Efficiency
Energy and Thermal Physics

Using an electric motor to raise a load

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

An electric motor lifts a load.

Apparatus and Materials

  • Switch unit (very useful to break circuit quickly)
  • Line shaft unit
  • Motor unit
  • Demonstration meter, 0 - 5 amp DC
  • Rubber band or driving belt
  • Cord
  • Mass, 1 kg
  • Power supply, 0–12 V

Health & Safety and Technical Notes

Read our standard health & safety guidance


The mass you select as the load must be suitable for the size of motor you have available.

Procedure

  1. Clamp the motor unit next to the line shaft unit and connect their pulleys with a rubber band or driving belt.
  2. Secure a length of cord to the axle of the line shaft and attach the other end to the chosen mass.
  3. Connect the motor to the power supply in series with the demonstration meter.
  4. Read the ammeter when the load is being lifted and compare this with the reading obtained when the motor is running light (that is, when disconnected from the line shaft}.

Teaching Notes

  • In this demonstration the current flowing in the motor produces a force that lifts the load.
  • When analysing the situation in terms of energy it is helpful to be clear about start and end points.
  • Before the motor is turned on, there is more energy stored chemically in the battery or the fuel (and oxygen) in the power station.
  • When the load has been lifted and is stationary, there is more energy stored gravitationally in the load, and a small amount is stored thermally in the surroundings due to heating in the wires, heating of the air, heating due to the effects of friction.
  • At a point, say halfway, when the load is moving at a steady speed, then there is energy is stored kinetically and gravitationally in the load, stored kinetically in the motor, and stored thermally in the surroundings.
  • It is important to point out that these changes are happening concurrently and not consecutively.
  • The energy stored kinetically only increases or decreases when there is a change of speed.
  • The current increases as the load increases showing that a bigger force needs to be used to raise greater loads. More work is done. More energy is stored gravitationally.
  • Even when the motor is running light with no load connected to it, a current flows and the motor turns. The energy stored chemically in the battery or fuel (and oxygen) is decreasing. You might ask students: "Where does this energy get transferred to?" The answer is that it warms up the motor, and the surroundings. Energy is now stored thermally.
  • You can analyse the energy transferred using the following equations:
  • Energy transferred by the electric current, E = I x V x t
  • Change in energy stored gravitationally, E = m x g x change in h
  • If you measure these, it is very unlikely that the two will be equal enough to satisfy students because of the energy losses (dissipated) in the system, but they could be used to measure the efficiency of the energy transfer.
  • Efficiency = (m x g x change in h /I x V x t) x 100%

This experiment was safety-tested in August 2007

Efficiency
is used in analyses relating to Engines
ONLINE COMMUNITY FORUM

Have a Physics Teaching Question?

Want to ask it in a safe, friendly, knowledgeable environment? TalkPhysics is an online community for anyone involved in the teaching of pre-19 physics.

Visit TalkPhysics