# Help with 2 questions coast of electricty and pressure

1. Nov 13, 2008

### ABR124

Hi

I am stuck on two questions here not sure if this is the right part of the forum to post it in, sorry in advance if it isn't. Ok the questions, my problem is that I'm not sure on the equation to use to solve these problems first problem is

A solar water heater utilizes a pump to lift 400 L of cold water per day through a vertical displacement of 7.0 meters. What is the daily cost of electricity, if the rate is set to be $0.065/kWh? Acceleration due to gravity is 9.81 m/s2 and the density of water is 0.996 g/L. second problem Calculate the pressure exerted by the air to the base of the column (area is 1.0 m2). The density of air can be taken a 1.3 kg/m3 and the volume of the container is 100L. Assume ideal gas behaviour and standard conditions (atmospheric pressure and 25 degrees C) Ok I don't really know where to start with these problems and I'm not looking for the solution to these problems just maybe someone to help me on the right track or maybe to provide me with the equations to do these problems. Thanks in advance. 2. Nov 14, 2008 ### Borek ### Staff: Mentor Electricity is used to do the work of lifting the water. Calculate this work. Note, that water density - as listed - i sworng. I am not sure if I get 2nd question. Is it a closed container? 3. Nov 16, 2008 ### cdr123 the container is open for the second question....the first question first find the mass then find Force 4. Nov 16, 2008 ### ABR124 alright I think I figured out the first problem w=m x g x deltaz =(400L/day)(9.81m/s^2)(7m) = 27468 J/day (27468/3.6 x 10^6) x 0.065 =$0.00049

the second one i'm still having problems with i think the equation is PV=NrT
so it would be P=nr(25 C)/100L I'm just not sure what N and R are....

5. Nov 16, 2008