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Electrical Forces help. Two questions

  1. Feb 26, 2014 #1
    Did a practice test but I would like to understand these two questions a bit more. I know the answers to both but that doesn't mean anything if i don't get it. Any help would be appreciated

    A very small 4.8-g particle carrying a charge of +9.9 μC is fired with an initial speed of directly toward a second small 7.8-g particle carrying a charge of + The second particle is held fixed throughout this process. If these particles are initially very far apart, what is the closest they get to each other? (k = 1/4πε0 = 9.0 × 109 N • m2/C2)

    Two tiny beads, each of mass 3.2 g, carry equal-magnitude charges. When they are placed 6.4 cm apart and released in outer space, they begin to accelerate toward each other at 538 m/s2. What is the magnitude of the charge on each bead? (k = 1/4πε0 = 9.0 × 109 N • m2/C2)
     
  2. jcsd
  3. Feb 26, 2014 #2

    Simon Bridge

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    Welcome to PF;
    Those are great questions - please tell us how you understand them so we can see how best to help you.
    i.e. there are a number approaches - how would you go about finding the answers and what is your reasoning along the way?
     
  4. Feb 26, 2014 #3
    In Classical Physics
    1. Force = mass x acceleration.
    2. Energy = ½ x mass x (velocity²) = Force x distance = ...
    3. Simple math requires Position(now) = Position(then) + (velocity(then))x(time interval between then and now) + ½ x acceleration x (time interval between then and now²)
    4. Velocity(now) = velocity(then) + acceleration(between then and now) x (time interval)
    5. Two common forces are electrical and gravitational. Both follow inverse square laws (in simple situations). The inverse square law is F = kPp/(r²) where F=force, k is a constant of Nature, r is distance between particles and P and p are quantities associated with the Law (mass or charge in these two cases) of Particle1 and particle2.
    6. momentum = velocity x mass
    7a. >>and most important<< energy (including potential energy) is conserved.
    7b. >>and just as important<< momentum is conserved.
    All the rest is algebra (and geometry).
     
  5. Feb 26, 2014 #4
    Its been over a year since I've taken physics 1. My practice was for physics 2 which I'm finally taking. All the other questions I was able to answer but these two seem to borrow concepts from physics 1 so I'm a bit clueless.

    I figured for the second question, I'd solve for force=ma and then solve for the charges given coulombs law since I have the force. Thats what I'm doing but I'm not getting 890 nC which is the answer.

    F= ( 3.2g)(538 m/s)
    F=1721.5

    F = kPp/(r²) and solve for P.

    Edit: Forgot to change the units for question 2. I guess thats what happens when you take so long to take physics 2. Help would still be appreciated for question 1! I noticed I was missing information when I posted it.

    A very small 4.8-g particle carrying a charge of +9.9 μC is fired with an initial speed 8.0 m/s of directly toward a second small 7.8-g particle carrying a charge of +5.2 μC + The second particle is held fixed throughout this process. If these particles are initially very far apart, what is the closest they get to each other? (k = 1/4πε0 = 9.0 × 109 N • m2/C2)
     
    Last edited: Feb 26, 2014
  6. Feb 26, 2014 #5

    Simon Bridge

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    Yah - for Q2 you can just use that the force is equal to the mass times acceleration - and Coulombs force law.
    The other approach is to say that the force is proportional to the gradient of the potential energy.

    Q1 uses conservation of energy - or the Work-energy theorem.
    Start out by describing the initial and final conditions in terms of potential and kinetic energy.
     
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