ASSOCIATE DEGREE IN ENGINEERING TECHNOLOGY RESIT EXAMINATIONS. SEMESTER 2 July 2012

Transcription

1 ASSOCIATE DEGREE IN ENGINEERING TECHNOLOGY RESIT EXAMINATIONS SEMESTER 2 July 2012 COURSE NAME: PHYSICS 2 CODE: GROUP: ADET 1 DATE: July 4, 2012 TIME: DURATION: 1:00 pm 2 HOUR INSTRUCTIONS: 1. This paper consists of SIX questions. 2. Candidates must attempt ANY FOUR questions on this paper. 3. All working MUST be CLEARLY shown. 4. Keep all parts of the same question together. 5. The use of non-programmable calculators is permitted. DO NOT TURN THIS PAGE UNTIL YOU ARE TOLD TO DO SO 1

2 Instructions: Answer any FOUR (4) questions. Question 1 a. State the three main effects of an electric current and give one practical application of the effect of each [6] b. Distinguish between direct and alternating current [ 4] c. R1 6.5Ω R2 40Ω R3 30Ω R4 20Ω R5 10Ω V1 120 V R6 15Ω R7 60Ω Determine, using the circuit above i. The total resistance [2] ii. The feeder current [2] iii. The voltage drop across the 10 ohms resistor [2] iv. The power drawn by the 10 ohms resistor [2] v. The energy dissipated in 6.5 ohm resistor in 3 minutes [2] d. A 12V battery of a certain car has a capacity of 80 ampere-hours (Ah) which means that it can furnish a current of 80A for one hour, a current of 40A for two hour and so on. i. How much energy is stored in the battery when it is fully charged? [2] 2

3 ii. If the car s lights required 60W of power, how long can the battery keep them lighted when the engine (and hence the generator) is not running? [3] [Total 25 Marks] Question 2 a. Define and state the units of self inductance and mutual inductance [4] b. Describe a simple experiment to show that a force is exerted on a current carrying conductor in a magnetic field [7] c. A conductor 350mm long carries a current of 10A and is at right angles to a magnetic field lying between two circular pole faces each of radius 60mm. if the total flux between the pole faces is 0.5mWb, calculate the magnitude of the force exerted on the conductor. [7] d. A flux of 25mWb links with a 1500 turn coil when a current of 3A passes through the coil. Calculate i. The inductance of the coil [4] ii. The average e.m.f induced if the current falls to zero on 150ms. [3] [Total 25 marks] Question 3 a. State Kirchhoff s rules [2] 3

4 b. Using the circuit below R2 4kΩ V1 12 V V3 60 V V2 80 V R1 3kΩ R3 2kΩ i. Find the current in each resistor [9] ii. Find the potential difference between points c and f [6] c. A dry cell of e.m.f 1.5 V and internal resistance 0.5 is connected to a component of resistance 7 ohms, find : i. The power input to the component [2] ii. The power wasted within the cell [3] iii. The power which would be wasted if the cell terminals were short-circuited [3] [Total 25 Marks] Question 4 (a) i) State Coulomb s law. [1] ii) Write an equation that represents Coulomb s law. [1] 4

5 (b) With respect to Figure 2, what is the magnitude and direction of the net force acting on charge D? ( Take k = 1 / 4πε = 9 X 10 9 Nm 2 C -2 ) [16] C D 7.0 μc 0.2 m 3 μc 0.2 m B A 10 μc 5 μc Figure 2 (c) i) Define electric field strength, and state whether it is a scalar quantity or vector quantity. [2] ii) Copy and complete Figure 3 to show the electric field lines in the region around two equal positive point charges. Mark with a letter N the position of any point where the field strength is zero. [5] + + Figure 3 [Total 25 marks] 5

6 Question 5 (a) i) State the formula for the capacitance of a parallel plate capacitor with a dielectric between its plates. Give the meaning of each symbol used. [5] A parallel plate capacitor has a capacitance of 25 μf, a plate separation of 1.5 mm and the area of each plate is 35.0 cm 2. Given that the potential difference between the plates is 75 V, determine: i) the dielectric constant K [2] ii) the charge on either plate [2] iii) the energy density between the plates. [2] (b) The circuit in Figure 5 shows a battery of e.m.f. 70 V connected to five capacitors C 1, C 2, C 3, C 4 and C 5 whose values are shown. Calculate: i) the effective capacitance of the circuit [10] ii) the energy stored in capacitor C 2 [4] 70 V 2 μf 4 μf 4 μf C 1 C 2 C 5 8 μf 8 μf C 3 C 4 6

7 Figure 5 [Total 25 marks] Question 6 (a) Explain what is meant by: i) mass defect. [2] ii) binding energy [2] (b) β + decay converts sodium 22 Na ( with a mass of u ) to 22 Ne ( with a mass of u ). Mass of an electron = u iv) Write the equation for the decay process. [2] ii) Calculate the energy (in MeV) released. [3] (c) In the radioactive decay series starting with thorium 228 Th and ending with Bi, calculate the total number of α particles and the total number of 83 β- particles that are generated. [7] (d) Define the terms: i) Activity [1] ii) half-life [1] In a medical laboratory a radioactive isotope sample of activity A 0 = 396 disintegrations per minute was introduced intravenously into the blood stream of a patient. The decay constant of the isotope is 4.62 X 10-2 min -1. 7

8 iii) Calculate the activity of the sample after 30 minutes. [5] iv) If 10cm 3 of the patient s blood had an activity of per minute, determine the patient s blood volume. [2] [Total 25 marks] ****END OF PAPER***** 8

9 ASSOCIATE DEGREE IN ENGINEERING SOLUTIONS SEMESTER DECEMBER COURSE NAME: PHYSICS 2 CODE: [8 CHARACTER COURSE CODE] GROUP: AD ENG 1 DATE: MAY 2012 TIME: DURATION: "[TIME OF PAPER]" 2 HOURS Solutions Question 1 a. Heating electric iron 2mks Magnetic electromagnets 2 mks Chemical electrolysis 2mks b. Direct current flows continuously in one direction. Alternate current flow first in one direction for a brief time, and then reverses to flow in the opposite direction for a similar time. 4 mks c. 9

10 R R R R 3,4, i R R R 3,4,5 3,4,5 7 2, R R R R 3,4,5 2, R R R T T R ii. I T E A R 23.5 T iii. Va voltage drop across R 5 Va IR I T I R R 6 2 R3,4,5 R7 R6 R I I 4.24A V Now V 23.3V 10

11 iv. 2 2 V 23.3 P 54.3W R 10 v. E E H H 2 2 I RT J d. E H i. E E H H VIT J ii. t E h P t 60 t 57600s [25 Marks] Question 2 a. Self inductance is when a change of current is accompanied by a change of flux and therefore by induced e.m.f. mutual inductance is when two coils are wound on the same magnetic circuit and one coil induces a voltage on the other when the current changes b. Faradays law 2 c. A r A A 0.011m 2 B A B B T 11

12 F F F BIl sin sin N d. i. ii. N L 12.5 H I 3 N e 250V t 0.15 [25 Marks] [25 Marks] Question 3 a. Hghhhjj b. i. ii. c. jjjjj 1 2EH EH QV V 240V 2 Q 0.01 Q 0.01 C 0.42 F V 240 [25 Marks] [25 Marks] 12

13 (Question 4) 13

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15 [25 Marks] 15

16 (Question 5) 16

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18 [25 Marks] 18

19 (Question 6) 19

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21 [25 Marks] END OF SOLUTIONS 21