Classroom energy experiment using RCX Current Sensor
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Classroom: Energy and Work

The current sensor can be used for experiments teaching the concepts of energy, work, energy conversion, and effieiency. In the setup shown below a simple winch lifts a 500 gram weight. Students calculate the electrical energy used and the mechanical work done. Comparing the two illustrates efficiency of energy conversion. Students may compare different winch models and motors for best efficiency.

Connector for Mindstorms relay control

Using Robolab Investigator create a program similar to the one below. This program configures the light sensor for data logging and starts data logging. It then starts a motor and waits 10 seconds while the motor lifts the weight. It turns off the motor (brakes) and waits four seconds, then stops data logging.

If you run this program and then display the data log it should look something like the one shown below. Current rises while the weight is being lifted. When the motor is shut off the current drops, but does not drop to zero. Notice the weight slowly returns to the floor. If you disconnect the motor (or turn off and float the output) the weight will fall rapidly. When the weight is falling in braking mode, the motor acts like a generator, causing current to flow while the weight drops to the floor.

 


Calculations

The following are rough calculation based on a single experiment with the numbers rounded.

In 10 seconds the motor lifted the 500 gram weight a distance of 500 mm. Acceleration of gravity is 10 m/sec*2. F=MA so the force of gravity on the mass is 0.5 Kg times 10 m/sec*2 or 5 Newtons. The weight was lifted a distance of 0.5 meter and mechanical energy is force times distance, so the mechanical energy is 0.5 meter times 5 Newtons or 2.5 Joules.

From the data log the average light sensor reading is about 75. The zero current reading is 50, so the difference is 25 units. Each unit represents 5 mA, so the current is 25*5 or 125 mA. A voltmeter was used to find the motor voltage was 6.8 volts. Power = voltage time current, so the power was 6.8 times 0.125 or 0.85 watts. Electrical energy is power multiplied by time or 0.85 watts times 10 seconds, or 8.5 watt-seconds. One watt-second equals 1 Joule, so the electrical power used is 8.5 Joules.

Efficency is represented as the work done, divided by the energy used. In this case it is 2.5 joules done, divided by 8.5 joules used, or an efficiency of 29 percent.

When the motor is shut off, it takes about 85 seconds for the weight to return to the floor. During this time the current is around 25 mA and the motor voltage about 0.1 volt. This is about 0.2 Joules. Our system is not very efficient!.