Download PHY_211_ADDITIONAL_REVISION_QUESTION_

PHY 211 ADDITIONAL REVISION QUESTIONS
Useful constants: Electronic charge, 𝑒 = 1.6 × 10−19 𝐶, mass of electron 𝑚 =
9.1 × 10−31 𝑘𝑔.
Data for questions 1
Material
n
p
Silicon
Germanium
GaAs
0.135
0.39
1.1
.05
0.19
0.05
Intrinsic
Atom
carrier
density
density
in
3
m
1.7 × 1016
5 × 1028
2.4 × 1019
4.41 × 1028
1.4 × 1012
4.43 × 1028
1.
A germanium rod has diameter 2mm and length 50cm. Calculate its
resistance before it is doped. Find also its resistance after it is doped
with (a) trivalent atoms and (b) pentavalent atoms. (see data above for
the parameters needed). Assume a doping ratio of 1:107.
2.
In a CRT, VA = 1135, l= 2cm, d = 1cm, L = 25cm. The p.d. across the yplates is 10V, calculate the velocity of the electron on leaving the
electron gun and the total deflection in the vertical direction when it
arrives at the screen.
3
Describe the structure and mode of operation of the cathode ray tube.
State four applications of the C.R.O (b)
The screen of a cathode ray
tube measures 15cmx15cm. The final anode voltage is 1135V. The
electron beam is to be magnetically deflected. The deflecting coil is
25mm long and its distance to the screen is 28cm. calculate (i) the
velocity of the electrons on leaving the final anode (ii) the flux density
required to deflect the spot of light to the edge of the screen.
4
A cathode-ray tube has an accelerating potential of 2kV. It is fitted with
two pairs of deflecting plates, the pair being mutually perpendicular.
Each plate is 1.5cm long, the spacing between the plates being 1cm.
Assume that the centres of the pairs coincide calculate the lengths of the
line produced if the input signals are Vy = 50V rms, and Vx = 30V rms.
From the centre of the deflection plates to the screen is 38 cm.
5
A 15cm cathode-ray tube is to be magnetically deflected. The final
Anode voltage is 2kV, the deflecting coil is 2.5cm long and the distance
from its centre to the screen is 38 cm. find the flux density required to
defect the beam to the edge of the screen. Through what angle is the
beam deflected.
6
Describe the Thomson’s experiment to measure q/m.
7
Explain in detail how to determine the variation thermoemf of a copper
– constantan
thermocouple with temperature In a copper/iron
thermocouple, the EMF generated at temperature θoC is given by the
table below. Deduce the constants a and b in the expression for the
thermo-emf
θ0 C
50
100
150
200
200
emf(mV)
0.65
1.20
1.65
2.0
2.25
8
Describe the construction of a practical light dependent resistor. Explain
its use in street light switching.
9
Describe the operation of the photo-electric tachometer
10
(a)
(b)
Explain the electron-gas model of a metal.
Calculate the end-to-end resistance of a rectangular bar of
intrinsic GaAs crystal at 3000K. Take w=2mm, t= 1mm, and l=
10cm. Intrinsic carrier density = 1.4x1012/m3
GaAs number density = 4.43x1028atoms/m3
µn=1.10, µp= 0.05
. If the doping ratio is 1: 108, find the
resistances of a p-type GaAs bar and an n-type GaAs bar
11
12
13
14
Describe the Hall effect experiment.
Explain the working of the Hall Effect fluid level detector and the Hall
effect probe
For a certain cathode material in a photoelectric experiment a resending
measures a stopping potential of 1V. 2V, 3V, 4V, and 5V for light of
wavelengths 400nm,300nm, 240nm, 200nm and 171nm respectively.
Determine the work function for this material and the value of the
plank’s constant. For each wavelength, calculate the maximum kinetic
energy and the velocity of the emitted electron.
Describe the Thomson’s experiment to measure q/m.
B = ITesla
1mm
1mm
10A
2mm
1cm
15
With the aid of a neat diagram explain the phenomenon of Hall effect.
Obtain the formula for VH.
An n-type silicon water is shown. The
number density of the electron is 4x1020/m3 Determine (a) The current
density (b) The Hall coefficient (c) The magnitude and polarity of the
Hall voltage
16
An electron moves in a vertical plane with a speed 1.41x107m/s and
enters a region where there is a uniform electric field of 3500N/C
directed downwards. At the point of entry of the electron into the
electric field, its velocity is along a path directed at angle 30 degrees
below the horizontal. Find (a) the velocity of the electron 5  10 8 s after
it enters the electric field and (b) The position at the electron at this
time. Draw a sketch of the orbit
17
A proton q  1.60  10 19 and m  1.67  10 27 kg is moving in a magnetic field
of intensity 1.5T directed along the x  axis. The initial velocity of the
proton has components u x  2.5  10 5 m / s, u y  0 and u z  2.0  105 m / s
Calculate (a) the force on the proton and the acceleration at time t  0
(b) the radius of the helical path (c) the angular speed of the proton (d)
The pitch of the helix (the distance travelled along the axis of the helix
per revolution Draw a sketch of the orbit