Is the Low Energy Loss to EMF in DC Motor Braking Typical?

[adlinkie1]

Homework Statement

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?

Homework Equations

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

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

Cheers

 

[anathema]

I would expect the majority of the kinetic energy to be lost due to frictional forces in the motor, rather than through back e.m.f. This is because frictional forces are typically much larger and more significant in slowing down a moving object. It is also possible that the back e.m.f. is not accurately measured or accounted for in your calculations. I would recommend double checking your measurements and calculations to ensure they are accurate. Additionally, you may want to consider using a different method or equipment to measure the back e.m.f. to confirm your results. Overall, the small fraction of energy lost through back e.m.f. is not unexpected, but it is important to verify the accuracy of your measurements and calculations to ensure your results are reliable.

 

[kerimek]

I would say that your experimental set up and calculations seem to be accurate and well thought out. It is expected that the majority of the kinetic energy lost during braking would be due to frictional forces in the motor, rather than the back electromotive force (e.m.f). This is because the back e.m.f only occurs when the motor is still generating electricity, whereas frictional forces are constantly acting against the motion of the disc.

Additionally, the small fraction of energy lost due to the back e.m.f is also expected, as it is a result of the motor's efficiency. A perfectly efficient motor would not generate any back e.m.f, but in reality, some energy is always lost in the form of heat due to resistance in the motor's components.

Overall, your results seem to align with what would be expected in a real-world scenario. However, as with any scientific experiment, it is always important to consider any potential sources of error and to repeat the experiment multiple times to ensure accuracy.