Work, Power and Energy
Senior 1 · Physics
Work, Power and Energy
Lesson 3 of 5
24 min
At dawn in Nyanza the sun warms the red earth. A girl eats a banana and walks to school. A moto starts with a roar and carries a passenger up the hill. In the classroom the ceiling light comes on, powered in part by water falling through a dam on the Nyabarongo river. The sun, the banana, the petrol, the falling water: each one can make something happen. Physicists say that each one has energy, and that in each case energy is being changed from one form to another.
Definition
Energy () is the capacity (ability) to do work. It is a scalar quantity, measured in joules (J), the same unit as work.
When work is done, energy is transferred: a body that does 50 J of work gives 50 J of energy to something else. Example: when you lift a 1 kg book through 1 m you do of work, and the book gains of energy.
| Form | What it is | An example in Rwanda |
|---|---|---|
| Kinetic energy | energy of a moving body | a moto on the road; wind turning the blades of a fan |
| Gravitational potential energy | energy of a body because of its height | water held behind the Nyabarongo dam; a jerrycan on a shelf |
| Elastic potential energy | energy stored in a stretched or squashed object | a stretched catapult; the spring in a mattress |
| Chemical energy | energy stored in the bonds of substances, released by reactions | food, charcoal, firewood, petrol, the methane gas under Lake Kivu, a battery |
| Thermal (heat) energy | energy of the random motion of particles | a hot charcoal stove; boiling water |
| Electrical energy | energy carried by an electric current | the grid supply from REG; a solar home kit |
| Light (radiant) energy | energy carried by light and other radiation | sunlight falling on the solar panels at Rwamagana |
| Sound energy | energy carried by vibrations in air or other materials | the beat of ingoma drums at a celebration |
| Nuclear energy | energy stored in the nuclei of atoms | the Sun, which shines by nuclear fusion |
Kinetic energy and gravitational potential energy together are called mechanical energy. You will calculate both in the next lesson.
Set up five stations around the lab. At each one, do the activity and write down the energy change as “form A → form B”.
| Station | Energy change |
|---|---|
| 1. Rubbing hands | kinetic → thermal |
| 2. Cell and bulb | chemical → electrical → light + thermal |
| 3. Rubber band | elastic potential → kinetic |
| 4. Solar calculator | light → electrical |
| 5. Drum | kinetic → sound |
Important
Safety. Never aim a rubber band at a person. Do not short-circuit the cell by joining its ends with a bare wire: the wire gets hot. A torch bulb can be hot to touch after a few minutes.
Definition
The law of conservation of energy: energy cannot be created or destroyed. It can only be changed from one form to another, or transferred from one body to another. The total amount of energy stays the same.
Example: a 60 W bulb that receives 60 J of electrical energy each second gives out 60 J each second as light and heat together.
Near Mushishiro in Muhanga District, a dam 44.5 m high holds back the Nyabarongo river. The stored water flows through a tunnel to a power house where it spins two turbines of 14 MW each. Since October 2014 the plant has fed up to 28 MW into Rwanda's national grid.
Figure 3.1: Every step changes energy from one form to another. At each step a little energy becomes heat and sound that are not useful, but the total energy is always the same.
Worked example 1: energy is conserved in a generator
Each second, a generator receives 500 kJ of kinetic energy from its turbine and gives out 460 kJ of electrical energy. How much energy is changed into other forms each second, and what are they?
The 40 kJ is not destroyed: it becomes thermal energy (the generator warms up) and sound energy (its hum). Because this happens every second, the generator's electrical power output is .
Worked example 2: the energy in a banana
A banana provides about 400 kJ of chemical energy. If a 50 kg pupil could turn all of it into gravitational potential energy, how high could she climb?
In reality muscles turn only about a quarter of food energy into work; the rest warms the body. So one banana gives enough energy for a climb of about 200 m.
| Source | Renewable? | Energy change | Rwandan example |
|---|---|---|---|
| Hydropower | yes | gravitational PE → kinetic → electrical | Nyabarongo I; Ntaruka; the Rusumo Falls plant shared with Burundi and Tanzania |
| Solar | yes | light → electrical | the Rwamagana solar field; solar home kits |
| Methane gas | no | chemical → thermal → kinetic → electrical | gas extracted from deep in Lake Kivu at the KivuWatt plant, Karongi |
| Peat | no | chemical → thermal → kinetic → electrical | peat-fired power stations in Rusizi and Gisagara districts |
| Firewood and charcoal | yes, if trees are replanted | chemical → thermal | cooking in most rural homes |
| Diesel and petrol | no | chemical → thermal → kinetic | motos, buses, lorries and back-up generators |
A renewable source is replaced naturally as fast as we use it; a non-renewable source, such as fuel formed over millions of years, will run out.
Warning
Energy is not “used up”. When a phone battery goes flat, its energy has not disappeared; it has become light, sound and heat. What we lose is useful energy. Also, energy is not a force: a force is measured in newtons, energy in joules.
Tip
Describe an energy change in three parts: the starting form → the useful form → the wasted form. For a charcoal stove: chemical → thermal (useful, in the pot) + thermal and light (wasted, into the kitchen air).
Try it: in PhET: Energy Forms and Changes (version 1.4.22), build a system on the Systems screen, such as falling water turning a generator that lights a bulb, and watch the energy chunks move and change form.
Where does energy come from? (TED-Ed, George Zaidan and Charles Morton). Watch for how energy changes form but the total never increases or decreases.
Practice
Key points
Sources and credits
Written by Ganzaa: the lesson text, the energy circus, the flowchart, tables, worked examples and practice exercises. Facts about the Nyabarongo I plant are from Rwanda Energy Group.
