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Homework Help: Electro-Magnetic Braking

  1. Jan 16, 2010 #1
    1. The problem statement, all variables and given/known data

    I have been using a dc motor, spinning it up to a given speed, not really important. Have used rotational inertia or a disc to work out rotational kinetic energy. After this i have braked the disc by using a load resistor circuit (LR), from which i measured the voltage over the resistor against time. From which i have worked out the power at each rotation. From which using a line of best fit (6th order polynomial), integrated to find out the kinetic energy lost due to the back e.m.f. This often turns out to be a very small fraction of the initial kinetic energy, i.e. 0.01 J lost through e.m.f and 1.6 J lost du to frictional forces in the motor. Is this what you would expect or should it be more like the other way on?

    2. Relevant equations

    I = V/R, P = I2R, Inertia = mr2/Radius, K.E = Inertia*Rotational Velocity2/2

    3. The attempt at a solution

    So i get 1.65 J of Kinetic energy with 10.9 Ohms in the circuit.

    During braking it takes 7.25s for the disc to come a complete stop.

    The power against time graph gives a line of best fit of:

    y = 6E-07x6 - 2E-05x5 + 0.0002x4 - 0.001x3 + 0.0034x2 - 0.0065x + 0.0059

    Integrated between o and 7.25 s as time is on the x axis, which gives 0.0191 roughly.

    Does this mean that only 0.0191 J of energy was dissapated over the resistor as heat?

    Any help will be greatly appreciated

    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
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