# pexam2S - Physics 21 Fall 2007 Solution Practice Exam#2...

This preview shows pages 1–2. Sign up to view the full content.

Physics 21 Fall, 2007 Solution, Practice Exam #2 Problem 1. For the following circuit, R =8 . 0Ω , L = 2 . 0mH ,and C = 500 µ F, and the frequency is 700 Hz. R L C V rms I rms / ω C I rms R V rms φ ω LI rms (a) (5 pts.) Write the loop equation for this circuit. V IR L dI dt Q C =0 (b) (10 pts.) Draw a phasor diagram for the voltages in this circuit. What is the rms value of the current if V rms =6V? For the phasor diagram shown above, use ωL =2 π × 700 × 0 . 002 H = 8 . 7965 Ω; R =8Ω 1 /ωC = ( 2 π × 700 × 500 × 10 6 F ) 1 . 4547 Ω The resultant vector of ωLI rms and 1 / ( ωC ) I rms is 8.3418 I rms , which points upward. Adding that resultant to I rms R gives V rms = p 8 . 3418 2 +8 2 I rms =11 . 5579 I rms Then V rms =6V=11 . 5579 I rms , so that I rms . 519 A (c) (5 pts.) What is the phase relationship between the current and the applied ac voltage? In other words, does the current lag or lead the applied voltage? by how much? The phase φ is given by tan φ = 8 . 3418 8 φ =46 . 2 . 806 rad . The current lags the applied voltage by 46 . 2 . Problem 2. A single rectangular loop of wire with the di- mensions shown in the ±gure is inside a region of magnetic ±eld B ( t ) directed into the page. The magnitude B ( t )ofthe ±eld is given by B ( t )=0 . 02 t T, where t is in seconds. x x xxxxxxx x x x x x x x x x x x x x x x x 0.4 m 0.8 m (a) (14 pts.) What is the current in the loop at time t =1s if the resistance of the loop is 0.1 Ω? From Faraday’s Law, E = d Φ dt = d dt ³ B ( t ) × 0 . 4m × 0 . 8m ´ = d dt ³ 0 . 02 t × 0 . × 0 . ´ = 0 . 02 × 0 . 4 × 0 . 8=0 . 0064 V Then I = |E| /R . 0064 / 0 . 1=0 . 064 A (b) (6 pts.) In what direction does the current in the loop ﬂow? (Specify clockwise or counterclockwise.) Brieﬂy explain your answer.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

## This note was uploaded on 02/29/2008 for the course PHYSICS 21 taught by Professor Hickman during the Spring '08 term at Lehigh University .

### Page1 / 3

pexam2S - Physics 21 Fall 2007 Solution Practice Exam#2...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online