Total Energy of a System
Energy and Thermal Physics

Moving energy from one thing to another 1

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

This series of activities is designed to give students experience of energy transfers.

Apparatus and Materials

  • G-clamps, 5 cm, 2
  • Malvern energy transfer kit
  • Variable low-voltage power supply
  • Mass, 0.5 kg
  • Brick, single
  • Cord
  • Rubber band or driving belt

Health & Safety and Technical Notes

Read our standard health & safety guidance


The Malvern energy transfer kit is also available as individual components from Beecroft & Partners and other suppliers. Individual items may also be substituted for those in the kit.

All motors can be driven by up to 12 V and can also be used as dynamos. Do not allow the dynamo to labour under too heavy a load.

Procedure

  1. Using the brick to operate a dynamo and light a lamp...
  2. Clamp the motor/dynamo unit to the bench with a G-clamp. Fix the line-shaft unit next to it. Tie the cord round the brick and wrap it round the axle of the line shaft. Connect the output terminals of the dynamo to the lamp unit. When the brick is released it will turn the dynamo which in turn lights the lamp, or several lamps in parallel.
  3. Using a battery to drive an electric motor...
  4. Connect the motor to a 4-6 volt battery.
  5. Using an electric motor to lift a load which then drives a dynamo to light a lamp(s)...
  6. Clamp the motor/dynamo unit and the line shaft unit next to each other. Connect the pulleys on each with a rubber band or driving belt. Secure a cord to the axle of the line shaft and attach the lower end of the cord to a 1/2 kg mass. Connect a 4-6 volt DC supply to the motor/dynamo unit via a two-way switch. The switch is thrown so that it connects the power supply to the motor and the load is raised. When the switch is thrown back and the load allowed to fall, then the dynamo lights the lamps.
  7. Using the motor to drive a dynamo, which can light a lamp...
  8. Clamp the motor and dynamo units next to each other and join their pulleys with the rubber band or driving belt. Connect the motor to the power supply and the dynamo unit to the lamps.
  9. Using the motor to drive a flywheel and the flywheel to drive the dynamo...
  10. Clamp the flywheel next to the motor/dynamo unit. Connect the pulleys on each with a rubber band or driving belt. Connect the power supply to the motor via the two-way switch so that when the switch is thrown the motor turns the flywheel. When the switch is thrown back the flywheel turns the dynamo and the lamps light.
  11. Using the water turbine to drive a dynamo, which lights a lamp. This requires practice...
  12. Position the turbine/pump unit next to the motor/dynamo unit and clamp both rigidly with G-clamps. Connect the pulleys on the two units with a rubber band or driving belt. Connect the output from the dynamo to a lamp unit with one lamp. The water from the mains enters the turbine at the top and the pressure drives round the turbine blades, which in turn drives the dynamo. If the water pressure is not very great, some form of force pump will be necessary to increase the pressure.
  13. Using a pump to raise water...
  14. Clamp the motor/dynamo unit next to the turbine/pump unit, which in turn is clamped next to the head of water unit. Connect the pulleys on the motor and pump units with a rubber band or driving belt. Apply 4-6 volts d.c. to the motor. This will drive the pump unit which takes water from the lower level to the higher one. (It is necessary to prime the pump by filling with water before use: this is achieved by sucking on the third connection to the pump unit with a finger over the output and the input under water.)

Teaching Notes

  • You can set up a selection of these activities as a circus so that groups of students can progress round them. At each station students record the main energy transfers, saying where the energy is stored at the start, where it is stored at the end, and the processes by which it is transferred.
  • In step 1, energy is initially stored chemically in muscles (due to food and oxygen), and at the end energy is stored gravitationally in the raised brick. When the brick is released, the force of gravity pulls it downwards. Just before it hits the floor the all (or very nearly all) the energy that was stored gravitationally is now stored kinetically. When the brick hits the floor it exerts a force on the floor warming it up. The floor and the surrounding air may be caused to vibrate and a sound will be heard too.
  • When the raised brick is connected to the motor/dynamo unit and allowed to fall then the lamp lights. The dynamo produces an electric current, which flows through the filament of the lamp. The filament becomes hot and glows, radiating light to the surroundings. At the end, the energy that was stored gravitationally is now stored thermally in the surroundings.
  • In 2 the energy stored chemically in the battery is transferred by the electric current to the motor, causing the motor to spin.
  • In 3 the rotating motor produces a force on the line shaft, raising the load. There is less energy stored chemically in the battery, and more stored gravitationally in the load. When the load is allowed to fall, it exerts a force on the dynamo which makes it spin. The dynamo produces an electric current, which flows through the filament of the lamp. The filament becomes hot and glows, radiating light to the surroundings. At the end, the energy that was stored chemically in the battery is stored thermally in the surroundings.
  • In 4 the power supply produces an electric current in the motor, making it spin. The motor exerts a force on the dynamo, via the belt, so that the dynamo spins and produces an electric current which flows through the filament, which gets hot and the lamp then radiates light to the surroundings.
  • In 5 the power supply produces an electric current in the motor, making it spin. The belt attached to the motor produces a force on the flywheel, which also spins. When the belt is transferred to the dynamo it produces a force on the dynamo causing it to spin. When it spins a current is produced, which flows through the filament, which gets hot and the lamp then radiates light to the surroundings.
  • In 6 water from a high reservoir (feeding the water main) or a pressurized water pump flows through the turbine, turning the turbine blades before flowing out of the system. There is more energy is stored gravitationally in the water because the water reservoir is at a higher level compared to the ground. The falling water exerts a force on the turbines, making them spin. A belt produce a force on the dynamo, which produces an electric current, which flows through the filament, which gets hot and the lamp then radiates light to the surroundings. At the end, the energy that had been stored gravitationally ends up stored thermally in the surroundings.
  • This demonstration depends critically on the water pressure and the pressure may only be high enough to turn a small generator and drive a very slender elastic band. If the lamp will not light, it can be removed and replaced with an ammeter to show that there is a small current flowing.
  • In 7 an electric current transfers energy from the power station to the motor, producing a force on the motor, which spins. A belt can be used to produce a force on the pump, which produces a force on the water and raises it up. If the power station burns coal, then energy has been transferred by an electric current and forces. Energy that was stored chemically is now stored gravitationally. This is the reverse of demonstration 6.
  • Steps 6 and 7 model a pump-storage hydro-electric power station such as at Dinorwig. There is only enough water in the reservoir to operate for about 3 hours. When there is spare capacity on the grid the water can be pumped back up to the reservoir. Power stations like this can be brought up to generating speed in a few seconds to cope with a dramatic rise in demand for electricity.

This experiment was safety-tested in November 2005.

Total Energy of a System
appears in the relation dU=dQ+dW
is used in analyses relating to Thermal Equilibrium
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