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To find the relationship between the weight of an object and the efficiency of an electric motor

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                Katrina Joseph

Aim: To find the relationship between the weight of an object and the efficiency of an electric motor


All real machines have an efficiency less than 100% because of the force need to overcome the frictional forces between the moving parts of a machine and to raise any of its moving parts.

There must be a weight which a motor is not capable of lifting. So, I also believe that although the motor can do more work as it draws more current, it becomes less efficient. Efficiency is defined as:

output energy /input energy.

If the output increases more slowly than the input energy the efficiency will decrease.

To test this we increase the load on a motor while measuring the electrical energy going into it.


The apparatus I will need for this experiment are:

  1. An electric motor
  2. Some wires
  3. A power pack
  4. An ammeter
  5. A voltmeter
  6. A piece of cotton
  7. Some weights
...read more.


You can now work out the output and the input and thus work out the efficiency of the motor for the chosen weight and length of the cotton. You can do this by using the equations:

Input = Power x Time (electricity into motor)

Output = Mass x Gravity x Height

  1. Once you have found out the input and output you can then work out the efficiency of the motor by using the equation:

Efficiency = output/ input x 100

  1. Repeat the experiment with different weights and record the results in a table. The weights I have chosen to use are: 10g, 20g, 30g, 40g, and 50g.                


  • Total Input Energy(Joules) = power(Watts) x time(s)
  • Total Output Energy(Joules)= mass(kg)x gravity(m/s/s)x height(m)
  • Efficiency = Output Energy / Input Energy x 100%




When a

...read more.


This experiment has shown that my prediction was correct and that as the input increases, the efficiency of the motor decreases, so a greater fraction of the power will be wasted as heat in the resistance of the motor.


Although my results were mainly correct and did support my hypothesis, my efficiency results were surprisingly low. I believe this may have been due to the inaccuracy of some of my readings, especially time. This was because it was very difficult to stop the clock at exactly the right time, as soon as the weight reached the top. Also, the readings taken from the ammeter and voltmeter may have had a slight inaccuracy. This was a result of not knowing when to take a reading off the metre, as the numbers did not tend to settle at a particular reading.

All in all, I think this was a successful experiment and my results from it prove my hypothesis.

...read more.

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