April 1st, 2015, 11:20 AM  #1 
Senior Member Joined: Dec 2014 From: Canada Posts: 110 Thanks: 4  Exam Question
Can someone confirm if I answered correctly? Question: According to rumours the next generation smart phone will be powered by a 3.82V battery that can deliver 1.81A of current for one hour. The phone automatically turns off when the battery has less than 2% of charge remaining. i) Calculate the amount of charge that can be delivered by the battery. Solution: I=Q/T Q=IT Q=(1.81*60*60)= 6516C ii) How much charge has passed before the phone loses power? 100%2% = 98% 6516*0.98= 6385.68C iii) Show that if a charge Q is transferred across a p.d V, the energy consumed is equal to QV. P=IV = (1.81*3.82) = 6.9142W Calculating Charge at 1 second: I=Q/T Q=IT (1.81*1) = 1.81C Therefore P=QV = (1.81*3.82) = 6.9142W iv) In an idle state the phone uses a constant 70mW of power. How many days will pass before the phone loses power completely? P=IV I=P/V I=70mW/3.82V = 0.0183A Therefore charge every second = 0.0183C Total time = 6516C/0.0183C = 356065.6s 356065.6/3600 = 98.9 hours. v) The majority of components inside a smart phone are made up of semiconducting material. Briefly explain how the electrical conductivity of an intrinsic semiconductor increases as the temperature rises. Upon temperature rise, valence electrons gain energy and break free becoming free electrons. The more free electrons the greater the conductivity. The semiconductor is known as negative temperature coefficient. Temperature rises and resistance decreases. Since resistance decreases so does the resistivity. Conductivity is the reciprocal of resistivity therefore conductivity will increase. vi) Explain the origin of holes in intrinsic semiconducting materials and explain the process by which holes participate in current flow. When the atom has absence of electrons it means it has a hole. Therefore the atom would lack any free electrons, this would will result in no current flow making it an insulator. 
April 7th, 2015, 12:47 AM  #2 
Senior Member Joined: Apr 2014 From: Glasgow Posts: 2,164 Thanks: 736 Math Focus: Physics, mathematical modelling, numerical and computational solutions 
Looks good to me.


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