Unformatted text preview: Physics 1112 Spring 2009 University of Georgia Instructor: HBSch¨ttler u Solutions to Additional Conceptual Practice Problems for PHYS 1112 Final Exam
Mon. May 4, 2009, 7:00pm10:00pm CP 4.01: Answer: (A) 1 Physics 1112 Spring 2009 University of Georgia Instructor: HBSch¨ttler u Detailed Problem Statement: A bar magnet is moved either towards or away from a circular conducting ring with either its northpole (N ) or its southpole (S) pointing towards the ring, and the respective pole being located either above the plane of the ring (in ”Top” position ”T”) or below the plane of the ring (in ”Bottom” position ”B”), as shown in the Fig. As also shown in Fig., the plane of the ring deﬁnes the xyplane, and the magnet’s N Saxis is oriented parallel to the zaxis while the magnet is being moved. Which of the ﬁve motion processes of the bar magnet, (A)(E) described above, will induce a clockwise (cw) current I in the ring? Solution: Above/below the poles of the bar magnet, B points approximately along the zaxis, out of the N  and into the Spole, with the ﬁeld strength B ≡ B decreasing with increasing distance from either pole. Hence, the magnetic ﬁeld B inside the ring points in +zdirection when the N pole is below the ring pointing up (in Bottom position ”B”) or when the Spole is above the ring pointing down (in Top position ”T”). Likewise, B inside the ring points in −zdirection when the Spole is below the ring pointing up (in Bottom position ”B”) or when the N pole is above the ring pointing down (in Top position ”T”). Also, the ﬁeld strength B inside the ring increases if either pole is moving towards the ring; and the ﬁeld strength B inside the ring decreases if either pole is moving away from the ring. By Lenz’s rule, the induced current I in the ring produces a secondary ﬁeld, BI , which opposes the change ∆B of the timedependent inducing ﬁeld B(t). Also, the direction of I is related to the BI direction by a righthand (RH) rule: if RH thumb is in direction of BI , the RH 4 ﬁngers point in the ﬂow direction of I. So, to produce a BI pointing in −zdirection requires a clockwise I, as deﬁned in Fig.; to produce a BI pointing in +zdirection requires a counterclockwise I. Hence, ∆B points in +zdirection, and by Lenz’s rule, BI points in −zdirection, and I ﬂows clockwise if Case (1): B points in +zdirection and its strength B increases in magnitude; or Case (2): B points in −zdirection and its strength B decreases in magnitude. Likewise, ∆B points in −zdirection, and by Lenz’s rule, BI points in +zdirection, and I ﬂows counterclockwise if Case (3): B points in +zdirection and its strength B decreases in magnitude; or Case (4): B points in −zdirection and its strength B increases in magnitude. Thus, in the answer key above, process (A) corresponds to Case (1), process (B) corresponds to Case (3), process (C) corresponds to Case (4), process (D) corresponds to Case (4), and process (E) corresponds to Case (3). So, only process (A) induces a clockwise current, while (B), (C), (D) and (E) all induce a counterclockwise current in the ring. 2 Physics 1112 Spring 2009 University of Georgia Instructor: HBSch¨ttler u CP 4.02: Answer: (A) The induced EMF amplitude is Eo = ABω where the loop’s angular speed is ω = 2πf , expressed in terms of the rotation frequency f . Hence, at ﬁxed ﬁeld strength B, Eo ∝ A × f . So increasing A = 10cm2 → A = 30cm2 = 3 × A and increasing f = 500RPM → f = 1500RPM = 3 × f will increase Eo = 20mV → Eo = 3 × 3 × Eo = 180mV 3 Physics 1112 Spring 2009 University of Georgia Instructor: HBSch¨ttler u CP 4.03: Answer: (D) By Kirchhoﬀ’s loop rule EB + E = 0 where E = −L∆I/∆t is the selfinduced EMF in the inductance L. Hence, ∆I/∆t = EB /L = 50V/10H = 5A/s. 4 Physics 1112 Spring 2009 University of Georgia Instructor: HBSch¨ttler u CP 4.04: t
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2 CoS (bOO) = CP 4.05 (=Q1), Answer: (B) For unpolarized light of timeaveraged (TA) intensity I0 passing through polarization ﬁlter (PF) 1, the TA intensity after the ﬁlter is I1 = I0 cos2 θ1 θ . Here, ... θ means averaging over all possible values of the random angle between an incident waves packet’s electric ﬁeld E0 and transmission axis T1 , denoted by θ1 ≡ (E0 , T1 ). For uniformly random angle θ1 (i.e., any value of θ1 between 0o and 180o is equally likely to be found among incident EM wave packets), cos2 θ1 θ = 1/2 and hence I1 = (1/2)I0 . The wave packets passing through PF1 (i.e. between PF1 and PF2) are then polarized with electric ﬁeld vector E1 T1 . But then the angle between E1 (incident upon PF2) and T2 is θ2 ≡ (E1 , T2 ) = 90o , since T2 ⊥ T1 . Hence, by Malus’ Law, the TA intensity transmitted 7 Physics 1112 Spring 2009 through PF2 is I2 = I1 cos2 θ2 = I1 cos2 (90o ) = 0. CP 4.06 (=Q2), Answer: (E) University of Georgia Instructor: HBSch¨ttler u As in CP 4.05, the intensity I1 after PF1 is still I1 = (1/2)I0 with E1 T1 . However, the angle between E1 and T2 is now θ2 ≡ (E1 , T2 ) = 90o − 30o = 60o , since T2 is 30o from the horizontal, but E1 T1 , i.e., E1 is vertical. So, I2 = I1 cos2 θ2 = I1 cos2 (60o ) = I1 × (1/2)2 = (1/2)I0 × (1/2)2 = (1/8)I0 ; and thus I2 /I0 = 1/8. Important to note here: It does not matter here if you add or subtract 180o from the ET angle θ, or change the sign of θ, since cos2 (θ) = cos2 (−θ) = cos2 (θ±180o ) = cos2 (180o ±θ). 8 The area vector of the loop A must be normal to the plane of the circuit loop, i.e., parallel to the zaxis. So, let’s choose it to point in −zdirection, i.e., into the paper, i.e. in the +Bdirection. The initial ﬂux through the circuit, before changing the ﬁeld/circuit conﬁguration, is then Φm = BA cos Θ = +BA > 0 since Θ ≡ (A, B) = 0o . The ﬁeld or circuit conﬁguration change processes (A), (B), (C), (D), (E) and (F) will then result in a timedependent change First note, for orientation, that for the x and yaxis shown, the zaxis is out of the paper. Answer: (A)=True; (B)=True; (C)=True; (D)=False; (E)=False; (F)=True
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