K.C.S.E Physics Q & A - MODEL 2011PP2QN19
(a) When a radiation was released into a diffusion cloud chamber, short thick tracks were observed. State with a reason, the type of radiation that was detected.
(b) The half-life of an element X is 3.83 days. A sample of this element is found to have an activity rate of 1.6 x 103 disintegrations per second at a particular time. Determine its activity rate after 19.15 days. (c) State what is meant by an extrinsic semiconductor. (d) Figure 12, shows a depletion layer in an unbiased p-n junction. 
State how a battery can be used to make the depletion layer narrower.
(e) Figure 13, shows an incomplete circuit of a full wave rectifier 
(I) Draw in the figure two more diodes to complete the circuit.
(ii) Show on the figure the points across which the output of the rectifier should obtained.
answers
(c) A semiconductor in which impurities have been added to change conductivity.
(d) By connecting it in forward biased mode (i.e. P to + and n to -) 
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K.C.S.E Physics Q & A - MODEL 2011PP2QN18
(a) State two differences between cathode rays and electromagnetic radiations.
(b) Figure 10, shows the main features of a cathode ray oscilloscope (CRO) 
(i) Name the parts labelled M and N.
M N (ii) Explain how electrons are produced in the tube. (iii) When using the CRO to display waveforms of voltages, state where the following should be connected: (I) the voltage to be displayed on the screen; (II) the time base voltage. (iv) state why the tube is highly evacuated. (c) Figure 11, shows the waveform of a voltage displayed on the screen of a CRO. The Y-gain calibration was 5V per cm. 
(i) Determine the peak-to-peak voltage of the Y-input.
(ii) Sketch on the same figure the appearance of the waveform after the voltage of the input signal is halved and it’s frequency is doubled.
answers
(a) -Cathode rays have charge but e.m radiations don’t have charge;
-Cathode rays are particles and have a mass but e.m radiations are waves; -Cathode rays travel at a speed depending on the accelerating voltage but e.m radiations travel at the speed of light in vacuum; - Different in the mode of production. (b) (i) M - grid; N - accelerating anode/anode/vacuum; (ii) Cathode is heated by filament; electrons are released from cathode; by thermionic emission (iii) (I) across Y-Y plates. (Il) across X-X plates. (2 m (iv) to reduce collisions, (hence ionization) with air molecules in the tube. (c) (i) peak-to-peak voltage = 5 x 2 =10v 
K.C.S.E Physics Q & A - MODEL 2011PP2QN17
(a) State what is meant by the term “electromagnetic induction”.
(b) Figure 9, shows a simple electric generator 
(i) Name the parts labelled P and Q.
P . Q (ii) Sketch on the axes provided, a graph to show how the magnitude of the potential difference across R, changes with the time t. 
(iii) State two ways in which the potential difference produced by such a generator can be increased.
(c) In a transformer, the ratio of primary turns to the secondary turns is 1:10. A current of 500mA flows through a 200 ohms resistor in the secondary circuit. Assuming that the transformer is 100% efficient, determine: (i) the secondary voltage; (ii) the primary voltage; (iii) the primary current.
answers
(a) The production of induced e.m.f when the magnetic flux linking a circuit is changed;
(b) (i) P - brushes Q - slip rings 
(iii) Increasing number of turns/coils;
Increasing speed of rotation; Increasing strength of field; Winding the coil on soft iron core. 
K.C.S.E Physics Q & A - MODEL 2011PP2QN16
(a) State the meaning of the term “principal focus” as applied in lenses.
(b) You are provided with the following apparatus to determine the focal length of a lens: - a biconvex lens and lens holder. - alit candle. - a white screen. - a metre rule 
(i) Draw a diagram to show how you would arrange the above apparatus to determine the focal length of the lens
(ii) Describe the procedure you would follow. (iii) State two measurements that you would take. (iv) Explain how the measurements in (iii) would be used to determine the focal length. (c) An object is placed 30cm in front of a concave lens of focal length 20cm. Determine the magnification of the image produced.
answers
(a) The point at which rays close to and parallel to the principal axis converge or seem to diverge from after striking the lens;
(ii) Candle is placed at a certain distance from the lens. The distance between the screen and the lens is adjusted until a sharp image is focused on screen.
(iii) The distance of candle from lens (U) is measured; The distance of screen from lens (V) is also measured; 
K.C.S.E Physics Q & A - MODEL 2011PP2QN15
(a) Figure 8, shows a graph of potential difference V (volts) against a current I (amperes) for a certain device.
From the graph:
(i) state with a reason whether or not the device obeys ohms law. (ii) determine the resistance of the device at: (1) 1 =l.5A (II) I =3.5A (iii) From the results obtained in (ii) state how the resistance of the device varies as the current increases. (iv) State the cause of this variation in resistance. (b) Three identical dry cells each of e.m.f. 1 .6V are connected in series to a resistor of 11.4 ohms. A current of 0.32A flows in the circuit. Determine: (i) the total e.m.f. of the cells; (ii) the internal resistance of each cell;
answers
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