7c_practice_8-7_solution.pdf

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Unformatted text preview: First Name Last Name ID Section ues 1011 : neec I'OIl raves a aspee 1} = . X — I‘Ollg areglono space W1 0 acons an magne 10 t' 5A 1t t It d 10106‘:th h ' f 'thbth tt t' and a constant electric ﬁeld, as shown in the picture below. The constant magnetic ﬁeld points out of the page and has a magnitude of 4.5 x 10—4 T. Both the magnitude and the direction of the electric ﬁeld are unknown. a). 56 lv=1.0x106§ e What is the magnitude and direction of the magnetic force on the electron? The magnitude is give by ‘F‘ = q|17l ‘3“ sin6. We are told that ‘E‘ = 4.5 x 10—4 T and |17| = 1.0 x 10—6 m/s. Since this is an electron, q = —1.6 x 10—19 C, and since the ﬁeld and the force are perpendicular, 0 = 90°. Hence the force has magnitude ‘13 = (1.6 x 10—19)(4.5 X 10—4)(1.0 x 106) = 7.2 x 10—17 N. For direction, note that right hand rule #2 gives us that F must point to the right, but since q is negative, we must ﬂip the force we get from our right hand rule. Hence, the force points to the left. . The electron is observed to travel with a constant velocity. What is magnitude and direction of the constant electric ﬁeld? If the electron moves with constant velocity, it must experience no net force. Hence the electric force must point to the right and have the same magnitude as the magnetic force. Equating these magnitudes and using F = qE gives: tu = qE :s E = 123 = (1.0 x 106)(4.5 x 10—4) = 450 N/C (7) Since the charge is negative, the force must point opposite the direction of the electric ﬁeld. Since the force is to the right, the ﬁeld must point to the left. . If the electric ﬁeld were to suddenly disappear, describe (qualitatively) the subsequent motion of the electron. If the electric ﬁeld were to disappear, the only force on the electron would be due to the magnetic ﬁeld. Since the force due to this magnetic ﬁeld is always perpendicular to the instantaneous velocity of the electron, this would cause the electron to travel in a circle. Since the initial net force is to the left, the electron would traverse the circle in the counterclockwise direction when viewed as drawn on the top of the page. ...
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• Fall '08
• MAHMUD
• Force, Magnetic Field, Electric charge, Lorentz force

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