sssamudra98 Posted July 2, 2015 Share Posted July 2, 2015 i = dq/dt so for a conductor of length L, maintained at constant potential difference, we know that q = enAL where e is charge on electronn is electrons per unit vol.A is area of cross section of conductorSO in q = enAL what is changing with time so that we come across rate of change of charge Link to comment Share on other sites More sharing options...
studiot Posted July 2, 2015 Share Posted July 2, 2015 (edited) This is a very important technique in Engineering and you have posted in the Engineering section. What you have is a control volume and your equation is not quite right for n, since they electrons at the start of the period are not the same as the eelctrons at the end ie you have electrons entering and leaving the control volume over the period so the current is the rate of electrons crossing a boundary surface. If you like n = dn/dt (leaving) - dn/dt (entering) this control volume idea is important in Thermodynamics (heat flow) Fluid mechanics Electricity and comes under the heading transport phenomenon. Momentum may be transported as well as mass and heat. Edited July 2, 2015 by studiot Link to comment Share on other sites More sharing options...
sssamudra98 Posted July 2, 2015 Author Share Posted July 2, 2015 you mean to say in q = neAL nis causing variation in q ?? AND what if A (area of cross section) is also varying ?? Link to comment Share on other sites More sharing options...
studiot Posted July 2, 2015 Share Posted July 2, 2015 I can move the goalposts too. What if L is varying? Is this a serious question or a windup? Link to comment Share on other sites More sharing options...
sssamudra98 Posted July 2, 2015 Author Share Posted July 2, 2015 i am not understanding what you are trying to say can you please define current in light of my doubt this doubt pinching me since long without having cleared this i cannot proceed please help me Link to comment Share on other sites More sharing options...
studiot Posted July 2, 2015 Share Posted July 2, 2015 You have not responded to my first answer. Link to comment Share on other sites More sharing options...
sssamudra98 Posted July 2, 2015 Author Share Posted July 2, 2015 What if L is varying ? varying L will vary q Actually i want to ask I=dq/dt? so, for every charge leaving the cross-section of the conductor, an equivalent one will enter. This means that across the cross-section dq/dt, the summed change of charge with respect to time, equals 0! Link to comment Share on other sites More sharing options...
studiot Posted July 2, 2015 Share Posted July 2, 2015 I=dq/dt? so, for every charge leaving the cross-section of the conductor, an equivalent one will enter. This means that across the cross-section dq/dt, the summed change of charge with respect to time, equals 0! Of course it does. The rate of charge entry equals the rate of charge exit. So the rate of accumulation of charge is zero. The summed charge is not the same as the accumulation of charge; the summed charge must include all charge entering and leaving the control volume in the time period. Link to comment Share on other sites More sharing options...
Sensei Posted July 2, 2015 Share Posted July 2, 2015 What if L is varying ? varying L will vary q If you will change length L, resistance R will change. The higher L, the bigger resistance R. Take a play with resistance wire, and check resistance with multimeter at different distances from the beginning.. They are sold per meters. Like f.e. 100 Ohms per meter. http://www.ebay.com/bhp/resistance-wire Link to comment Share on other sites More sharing options...
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