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Cork Institute of Technology
Bachelor of Engineering (Honours) in Mechanical
Engineering – Stage 1
(NFQ – Level 8)
Autumn 2007
Physics
(Time: 3 Hours)
Answer FIVE questions Examiners: Ms E.M Baldwin Mr. P. Clarke All questions carry equal marks Prof. M. Gilchrist
Physical constants Charge on electron: e = 1.6 x 10 -19^ C Permittivity vacuum: ε 0 = 8.85 x 10 -12^ C 2 N-1^ m- Density of water: ρ = 1000 Kg m - Speed of light: 3 x 10 8 ms-
Specific Heat Capacity of water: 4200 J Kg -1^ K- Planck’s Constant: 6.626 x 10 -34^ Js Rest mass of electron: 9.11 x 10 -31^ Kg Velocity of sound in air: 343 ms -
Q
a) Define the terms electric field and electric field strength. State the two S.I. units in which electric field strength may be measured and show that they are equivalent. (6 Marks)
b) Write down a mathematical expression for the electric field strength at a distance r from a point charge Q. Sketch how the field varies with r. (4 Marks)
c) Write a note on the use of field lines as a means of visualising electric fields. (4 Marks)
d) Three charges are placed in a straight line as shown below. Determine the magnitude and sign of the charge at C if there is no net force on the charge at B. (6 Marks) A (^) 10cm B (^) 6cm C
+6 μ C -10 μ C
Q
a) Two resistors one 3Ω and one of 0.3Ω are connected in series with a cell of emf of 1.5V. A current of 0.3A flows in the resistors. Calculate (i) The internal resistance of the cell (ii) The potential difference between the cell terminals (iii)The power generated in the 3Ω resistor (iv) The maximum current which may be drawn from the cell. (10 Marks)
b) Give an account of the Wheatstone bridge including a diagram and relevant equations (10 marks)
Q
a) Describe clearly, with the aid of an appropriate diagram, the operation of a transformer. If a transformer with 2400 turns in its primary coil and 120 turns in its secondary coil is supplied from the mains, calculate the peak current in a simple resistive load of 120Ω in the secondary circuit. What is the value of the peak current in the primary circuit? (10 Marks)
b) Outline the main sources of power loss in a transformer. (4 Marks)
c) Write a brief note on how the resistance of a conductor is used in temperature measurement. (6 Marks)
Q
a) An object of height 10cm is placed 30cm to the left of a convex lens of focal length 20cm. Using optical ray tracing techniques and the graph paper provided, determine the size and position of the image produced. State whether the image is real or virtual, upright or inverted. If the convex lens is replaced by a concave one of the same focal length, determine the position size and type of image produced for the same object size and position, again using optical ray tracing and graph paper. (10 Marks)
b) Use the thin lens equations to verify your answers from (a) above (8 Marks)
c) Explain why the apparent depth of an object at the bottom of a pool is different from the real depth (2 marks)
