Results 1 to 4 of 4

Math Help - Simple physics help

  1. #1
    Member Jones's Avatar
    Joined
    Apr 2006
    From
    Norway
    Posts
    170

    Simple physics help

    Hi, i have a physics problem i can't solve....

    In one point A there is a positive charge Q1=50nC and in another point a negative charge Q2=-80 nC. AB=0,20m Determine the acceleration(size and direction) for an electron situated in the middle of AB.

    I don't even know how to begin to solve this one
    Last edited by Jones; September 9th 2006 at 08:04 AM.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Forum Admin topsquark's Avatar
    Joined
    Jan 2006
    From
    Wellsville, NY
    Posts
    9,939
    Thanks
    338
    Awards
    1
    Quote Originally Posted by Jones
    Hi, i have a physics problem i can't solve....

    In one point A there is a positive charge Q1=50nC and in another point a negative charge Q2=-80 nC. AB=0,20m Detirmine the acceleration(size and direction) for an electron situated in the middle of AB.

    I don't even know how to begin to solve this one
    You know how to calculate the force on a charge due to another charge, right? (Coulomb's Law) Now recall that force is a vector. So we need to find the vector sum of the forces on the electron due to the charges at points A and B.

    Define midpoint of AB to be the origin (ie. I'm placing the origin at the electron.) I am going to define a positive direction to the right. (Point A is at -0.10 m and point B is at 0.10 m.) NOTE: I am taking the absolute value of the charges in the force formula and figuring out the direction of the force on the electron by whether the electron is attracted or repelled by the charge.

    The force on the electron by the charge at A is:
    F_{eA} = -\frac{kq_eQ_1}{r_{eA}^2} = -\frac{9 \times 10^9 \cdot 1.60 \times 10^{-19} \cdot 50 \times 10^{-6}}{0.10^2} N
    (This force is in the negative direction since the two charges attract, indicated by the "-" sign out front.)

    The force on the electron by the charge at B is:
    F_{eB} = -\frac{kq_eQ_2}{r_{eB}^2} = -\frac{9 \times 10^9 \cdot 1.60 \times 10^{-19} \cdot 80 \times 10^{-6}}{0.10^2} N
    (This force is in the negative direction since the two charges repel, indicated by the "-" sign out front.)

    Now just add the two forces. Finally, to get the acceleration of the electron, use F = ma. I would describe the acceleration as being in the direction of A or B. (Ans. The acceleration is in the direction of point A.)

    -Dan

    NOTE: In case you are using the "other" form of Coulomb's Law, use
    F = \frac{1}{4 \pi \epsilon _0 }\frac{q_1q_2}{r_{12}^2}

    The Coulomb constant, k is defined as k = \frac{1}{4 \pi \epsilon _0} = 9 \times 10^9 \, \frac{N m^2}{C^2}.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member Jones's Avatar
    Joined
    Apr 2006
    From
    Norway
    Posts
    170
    Thank you very much for the help.

    Funny i have never seen the definition of k before
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Forum Admin topsquark's Avatar
    Joined
    Jan 2006
    From
    Wellsville, NY
    Posts
    9,939
    Thanks
    338
    Awards
    1
    Quote Originally Posted by Jones
    Thank you very much for the help.

    Funny i have never seen the definition of k before
    My general experience is that engineering books tend to use \epsilon _0 and physics books use "k," but it really depends on the book. Obviously, the two approaches are equivalent.

    -Dan
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Physics Simple Calculation
    Posted in the Math Topics Forum
    Replies: 0
    Last Post: April 27th 2009, 04:25 PM
  2. Replies: 6
    Last Post: October 14th 2008, 12:46 AM
  3. simple Physics
    Posted in the Math Topics Forum
    Replies: 1
    Last Post: September 17th 2008, 04:18 PM
  4. how do solve quantum physics or physics equations
    Posted in the Advanced Applied Math Forum
    Replies: 2
    Last Post: March 26th 2008, 04:33 AM
  5. physics, acceleration, physics problem
    Posted in the Math Topics Forum
    Replies: 3
    Last Post: September 29th 2007, 03:50 AM

Search Tags


/mathhelpforum @mathhelpforum