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What really is a Pascal and a Joule?

  1. Jan 21, 2015 #1
    Hi forum,

    First time poster. I'm in thermo this year and it's the first time working with pascals and joules. My teacher and most post just jump right into the equations, 1Pa = 1N/m^2 and 1J = 1N * M. But what really is a pascal? I get what a N is, force to move 1kg*m/s^2. If I convert N, I get 1Pa = (1kg*m/s^2)/m^2.

    Solving further I get 1Pa = 1kg/s^2*m (1m on top cancels 1m on bottom, if I recall algebra correctly).

    Can someone please explain what that means, in terms of a sheet of metal or something?

    Same with a joule, convert N and its 1kg*m^2/s^2.

    When I see something ^2, I immediately think "Ok, this is 2d we are talking about over the surface of a meter^2 ect.", same with ^3 "Ok, this thing has a volume in 3d space we are talking about". It's just how my mind seems to think about it.

    Sorry if I'm approaching this wrong or should think of a N more abstractly. I just like to understand things better in 3d/2d algebraic sense and physics is based in that. Can anyone explain those 2 in context or real world? I just hate plug and chug, not knowing really what I'm "saying" to myself with a final answer.

    Thanks,
    Paul J.
     
  2. jcsd
  3. Jan 21, 2015 #2

    SteamKing

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    Staff Emeritus
    Science Advisor
    Homework Helper

    A pascal is a unit of pressure, something which will come up a lot in your thermo classes. Pressure is defined as force (N) per unit area (m2). Remember, in SI, the force unit is also a derived unit.

    If you check the pressure in a car or bicycle tire, the gauge will read in units of kPa probably. Atmospheric pressure is approximately 101 kPa.

    The joule is the SI unit of energy or work. It is defined as the amount of work performed by a force of 1 N applied over a distance of 1 meter. You will also encounter work quite frequently in your study of thermo.

    I find it quite unlikely that you have not studied or encountered pressure or work before taking your first thermo course. Typically, an introductory physics course covers these topics.
     
  4. Jan 21, 2015 #3

    jedishrfu

    Staff: Mentor

    Newton is a unit of measure for force so think F=ma meaning that the newton is related to accelerating a mass and where accelerating is a changing velocity over time hence (m/s)/s or m/s^2 so the newton then is kg m/s^2

    http://en.wikipedia.org/wiki/Newton_(unit)

    The Pascal is a unit of measure for pressure and pressure is force on an area. For one pascal its one newton of force per square meter or ( kgm/s^2 / m^2 ) or (kg/ms^2)

    http://en.wikipedia.org/wiki/Pascal_(unit)
     
    Last edited: Jan 21, 2015
  5. Jan 21, 2015 #4

    BiGyElLoWhAt

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    Gold Member

    It seems as thought you have the idea of a newton down, but I think what's confusing you is you're canceling units within Netwons.

    Just think of a pascal as N/m^2. the m^2 comes from the fact that a pascal is force per unit Area (that's where that 2d thing you were talking about comes in). So if I have a piece of metal, like a bucket or something, and I let water sit inside of it, that water is going to exert a force on the bucket, but sometimes it's more useful to look at that force divided by the area rather than the total force.

    The same with Joule's. Don't think of it as Kg*m^2/s^2, think about it as a force being applied over a distance. Joule is energy, and an external force does work on an object by an amount of ##W=F\cdot ds## where F is the force and ds is a small chunk of the path that it travels (a line is 1d).
     
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