Hands-On-Physics
MECHANICS

Extensions

- Motor Power -
The batteries on the Aircart supply energy to the motor which spins the propeller to provide the force which does work on the Aircart, increasing its speed. What percentage of the energy supplied by the batteries is eventually transformed into kinetic energy of the Aircart? You can study the efficiency of Aircart system by measuring the motor power. When you know the energy output of the motor you can compare that with the increased kinetic energy of the Aircart.

Measuring Motor Power

You can determine the motor's energy use by measuring power and time. Power is the rate of working, so work (energy) is the product of this rate (power) with time. The longer the motor runs, the more energy it uses up. Electrical power is the product of voltage and electrical current. Both voltage and current can be measured with your multimeter. Both measurements should be taken with the motor running.

To measure voltage, switch the meter to DC Volts and touch the leads to the electrical contacts on the back of the motor. Expect to measure about 3 Volts (if you used two 1.5 volt batteries for your Aircart).

Measuring current (Amperes) is a little trickier. Your series circuit must be taken apart somewhere and reconnected to an ammeter. To have the multimeter measure current, switch it to DC amps, making it into an ammeter. Now the meter must be connected in series with everything else, completing the circuit. One lead must be connected to one of the disconnected wires, the second lead connected to the other disconnected wire.

If you do not have an ammeter, you can connect a 1 ohm resistor in series and then measure the voltage drop across the one-ohm resistor with a voltmeter. The voltage reading will indicate the current. (V=IR: here R=one, so V=I)

The motor should run when all your connections are finished. The current (Amps) can be read from the multimeter. The product I*V is the power (Watts) being used by the motor. Note: the unit of current is the Ampere, and the symbol generally used for current is I.

Once you have determined the power of the motor, you can study the efficiency of the Aircart by comparing electrical energy used by the motor with the a corresponding kinetic energy increase of the Aircart. You can choose some specific time and measure the Aircart's increase in speed during that time or choose a distance and measure the Aircart's increase in speed. The energy used by the motor during this run is proportional to the time required for the run.

When everything seems to work consistently, improve your experiment by trying several different Aircart distances and times. How accurately can you determine the power? How accurately can you determine kinetic energy? What is the efficiency of the Aircart?

Previous Page || Up a Level || Index|| Next Page