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Sph2174: Physics For Engineers Ii Question Paper
Sph2174: Physics For Engineers Ii
Course:Bachelor Of Science In Mechanical Engineering
Institution: Dedan Kimathi University Of Technology question papers
Exam Year:2010
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KIMATHI UNIVERSITY COLLEGE OF TECHNOLOGY
UNIVERSITY EXAMINATIONS 2010/2011
FIRST YEAR SECOND SEMESTERS EXAMINATIONS FOR THE DEGREE OF
BACHELOR OF SCIENCE IN MECHANICAL ENGINEERING
SPH2174: PHYSICS FOR ENGINEERS II
DATE: 15TH DECEMBER 2010 TIME: 2.00 PM – 4.00 PM
Please, answer the first question and any two other questions
Question 1
(a) State in words and write down Coulomb’s law of electrostatics and write it down in SI in a way
that fully reflects the characteristics of interaction of two point charges. Give the name and SI
unit of each symbol represented (6 marks)
(b) (i) Define the quantity “electromotive force”. Give its SI units and express it in SI base
units.
(ii) Write down Ohm’s law for a closed DC circuit. Give the name and SI units of each
symbol used (6 marks)
(c) Write down the Biot-Savart law for the magnetic field. Give the name and SI unit of each
symbol represented. Express the SI unit for magnetic field in SI base units (6 marks)
(d) (i) Give in words the definition of “inductance” of a conductor. Give its SI unit and express
it in base SI units.
(ii) Using the technique of dimensions, prove that the quantity
R
L
has dimension of time.
(6 marks)
(e) (i) Describe the effect of “total internal reflection” of light and define and find the
expression for the “critical angle”. Describe one application of total internal reflection”
of light
(ii) Explain the effect of “dispersion” of light and one of its applications (6 marks)
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Question 2
(a) Two points charges Q1 andQ2 are 3mapart, and their combined charge is Q ? 20?C . If one
repels the other with a force F ? 0.075N , what are the two charges. (6 marks)
(b) The heating element of a kettle has two sections of resistance R ? 25?each. The volume of
kettle is 2? and its efficiency80%. Calculate the time it takes for water to start boiling if (i)
only one section is on (ii) both sections are connected in series (iii) both sections are connected
in parallel. Mains voltage is 240V and initial temperature of water is C 0 20 (8 marks)
(c) Light beam from a GaN diode laser? ? 405nmilluminates a potassium surface. Work function
of potassium is W ? 2.0eV . Find the speed of ejected photoelectrons. (6 marks)
Question 3
(a) An electron beam, in space between plates of a plane-plate capacitor while travelling a distance
of 5cmexperiences a deflection of1mm. Calculate the velocity of electrons in the beam, if the
distance between plates is1cm and the potential difference is V ? 250V (8 marks)
(b) Compute the internal resistance of an electrical generator which has an emf E ?120V and a
terminal voltage of V ?110V when supplying a current I ? 20A (4 marks)
(c) (i) From the Ampere’s circuital law, derive the expression for the force of interaction of
two straight conductors with current.
(ii) From there, state in words the “official” definition of Ampere as one of base units of SI
system (8 marks)
Question 4
(a) A 30 ? cm-diameter sphere hangs from a thread in a very large room, so its surroundings are
essentially at infinity. If the electric field at its surface is to be equal to the breakdown strength
of air ( E ? 3MV /m, what is the absolute potential of the sphere? (4 marks)
(b) In an 5 ? km long copper power line the current density is 2
5 2.5 10
m
A
X . What must be the
minimum transmission voltage so that the power losses in the line do not exceed 1%of the total
transmitted power. X m Cu 1.7 10 . 8 ? ? ? ? (6 marks)
(c) (i) With the aid of a simplified diagram of an experimental set-up and explain how we can
explore the photoelectric effect experimentally.
(ii) Write down Einstein equation for photoelectric effect. Give the name and SI unit of
each symbol involved
(iii) Draw the intensity-intensity characteristic curves and the volt-ampere characteristic
curves and comment on them (10 marks)
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Question 5
(a) A charge Qis divided into two parts q and Q ? q , which are then separated by a
certain distance. What must q in terms of Qto maximise the electrostatic repulsion
between the two charges (6 marks)
(b) Nine copper wires of length ? and diameter d are connected in parallel to form a single
composite conductor of resistance R . What must be the diameter D of a single copper
wire of length ? if it is to have the same resistance? What is the best solution in terms of
economy of material (copper is expensive!!!) (6 marks)
(c) The electron beam in a TV tube consists of electrons accelerated from rest through a
potential difference of about 20kV . Supposing conservation of energy,
(i) What is the speed of electrons when they reach the screen?
(ii) What is wavelength of X-rays produced by electrons on impact on the screen?
(ii) Do think these X-rays are harmful for the viewer? Explain (8 marks)
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