Saturday, March 27, 2010
WD on or by electric force
I am pleased that Anirban spoke his mind. I am also pleased to have an opportunity to discuss the confusing part. Now as this topic involves positive and negative charges you have to be more careful. The electron moving upwards is decelerating as the electric force is opposite to motion. If you hold on to basic from here on you would have got the final velocity correct. In fact many just gave a wrong answer by just not putting negative sign for acceleration. You can sense something wrong when your final velocity is larger than initial. So The KE is decreasing so where is the energy ... there is increase in EPE. No need for you to deduce this as the question ask you to calculate the increase in EPE which is final -inital KE (drop the minus sign) . So far so good. In the final part , the graphs should be drawn for the entire journey until the electron comes to a rest. This is the part where you have to use the final -initial KE (drop the minus sign) but the final is zero. So only KE(initial) = qV where V is the potential difference. Answer should be 160 V marked on the y axis to get one mark. A few students got 35 V which is carelessness i think but good try. The distance no need to mark with value . Just note that it is not 0.0177m as obtained in earlier part . The electron would have traveled greater when it comes to a rest. Any more queries?