumb points. This is flipped for a proton (direction of magnetic force extends outward from back of the hand)

RHR - 2 --> Refers to the magnetic force produced by a current of wire. Wrap your right hand around the wire, so that your thumb points in the direction of the current. The tips of your fingers will point in the direction of of the magnetic field (it will be spherical around the wire).

RHR - 3 ---> Refers to the direction of the induced magnetic field that a circular loop of wire creates. With your right hand, curl your fingers, so that they resemble the direction of the current in the circular loop of coil. Your thumb will point in the direction of the produced current.

#### olygt

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ok...so I just finished berkeley's ten passages on electrostatics and electromagnetism and did horrible. I'm averaging 9's on all the other sections and got a 6 on this test. I practically guessed on most of the questions b/c I didn't have a good understanding of the subject. Is there any hope? I have 4 more berkely books to go through (orgo and chem.) and don't want to get stuck trying to master electromagnetism. I'm starting to stress, please tell me I have enough time to get through the rest...test is in a month ahhhhhh

#### RSAgator

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Of course there's hope, but they're big topics and it would really be a lot easier to help you if you gave us some particular things you're having trouble with.

To toss some concepts out, you can draw a lot of analogies between electrostatics and gravitational forces.

For gravity:

F = G(m1)(m2)/r^2

For electrostatics:

F = k(q1)(q2)/r^2

G and k are both constants. As you can see from these two equations, the force on a certain mass is related to both the mass of the object of interest, the mass of the other object (in our case the earth), and the distance between the two (between the two centers of mass).

This is very similar to the force on a charge. The force on a charge or a charged object (q1) is related to the charge on the first object, the charge on the second object (q2) and the distance between both their centers of mass.

Now we extend the concept further into the electric field and the gravitational field.

The gravitational field, we'll call g

g = G(m2)/r^2

where G is the same constant, m2 is once again equal to the mass of the earth, and r is again the distance between the two centers of mass. g is the force per unit mass. If you want to see the force exerted on a mass by the gravitational field you would multiply both sides by m1 giving you

m1*g = G(m2)(m1)/r^2 = F

The electric field is defined similarly, it is the force per unit charge (as oppose to the gravitational field which is the force per unit mass).

E = k(q2)/r^2

So you have a charge, q2, which in this case is analogous to the earth in that it creates a field that can exert a force on another charge (as oppose to the earth creating a field that can exert a force on another mass). If you place a second charge, q1, within an electric field, a force will be exerted on it that is related to the electric field. Much like we did for the gravitational field, if we multiply each side by q1 we come to:

q1 * E = k(q1)(q2)/r^2 = F

The electric field is defined as the force per unit charge, so multiplying E by q gives you F.

Now we have electric potential energy which is analogous to gravitational potential energy but slightly less obviously so I'll just explain electric potential energy, U.

As shown above, F = k(q1)(q2)/r^2

We know that work or energy has units of N*m (force * distance). With this in mind, if we multiply each side by the distance, r, we get:

U = k(q1)(q2)/r

What this equation basically says is that you have two point charges. If they are both the same charge (both + or both -) then they repel one another and if they are opposite charges they attract. In nature things tend to move towards their most stable state, which in the case of like charges is when two molecules are farthest away from one another, and in the case of opposite charges is when they are closest. Any movement against this stable state (bringing like charges together, pulling unlike charges apart, lifting a mass off the ground) requires an input of energy which is stored as potential energy. This equation gives that potential energy in the case of point charges.

V = k(q1)/r

Voltage is the potential energy per unit charge (as you can see from dividing potential energy by q2). Quite honestly, I don't exactly know how to explain voltage aside from that lol.

Hope that helps clear some things up.

#### MarathonsRcool

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i did tpr, first read i understood very little, second reading, everything was so much clearer.

#### bubbles17

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thanks for typing that out dcohen!
i've been having trouble with this topic for a while...and just reading your explanation and relating concepts I'm already familiar with- it makes more sense!
yay!

#### olygt

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Thanks you all for the help...I'll post some specific questions from berkeley relating to this topic.

#### physics junkie

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Kind of inappropriate but you probably won't forget it now.

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