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Unformatted text preview: Physics 6B Practice Final Solutions . Two speakers placed 4m apart produce sound waves with frequency 425Hz. A listener is standing 3m in front f the left speaker. Describe the sound that he hears. Assume the speed of sound is 340 m/s. 4m 3m irst find the distance from the right speaker (use the ythagorean theorem or recognize the 345 right triangle). ow find the difference in the distances to each speaker: (5m – 3m) = 2m e need to compare this to the wavelength of the sound. his means the difference is 2.5 wavelengths, yielding estructive interference (a quieter sound) 5m 5 . 2 8 . 2 8 . 425 340 = = = = m m m Hz f v s m λ . Charge q 1 = 5.4μC is placed at the origin, and charge q 2 = 2.2 μC is on the xaxis at x=1m. Where should a charge q 3 be placed between q 1 and q 2 so that the net force acting on it is zero? 1x x q1 q3 q2 ere is a diagram, with q 3 in between. We just eed to find the forces on q 3 due to the other charges, and make them equal. e are not given the sign of charge q 3 , but it shouldn’t matter. The forces due to q 1 and q 2 will e in opposite directions in either case. F(q1 on q3) F(q2 on q3) ( ) 2 3 2 2 3 1 2 1 ' 3 2 3 1 x q kq F x q kq F r kqq F q q q q elec − = = = → → etting these equal, and plugging in the given values: ( ) ( ) ( ) 2 2 2 2 2 1 2 3 2 2 3 1 1 2 . 2 4 . 5 1 1 x x x q x q x q kq x q kq − = ⇒ − = ⇒ − = ( ) ( ) m x x x x x x 61 . 32 . 2 8 . 3 1 32 . 2 48 . 1 1 4 . 5 2 . 2 2 2 = ⇒ = ⇒ − ⋅ = ⋅ ⇒ − ⋅ = ⋅ . An object with a charge of – 3.6μC and a mass of 12g experiences an upward electric force, due to a uniform lectric field, equal in magnitude to its weight. Find the direction and magnitude of the electric field. F elec mg lectric force is simply charge times electric field. ( ) ( ) C N s m elec C kg E q mg E mg qE F 000 , 33 10 6 . 3 8 . 9 012 . 6 2 ≈ ⋅ = = ⇒ = = − his is the magnitude of the field. The direction must be downward to give an upward force on a negative charge. One way to remember this is that the direction of the field is lways the direction of the force on a positive charge. Since our charge is negative, it ust be the opposite direction. . Two conducting spheres have net charges q1=+8 μC and q2=2 μC. The spheres touch and some charge is transferred. How many electrons are transferred, and to which sphere? q1 +8 μC q22 μC BEFORE q1 +3 μC q2 +3 μC AFTER hen the spheres touch, charges are transferred until both spheres have equal charge. he total charge before they touch is +6μC, so the charge on each sphere after they touch must be +3μC. o there must be a total of 5μC of charge transferred. e need to divide by the charge on the electron to find the # of electrons transferred: ince sphere 1 gets less positive, it must be gaining the negative electrons. Conversely, sphere 2 is getting more ositive, so it is losing electrons. Thus the electrons must be transferred to sphere 1....
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 Spring '11
 Staff
 Physics, Electron, Charge, Force, Magnetic Field, Electric charge

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