Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Defining the 'joule'


Introducing the concept of conservation of energy and a definition of the 'joule'.

Apparatus and materials

Steps a,b and d

Masses, 1kg, 2

Wooden lath*, 1 m

Shelves, 3, equally-spaced, e.g. on the lab wall

Motor/generator unit

Line shaft unit and string

Low voltage power supply, set at 6 V

G-clamps, 5cm, 2

Step c

Bricks, 2

Strong cardboard box

Retort stand

Single pulley on clamp

G-clamp, 10cm

Length of strong cord

Metre rule

Step e

Retort stands and bosses, 2

Nail, 15cm, 2


Compression spring

G-clamps,10cm, 2

Single pulley on clamp (If it is difficult to clamp the stands on the bench, a large board can be placed on the bench to carry everything).

Slotted masses, 6 x 100 g

Health & Safety and Technical notes

The hazard presented by these activities is that of 'working at heights'. No one must stand on a stool or bench to set up the apparatus on a high shelf. If apparatus (e.g. motor and line shaft unit) can be clamped onto the shelf at bench level, it can be lifted into place by two tall persons standing on the floor. The shelf supports must be rigid. Otherwise, the demonstration(s) must be done from the floor level.

* The 1-m wooden lath should be marked in centimetres, like a ruler. A metre rule with very clearly marked centimetre lines is an alternative.
For step d you should fit the three shelves approximately 1/3 metre apart, so that the top shelf is at the same height (about 1 metre) as in step a.



putting the load on a shelf

a Fix one shelf approximately 1 metre above the bench top. Lift the 1kg load from the bench to the 1m shelf. Refer to the teaching notes below for a possible script.

load pulley dynamo shelf

b Clamp the dynamo unit to the shelf, with the line shaft unit clamped beside it. Secure one end of a length of cord to the axle of the line shaft. Attach the other end to the 1kg load on the bench-top.

When you connect the motor to the supply, it will wind up the 1-kg to the shelf height.

pulling the load up to the shelf

c You can use a brick or a block of wood to represent a lift. Alternatively, a more realistic model can be made by using a cardboard box strong enough to take a brick, or two bricks on end, side by side.
The retort stand base should be fixed to the bench with a G-clamp. Fix the pulley to the top of the stand, pass the cord over the pulley, and tie it around the cardboard box to form the lift.

Raise the load from the bench top by pulling on the cord and raising the model 'lift'.

several shelves so load at different heights
d Lift the 1kg from one shelf to the next, reaching 1 metre high, but in three equal steps. (See the teaching notes on 'adding energies'.)

clamp stands, spring, bench

NB Clamp the stands to the bench.
e Use the G-clamps to clamp two retort stands about 50 cm apart on the edge of the bench. Use a boss to anchor one end of the compression spring to one of the retort stands. Use a second boss to attach the nail to the other retort stand as an anchoring pin.
Clamp a pulley to the end of the bench. Pass the string over the pulley, tying one end to the brick resting on the floor, and the other to the S-hook attached to the end of the spring held by the nail. Stretch the spring horizontally and anchor the free end to the nail.
Connect the spring to the string, using the S-hook. When you release the spring, the load will be pulled up by some distance. The spring and load must be such that the tension produced when the spring is stretched is sufficient to lift the load.


Teaching notes

1 Teachers and students often take it for granted that energies can be added together like masses. It is a basic property at the root of all ideas of conservation of energy. These demonstrations reinforce this principle.
Step a is the starting point. This is defining the job we want to get done (lifting the load).
Step b emphasizes the need for a fuel of some sort by using an electric motor to do the same job.
In step b the electric motor transfers the same amount of energy as in step a when it winds the kilogram up to the shelf. (The speed might drop as the layers of string increase the effective winch diameter.)
Instead of lifting the load by hand you could use an electric motor for the job. You would then have to pay for the fuel that the power station uses. If a person is used to raise the bricks, then they will need to be given extra food in order to do the job.
In step c students may be distracted by the very obvious 'manual' method of step a. Emphasize that whatever way we raise the load, fuel is needed from somewhere. If by hand, fuel comes from the food we eat.
When I haul up this model of a lift, do I use fuel? I certainly don't take in petrol or coal but I do eat food. If I had to do a lot of hauling I should get very hungry and have to eat more.
As you are learning in biology, the food you eat goes to provide many things that you need: keeping you warm, mending damaged tissue, producing growth and providing energy for movement. Some chemicals in food provide 'fuel' to run your muscles like an engine. When you haul up a load, your muscles 'burn' some of those chemicals hauling up the load and producing waste 'heat'. When your muscles work you are drawing on fuel, (chemicals), which came from your food.

In step d introduce the concept of adding energies. Show that the same job is done whether it is achieved in one step, or broken down into stages. This is the point at which to introduce the term 'joule'.
Suppose I now lift one kilogram up a 1/3 metre step, then a second 1/3 metre step and finally a third 1/3 metre step. How will this compare with the amount of fuel needed to raise the one kilogram through 1 metre in one go? The same amount of 'food energy' would be needed for both jobs.
In step e, show that the energy we need to do the job can be stored in a spring, just as it is stored in the food we eat. The spring does not pull all the way back to its original unstretched length because the load is applying a force to the spring. The energy ends up being stored partly in the stretched spring and partly in the raised load.
2 Introducing the joule
At the Earth's surface one kilogram has a force on it of about 10 N. When it is raised one metre we say that 10 J ( 10 joules) of energy have been stored as 'uphill energy'of the weight. Two kilograms raised through one metre will store 20 J of energy. Two kilograms raised three metres will store 60 J.
This experiment was safety-checked in November 2005


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