Phy 133 notes on potenital

Phy 133 notes on potenital - Electric forces and...

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Unformatted text preview: Electric forces and gravitational forces have many similarities, one of the most important is that both are conservative. As a result there must be an electric potential energy UE associated with F 5, just as there is a gravitational potential energy Ug = mgy due to Fg. Consider a positive test charge q0 placed in a vertical electric field. It experiences a downward electric force F = qu, if the charge is moved upward through a distance d, the electric force and the displacement are in opposite directions, therefore the work done by the electric force is negative. ‘ W = -qud Using the definition of potential energy , i.e. AU = —W we get AU = -W = qud Note: that UE increases just as Ug when it is raised against the field, on the other hand if qo is negative the FE acting on it will be upward, in this case the FE does positive work as the charge is raised through the distance d and the change in potential energy is negative. Thus the change in potential energy depends on the sign of the charge as well as on its magnitude. The electric force depends on qo the same way as does the change in electric potential energy é AU —W qu qr) i.e. (E = —— it is useful to use AV = where the SI unit of V is the volt go 1 volt = lJou'le/Coulomb As with US we deal only with the changes in potential AV, i.e. V can be set to zero at any desired location, just as height can be set to zero in gravitational problems. In addition, both V and U are scalar quantities, so combining them is a matter of simple algebra. There is a connection between E and V that is both straight forward and useful. To obtain this relation apply the definition AV = -W/qo to the case of a test charge that moves through a distance As in the direction of the field. The work done _by the electric field is the force times the distance W = qud , therefore the change in electric potential is AV : —W/qo = — (qud)/q0 = - EAs Solving for the field E = -AV/As this shows that the field which can be expressed in terms of Force/charge i.e. N/C also has units of Volts/meter i.e. l N/C = l V/m Therefore if the potential (V) is known then the electric field (E) can be found, or if E is known, (such as from Gauss’s Law) then V can be found To summarize: the electric field depends on the rate of change of the electric potential with position (in terms of gravity you can think of the potential V as the height of a hill, and the electric field E as the slope of the hill) in addition it follows that V decreases as one moves in the direction of E (i.e. V = - EAs is ‘ negative when both E and As are in the same direction) Example: If you have a constant potential in a region of space then what is the electric field in this region? (a) positive, (b) negative, (0) zero (the answer is c zero) Reasoning: since E is the rate of change of V with position and the rate of change of a constant potential is zero, so must be the field. Energy Conservation: when a ball is dropped in a gravitational field its U3 decreases as it falls, at same time its KE increases. If non-conservative forces are ignored then the energy is conserved Adeecrease = AI<Eincrease Because F E is also conservative the same considerations apply to a charged object in an electric field KEA+ PEA : KEB + PEB Recall that if forces are due to gavity PE = Ug = mgy springs PE = Usp = ‘/2 kx2 electric PE = qV i.e. (l) example: if an object of mass m = 1.75 x 10'5 kg and charge q = 5.2 x 10'5C is released fi'om point A at rest, as it moves to point B its electric potential decreases by 60Volts (i.e. VA-VB = 60V) the speed of the object at point B is? KEA + PEA :KEB + PEB V2 mvbZ = V2 mva2 + q(VA-VB) vB = «I 2q(VA—VB)/m = 18.9 m/s therefore a decrease in V appears as an increase in KB, if a negative charge is used the situation is the opposite. A negative charge will move to a region of higher electric potential with an increase in speed, i.e. ifq = —5.2 x 1050 and AV = -60V, the final speed is the same in general, positive charges accelerate in the direction of decreasing V negative charges accelerate in the direction of increasing V in both cases however, the charge moves to a region of lower electric potential energy example: an electron is released and accelerates through V, a proton is released and accelerates through —V, the final speed of the electron is (a) faster, (b) slower, (c) the same as the proton? The answer is (a) faster, since masses are different, but there is still the same magnitude change in potential V for a point charge: Consider a point charge +q that is fixed at the origin of a coordinate system Suppose a test charge +q° is held at rest at point A, here it experiences a force F = quD/rAz. If the test charge is now released it will accelerate away from the origin when it reaches point B its KE will have increased by the same amount as the decrease in electric potential energy _ quo quo i.e. UA is greater than U3 and UA — U3 — rA r B l and VA- VB 2 —‘ (UA-UB)= E— -—k£1_ qo r A ’13 if we choose r = 00 as a reference point then U = 0 at this point qu therefore V = — for a point charge and U = —— for a pair of point charges separated by r r r Superposition of V and U If more than one point charge exists in a system, then use superposition principle to find the total potential kq of the system. V = 2V,- where V‘- = —- The total work is the sum of the work done due to the individual r k forces. W. = 2W,- = 2 (14° = qAV r Example: if q = 4.11 x 10'9C is at 0 m and -2q is at 1m then, (a) what is V at the midpoint +1.; ~Zj of the between the charges, and (b) where does V = 0 ~ gm ). k —2 - ' - M solution (a) V = E + M ifx = 0.5 m then I x (1 — x) v = kq/O.5 + k(—2q)/(1~0.5) = -73.9 Nm/C = —73.9 v (b) V = —-kq/x + k(-2q)/(l-x) = 0 l/x =2/(l-x) x = 0.33m and V = O at x = —1m (11» f l l 1 l l \\\ \"‘ 9.... " 1m 0 im If the system is a continuous object, then break the object into parts, with each element having a charge dq, find the element of potential due to dq , and then integrate j V: V 7- ~43 Equipotential surfaces and electric field A contour map is a useful tool, it is a series of closed curves, the contours each denoting a different altitude, . When the contours are closely spaced the altitude changes rapidly indicating steep terrain, widely spaced ' contours indicate a fairly flat surface. A similar device can help visualize V ‘ __ , 19:6 o i/ojvnlixp 1 Wm” I Any surface for which all points are at the same potential 0) fie; wt, t is called an equipotential surface decreasing in magnitude with ‘ L "‘\1\~ . Increasing distance from the source charge from the source charge #144,. f a Note: The choice of specific potentials is arbitrary, an equipotential surface can be drawn through any point in the field. They are perpendicular to the electric field, if it were not perpendicular then motion of charges along the surface would correspond to a change in PE due to work done by the field on the charge. If excess charge is on a conductor it resides on the surface, if the charge is at rest, whatever E is present must be perpendicular to the surface, since any field parallel to the surface would cause the charge to move. Because the field is perpendicular to the surface all points along the surface must be at the same potential, therefore, when all charges have come to rest, the surface of a conductor is always an equipotential surface, and inside the conductor V = constant (that of the surface) Problem solving hints for potentials 1. Recall potentials are scalar quantities, therefore add algebraically for a discrete system of charges (Keep track of signs) The potential is positive for positive charges and negative for negative charges. 2. The choice of zero potential is arbitrary, since only changes in potential are significant (just like mechanics) 3. To find the electric potential at some point P due to a continuous char? distribution break into parts and then integrate V _ ‘5 (D V W h Cf» ‘ w 4. If E is known, then V can be found by ‘ M j CE J“; s. ,. 5 00 5. If V is known then B can be found since E is equal to the negative derivative of potential with respect to some coordinate J) .1 Z? 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This note was uploaded on 10/24/2010 for the course PHY PHYSICS 13 taught by Professor Mylander during the Spring '09 term at Cal Poly Pomona.

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Phy 133 notes on potenital - Electric forces and...

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