K.C.S.E Physics Q & A - MODEL 2015PP2QN18
​(a) Three resistors of resistance 2 ohms, 3 ohms and 4 ohms are to be connected to a cell such that they have the least effective resistance.
(I) Draw a circuit diagram to show how they can be connected to achieve this. (ii) Determine the least effective resistance of the three resistors. 
​(b) A real object of height 1 cm placed 50 mm from a converging lens forms a virtual image 100 mm from the lens.
(i) Determine the: (I) focal length of the lens; (II) magnification. (ii) On the grid provided draw to scale the ray diagram for the set up, to show how the image is formed.
answers
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 K.C.S.E Physics Q & A - MODEL 2015PP2QN17
​(a) State three factors that affect the capacitance of a parallel plate capacitor.
(b) Figure 5 shows the circuit used to charge a capacitor C 
​(i) State what would be observed on the following when the switch is closed:
(I) the milliammeter; (II) the voltmeter; (ii) Explain how the capacitor gets charged. (iii) State the purpose of the resistor R. (iv) On the axes provided, sketch the graph of voltage (V) against time (t) 
answers
​(a) distance of separation between plates
area of overlap of plates type of dielectric between plates (b) (i) (I) Current rises to maximum and then drops to zero (II) Potential difference between the plates increases to a maximum (ii) Negative charges flow from the negative terminal of the battery to one plate of the capacitor. Negative charges flow from the other plate of the capacitor to the positive terminal of the cell hence equal positive and negative charges gather on the plates, opposing further flow of electrons when fullycharged (iii) Resistor - to slow down the charging process so that current and voltage are is observed. 
K.C.S.E Physics Q & A - MODEL 2015PP2QN16
​(a) It is observed that alpha particles have a lower penetrating power than beta particles. Explain this observation.
(b) A radioactive substance has a half life of 12 years. Determine the time it would take to decay to 12.5% of its original value.
​(c) A Geiger Müller (GM) tube is used for detecting radiations from a radioactive source.
State the function of: (i) the mica window; (ii) bromine gas in the tube. (d) (i) In a diffusion chamber, explain why some of the tracks formed are observed to be; (I) Short, (II) Straight. (ii) State two advantages of using a GM tube instead of a diffusion cloud chamber to detect radiations from radioactive substances.
answers
(a) Alpha particles are heavier and move at lower speeds hence less penetrating power than
Beta particles which are lighter and move faster 
​(c) (i) allows the radiations into the tube
(ii) absorbs kinetic energy of positive ions so that they do not cause secondary ionization in the tube (d) (i) Short - They ionize heavily loosing most of the energy hence cannot travel far. Straight - They are massive compared to air molecules hence collision with air molecules cannot change their path. (ii) GM is easily portable than a cloud chamber. GM is more sensitive. GM tube detects radiation at very low intensity and cloud chamber cannot detect radiation at very low intensity. K.C.S.E Physics Q & A - MODEL 2015PP2QN15
​.(a) State two factors that determine the resistance of a metallic conductor.
(b) Explain how a fuse safeguards electrical appliances against excessive currents.
​(c) A hair dryer is rated 2.5 kW, 240 V.
(i) Determine whether a 10 A fuse may be suitable for the hair dryer. (ii) Determine the cost of using the hair dryer for 3 hours if the cost of electricity is Ksh 0.80 per kilowatt hour.
answer
​(a) length of conductor
area of cross-section temperature resistivity of conductor (b) When excessive currents flow through the circuit, the wire gets heated and melts hence breaking the circuit. 
​Fuse not suitable since current through the appliance is higher than the fuse rating
K.C.S.E Physics Q & A - MODEL 2015PP2QN14(a) State two factors that affect photoelectric emission.
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