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Fce 132: Engineering Physics 1B Question Paper
Fce 132: Engineering Physics 1B
Course:Civil And Construction Engineering
Institution: University Of Nairobi question papers
Exam Year:2013
SECTION A
Question 1 (20 marks)
a) State Coulombs law and give the equation involved. How is the equation different from the gravitational case? ( 5 marks)
b) A potential difference of 10kV is established between parallel plated in air
i) If the air becomes electrically conducting when the electric field exceeds 4 x 10^6 vm^(-1), what is the maximum separation of the plates (2 marks)
ii)When the separation has the minimum value calculated in part (i), what is the surface charge density on each plate?
c)Three capacitors c1, c2 and c3 connected in series are shown below. A potential difference V is applied across all three
Work out through (i) to (iv) below in order to obtain the value on terms of c1, c2, c3 of a single capacitor c that would replace all three. Hence or otherwise work out (v) below
i) The diagram above shows that each capacitor has an identical charge. Explain how this comes about (2 marks)
ii) Write down expression for the following:
V1 in terms of Q and C1
V2 in terms of Q and C2
V3 in terms of Q and C3
V in terms of Q and C
iii) What is the total potential difference V across all three capacitors in terms on V1, V2 and V3?
iv) Using your answers to (ii) and (iii) obtain an expression for c in terms of C1, C2 and C3. (1 mark)
v) What is the combined capacitance of a 10 microfarad, 20 microfarad capacitor and a 30 microfarad capacitor connected in series? (1 mark)
d) Draw and explain a circuit which can be used for charging and discharging a capacitor through a resistor. Hence or otherwise write equations for charge for both cases. (3 marks)
Question 2 (20 marks)
a) Briefly explain the following terms and write their units
i) Potential Difference
ii) Resistance
iii) Resistivity
iv) Conductance
v) Conductivity
vi) Current density
b) A 150W light bulb is connected to a 120V line. What is the current drawn from the line? What is the resistance if the bulb whilst it is burning? How much energy is consumed if the light is kept on for 6 hours? What is the cost of this energy at 8 shillings/kWh? (5 marks)
c) i) State Kirchhoff Laws (2 marks)
ii) Consider the circuit shown where R1 = 20 ohms, R2 = 40 ohms and V1 = 20V. Find the current through R1 (7 marks)
SECTION B
Question 3
a) An electron in a television picture tube moves toward the front of the tube with a speed of 8 x 10^6 m/s. Surrounding the neck of the tube are coils of wire that create a magnetic field of magnitude 0.025 T directed at an angle of 60 degrees to the initial direction of the electron.
i) Calculate the magnitude if the magnetic force on the electron. (3 marks)
ii) Explain why the magnetic force in (i) above does not do any work when the electron is displaced? (2 marks)
b) State three differences between Electric and Magnetic forces. (3 marks)
c) The electric field in an electromagnetic wave is given by
E=100sin(1x10^7x -(omega x t))
where all the values are expressed in SI units. Find
i) The amplitude if the corresponding magnetic field oscillations. (2 marks)
ii) The wavelength of the EM wave. (2 marks)
iii) The frequency of EM wave. (2 marks)
d) A long, straight vertical wire carries a downward current of 4A. The earth''s magnetic field is which this wire is placed has a horizontal component of 1.6 x 10^-5 T. Calculate:
i) The resultant horizontal magnetic flux density at a point 10cm to the west of the wire. (3 marks)
ii) The distance from the wire to a neutral point. (3 marks)
Question 4
a) A 50- turn rectangular coil of dimensions 5cm x 10cm is allowed to fall from a position where magnetic field B is zero to a new position where B = 0.5T and the magnetic field is directed perpendicular to the plane of the coil. Calculate the magnitude of the average emf that is induced in the coil of the displacement occurs in 0.35s. (3 marks)
b) A proton moves with a velocity of V =(2i-4j+k)m/s B =(i+2j-3k)T. What is the
magnitude of the magnetic force this charge experiences?
c) A wire 2-8 m in length carried a current of 5A in a region where a uniform magnetic filed has a magnitude of 0.39T. Calculate the magnitude of the magnetic force on the wire if the angle between the magnetic fields and the current id
i) 90 degrees. (2 marks)
ii) 120 degrees. (2 marks)
d) From Faraday''s law of electromagnetic induction, state three ways in which emf can be induced in a conductor loop. (3 marks)
e) A sinusoidal electromagnetic wave of frequency 40MHz travels in free space in x-direction the figure below.
i) Determine the wavelength and period of the wave (4 marks)
ii) At some point and some instant, the electric field has a maximum value of 750 N/C and is along the y-axis. Calculate the magnitude of the magnetic field at this position and time. (3 marks)
iii) Arrange the following regions of electromagnetic spectrum according to increasing energy: X-rays, radio waves, gamma rays, microwaves, visible light. (1 mark)
SECTION C (20 marks)
Question 5
a)
i) In a radioactive decay, the number of number of radioactive nuclides at time t=0 was No, and after time t, the number remaining was N. Show that the fraction of the particles remaining is given by:
(N/No)= (1/2)^(t/T0.5) where T0.5 is the half time of the element. (6 marks)
ii) A sample of radioactive material has 10^(18) atoms. The half time of the material is 2 days. Calculate the fraction remaining after 5 days. (4 marks)
b) Electrons are accelerated from rest through a p.d of 10kV in a X-ray radiation generated. (3 marks)
Question 6
a) Light caused by a transition between n=2 and n=1 levels is shown on the surface of magnesium causing emission of electron. Magnesium has a work function 3.66eV. Calculate the maximum K.E. of the emitted electrons in Joules. (5 marks)
b) When X-ray photons hit calcium, electrons are released. The frequency of a photon will have to be more than the threshold frequency of an electron is to be released. Discuss this statement in terms of the underlying physical principles. (5 marks)
c)
i) X-rays of frequency 1.53x10^(16) Hz cause the emission of electrons from a material with a maximum kinetic energy of 2.18x10^(-18)J. Calculate the threshold frequency for the release of electrons from the material. (5 marks)
ii) Describe and explain one piece of experimental evidence which shows that light is not simply a wave, but that it also has a particle nature. (5 marks)
iii) Explain why, if a photon causes an electron to jump to a higher energy level, the exact energy of the photon is critical, but if it is used to release an electron from the atom, it is the minimum energy of the photon that is critical. (5 marks)
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