Energy Sources and Conservation
What is energy?
Energy is the ability to do work, measured in joules (J). It exists as kinetic, potential, heat, light, sound, chemical, electrical and nuclear energy. In the KICD General Science approach, you link energy ideas to everyday Kenyan situations like cooking, lighting and transport.
Sources of energy
Renewable (replenished naturally):
- Solar, wind, hydroelectric, geothermal, biomass, tidal
- Kenya generates over 70% of its electricity from renewables, mainly geothermal, hydro and wind.
Non-renewable (finite):
- Coal, petroleum, natural gas, nuclear fuels
| Aspect | Renewable | Non-renewable |
|---|---|---|
| Supply | Unlimited | Limited |
| Pollution | Low | High (CO2) |
| Start-up cost | Often high | Varies |
Energy transformations
- Solar panel: light to electrical
- Hydro dam: gravitational potential to kinetic to electrical
- Charcoal stove: chemical to heat
- Electric bulb: electrical to light and heat
- Biogas digester: chemical (waste) to heat for cooking
Conservation of energy
Energy cannot be created or destroyed, only converted. Some becomes wasted heat, which is why no machine is 100% efficient.
Efficiency
Efficiency = (useful energy out / energy in) x 100%.
Worked example 1: A motor uses 2,000 J and does 1,500 J of useful work. Efficiency = 1,500 / 2,000 x 100 = 75%.
Work, energy and power
- Work = force x distance (J)
- Kinetic energy = 1/2 mv^2
- Potential energy = mgh (g = 10 m/s2)
- Power = energy / time (W)
Worked example 2: A pupil of mass 50 kg climbs stairs 4 m high in 5 s.
- PE gained = 50 x 10 x 4 = 2,000 J
- Power = 2,000 / 5 = 400 W
Electrical energy cost
Energy (kWh) = power (kW) x time (h). A 2 kW heater used for 3 hours uses 2 x 3 = 6 kWh. At KES 25 per kWh the cost is KES 150.
Energy conservation practices
- Use energy-saving bulbs and switch off appliances.
- Use improved cookstoves (jikos) and biogas to cut firewood use.
- Insulate buildings and use natural lighting.
- Plant trees and adopt solar water heating.
- Use public transport or walk where possible.
Environmental concerns
Burning fossil fuels releases carbon dioxide which causes global warming. Deforestation for firewood leads to soil erosion. Switching to renewables supports Kenya's climate commitments.
Try it
- List three energy transformations in a mobile phone being charged by solar power.
- A machine takes in 800 J and loses 200 J as heat. Find the efficiency.
- Find the energy in kWh of a 60 W television used for 5 hours.
- Suggest three ways your home can save energy.