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Whats the difference?

  1. Oct 21, 2003 #1
    ok, im taking engineering physics II ... unfortunately, im forced to take this as part of my COMPUTER SCIENCE degree ... (why i have no idea..stupid, as i dont really care how many newtons of force it takes me to push a key on the keyboard, and how much current it takes to go from the keyboard to the computer to put the key on the screen) .... sorry, just im not the most science-oriented person. which is why im not a computer engineer.

    so i have a question.

    the question on my homework is this:

    A sloar cell generates a potential difference of 0.10V when a 500-Ohm resistor is connected across it and a potential difference of 0.16V when a 1000-Ohm resistor is substituted. What are (a) the internal resistance and (b) the emf of the sloar cell? (c) The area of the cell is 5.0 (cm^2). What is the efficiency of the cell for converting light energy to internal energy in the 1000-Ohm external resistor?

    question. What is the difference between the potential difference, and its emf? i know V=IR . isnt the total V what comes out of the power source? and isnt the emf the power source? I'm lost on what this is.

    also, for part c ... how would i approach in solving this? The chapter deals with the circuits, resistors, etc. but effeciency?

    any help would be appreciated!
  2. jcsd
  3. Oct 22, 2003 #2
    The EMF can be thought of as the Potential Difference at the terminals of the cell WHEN NO CURRENT is flowing. However, the cell has some resistance, so if a current flows through it, then there is a voltage drop inside the cell. The voltage at the terminals will then drop. The emf is the value without this voltage drop, the PD is the Voltage at the terminals when the voltage has dropped (start you car with the lights on and see them dim as the starter turns - that is internal resistance at work). Draw a diagram of the system, including a resistor next to the cell (the internal resistance, r) and then use simple circuit rules to solve it.

    Efficiency is the ratio of useful energy out, divided by energy into a system. Work out the solar energy total into the cell and then the energy out.

    Hope this helps.
  4. Oct 22, 2003 #3
    Re: Re: whats the difference?

    i am somewhat lost on the last part ... i am going to think that the energy in is 10 mW/(cm^2) ....

    but i am lost on how to get the energy going out into the circuit ...

    ugh, i hate my physics class...im so lost everyday .....
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