Energy

Energy exchanges

Energy moves between objects by transfer or by conversion.

• Transfer: the same form of energy moves from one object to another (e.g. heat from a radiator → air in the room).
• Conversion: energy changes form (e.g. solar panel transforms light → electricity).

To describe these exchanges, an energy chain 

  Example of an energy chain:

Thermal energy transfer by radiation

A hot object emits radiation, often in the form of "invisible light" called infrared. 
An object that receives this radiation energy absorbs it and warms up.
The emission and absorption of radiation energy therefore correspond to an energy transfer. Unlike other thermal energy transfers, heat transfer by radiation does not need matter: it can travel through a vacuum.

  Examples :
- Radiation from the Sun travels through the vacuum of space and warms the Earth.
- A thermal imaging camera (thermal image) "sees" the radiation emitted by a warm body.

Measuring energy

All forms of energy are measured using the same unit: the joule (J). The kilowatt-hour (kWh) is also used for electrical consumption.

Benchmarks: lifting 1 kg by 1 m ≈ 10 J; 1 h of LED lighting ≈ 36 000 J.

Conservation of energy

In an isolated system: energy received = energy sent out. 
  Example: a lamp receives 100 J of electricity → 20 J light + 80 J heat = 100 J.

Mechanical energy

Mechanical energy (Em) is the sum of energy related to motion (kinetic) and energy related to position (potential).

  Formulas: 

• Mechanical energy: $E_m = E_c + E_p$
  • With all energies in Joules (J)
• Kinetic energy: $$E_c = \frac{1}{2} \times m \times v^2$$
  • m in kg
  •     v in m/s
• Gravitational potential energy: $$E_p = m \times g \times h$$
  • m in kg
  • g ≈ 9.81 N/kg ≈ 10 N/kg
  • h in m 

 Example: ball of mass 0.5 kg at 2 m height and thrown at 4 m/s:

• Ep = 0.5 × 10 × 2 = 10 J
• Ec = ½ × 0.5 × 4² = 4 J
• Em = 10 + 4 = 14 J

Mechanical Energy Simulation

Energy and power

Power indicates the speed at which energy is used or produced.

Analogy: energy = volume of water in a tank; power = flow rate of the tap.

  Fundamental formula

$$E = P \times \Delta t$$

• E: energy (J) or (more practical for billing) kWh
• P: power (W or kW)
• Δt: time taken (s or h)

  Examples

• 60 W light bulb on for 2 h → E = 60 W × 2 h = 120 Wh = 0.12 kWh
• 2 kW heater on for 3 h → E = 2 kW × 3 h = 6 kWh
• 1500 W oven for 45 min → 1.5 kW × 0.75 h = 1.125 kWh