1) For a positively charged sphere, the electric field lines point outward. For a negatively charged sphere, the electric field lines point inward. Why is it always like that? 2) A charge (3.6x10^-6 C) is 30cm to the left of another charge (-2.7x10^-6 C). Point A is 20cm to the right of the charge with -2.7x10^-6. a) Find the magnitude and direction of the net electric field at point A. b) What electric force is exerted on a charge 4.5x10^-6 C placed at point A? c) What electric force is exerted on a charge -4.5x10^-6 C placed at point A? [For part a, I got an answer of 4.8x10^5 N/C For part b, would the electric force have the same direction as the electric field at point A? (i.e. left?) I assumed so, and got an answer of -2.2N The main problem is part c. I don't know how to calculate this. We know that E = Fe/q, where E represents the electric field, Fe is the electric force, and q is the magnitude of the charge at point A. Note that in the equation, q is substitute as a magnitude. So would the negative charge affect anything, will the answer be the same as part b?] 3) A negative charge of 2.4x10^-6 C experiences an electric force of magnitude 3.2 N, acting to the left. a) Calculate the magnitude and direction of the electric field at that point. [The answer is 1.3x10^6N/C. Why is the direction to the right? The electric force has a direction to the left, and since electric force and electric field always have the same direciton, shouldn't the direction of the electric field be to the left also?] b) Calculate the value of the field at that point if a charge of 4.8x10^-6 C replaces the charge of 2.4x10^-6 C. 4) Consider the setup of two large, equally charge, parallel, flat conducting plates close together, the top plate positive and the bottom plate negative. -The electric field is constant everywhere in the space between the parallel plates. [WHY?] -The magnitude of the electric field at any point between the plates depends and is directly proportional ONLY on the magnitude of the charge on each plate. [Why is it so? I thought that E=Fe/q is the formula for electric field, shouldn't E be inversely proportional to q? (E=electric field, q=charge] -The plate separation has no effect on the electric field. [WHY?] Could anyone explain? I will really appreciate for your help! Please Register or Log in to view the hidden image!
1) Electric field lines demonstrate the general direction which a positive test charge will move in. for isolated source (fixed) and test charges, the test charge moves in a straight line (along the electric field line). Always remember: when the electric field line is curved (and the net force is the electric force), the test charge does NOT move along the electric field line. 2b) using 1 you can figure out what direction the positive and negative charges would move in. c) the magnitude of the force remains the same but direction changes as specified above. 3a) a negative charge will always move in the opposite direction that an electric field line points. b) E = F/q = 3.2/2.4E-6 Shouldn't you be out of school? I can hardly remember my physics Please Register or Log in to view the hidden image!
'''''''''''''''''1) For a positively charged sphere, the electric field lines point outward. For a negatively charged sphere, the electric field lines point inward. Why is it always like that?'''''''''''''''''''''''''''''' all positive charges.. are protons... and all negative charges are electrons... the charge.... or field of force... eminates from the proton.. to the electron. where the electron converts the potential of the field into velocity...(TOWARDS THE PROTON) alll fields are the result, of taking an electron away from a proton..... q is the total number of electrons moved.. or displaced... between two plates for example... since we moved electrons from one plate to the other... the field exists only between the two plates... if we scattered the electrons to the wind, all around the positive plate then the field would be form in all directions..... and the uncharged second plate would be less attractive... putting all math aside... youve been asking why... and it is very difficult to find an answer to this question.... i too have asked this question alot... and out of frustration i was forced to delve into the realms of theory... and i believe i found an answer... as far as i know.. im the only one who is promoting the idea of a universal natural design for the electron... the design of which is related to the design of the proton... Please Register or Log in to view the hidden image! along this line... electrons are rolled up balls of electrostatic field energy... remember when they said... as objects move faster... they get heavier... well electrons move fast... very fast... well.. as a non-moving electron attaches to a static field from an proton... it can then.. by design... roll up said field lines onto itself, and does two things.. it increases velocity towards the proton,, and doing so... increases its mass... then one of a few things can happen. 1) it will reach a maximum speed, as in orbiting an atom... and be thought of as stable..... 2) it can, due to other forces...(atoms, field etc.) slow down.... and if it slows down... it must shed its extra mass.... and how does it do so??? by emitting layers of its mass as photons... or inductive fields.... 3) it can hit the proton, and result in electron capture, and form a nuetron. all of which suggests, that since nuetrons emit electrons to form protons... that the electron was... a layer... on the surface of the nuetron mass... its emission... resulted in the exposure of a different layer of mass/charge on the nuetron surface.... one which makes it into a proton... in this way, hydrogen... forms... being a nuetron with its outer layer, emitted and condensed into an electron who then naturally orbits said proton due to the field of force created when the electron was emitted... connecting said electron to said proton by one line of electrostatic tension..(force) so... there really is only one field... the one emitted by the proton... '''''''''''''-The plate separation has no effect on the electric field. [WHY?]'''''''''''''' not so... as we seperate the plates... the voltage, which is the potential of the field between the plates grows as we move them apart.... the q stays the same... as do the number of lines of force... remember its one line of force per electron /proton pair.... so the field... stays the same... but the potential , or lenght of each line of force in the field... gets longer... and so they each have more potential.. more voltage. '''''''''''''''-The electric field is constant everywhere in the space between the parallel plates. [WHY?]''''''''''''' its not... for our purposes.. it may as well be... since we have a hell of a time dealing with one electron at a time.... we have no choice but to deal with billions at a time... a when you have a billion lines of force.. side by side... its easy to say, then field is constant and uniform betwen the plates... but in reality... the force... at both ends.. both plates.. focuses down on individual electrons, and individual protons... all of which are very small. -The magnitude of the electric field at any point between the plates depends and is directly proportional ONLY on the magnitude of the charge on each plate. [Why is it so? I thought that E=Fe/q is the formula for electric field, shouldn't E be inversely proportional to q? (E=electric field, q=charge] magnitude... as in a meausrement of the total number of lines of flux.... and it maybe true if the plates are equally spaced.. perfectly.. but if one corner of the twoo plates are closer together than the rest, then the over all field... will be changed.. and more electrons will collect at the point where the two plates are closest.. but since electrons repell each other, the amount of effect at at that corner, will depend on the charge and voltage involved.. q= # of electron /proton pairs involved. = number of lines of force. Please Register or Log in to view the hidden image! -MT
The field strength at a point is independent of the amount of charge there. If it helps, you can compare the electric field strength with it's gravitational equivalent. The gravitational field is given by F<sub>g</sub>/m = a. Close to Earth, its magnitude is 9.81 m/s<sup>2</sup>.
przyk is right. The electric field has NOTHING to do with the value of the test charge. Say you had a fixed charge Q and you brought a test charge of 3Q close to it. The value of the electric field felt by the 3Q is no different when you replace it with a test charge of 1000000Q or 1 million Q. The electric field is independent of test charges and only relies on the magnitude of the source charge.
I think the question is ambiguous in it's phrasing. You are told what force a test charge is experiencing at a given point, due to the field of some source charge. You are asked to calculate this field strength: E = F/q = 3.2/2.4x10-6 The question assumes that the first answer establishes the source charge field strength - not an unreasonable assumption. However, if you take the question at face value, then substituting the new test charge value along with the only other given - the force - then: E = F/q = 3.2/4.8x10-6 Clearly a different result. And not an unreasonable assumption either. Restate 3b as: This should lead the student to realize that the established field is not determined by the presence or magnitude of the test charge. (I truly hate ambiguous test questions).
What do the field lines look like near a large flat plate? What do you know about the relationship between the field strength and the density of the field lines?
The field lines have the same density between the 2 plates. But we know that Electric field = kq/r^2, then why is the electric field INDEPENDENT of the plate spearation r (distance)?
E = kq/r² is the formula for point charges. A flat plate acts like a lot of point charges acting together, all pulling in slightly different directions. If a charge (Q) is close to a horizontal plate, the parts of the plate very far from Q (at the edges of the plate) exert a very small force in the vertical direction (most of it acts along the plate), so it's only a small area of the plate that has a significant effect on Q. As Q moves further away, the force becomes more nearly vertical from the edges of the plate - which compensates for the increase in r from each part of the plate. The end result is that, for flat plates, the electric field remains more or less constant up to a certain distance away from the plate. In theory, it's constant at all distances if the plate is infinite. Dunno if that was clear enough.
