Physics Study Notes: Energy and Use of Energy

Hello! Welcome to your study notes for "Energy and Use of Energy". This chapter is super practical because it's all about the physics happening in your home, in your city, and how it affects your wallet and our planet. We'll explore how appliances work, what makes a building energy-efficient, and where our electricity comes from. By the end, you'll be a smarter energy consumer and understand the science behind it. Let's get started!


Section A: Electricity at Home

Ever wonder how much it costs to run your air-conditioner or why some light bulbs are better than others? This section answers those questions. We use electricity for almost everything, so let's see how it works.

1. Energy in Our Appliances

Almost every device at home is an energy converter. It takes electrical energy and changes it into other forms.

  • An electric fan converts electrical energy into kinetic energy (the moving blades) and sound energy.
  • A kettle converts electrical energy into thermal energy (heat).
  • A light bulb converts electrical energy into light energy and thermal energy.

But are they good at their job? That's where efficiency comes in.

End-use Energy Efficiency

This is a measure of how much of the input energy is converted into the useful form we actually want. No machine is 100% efficient; some energy is always 'wasted', usually as heat.

Analogy: Imagine pouring water into a leaky bucket to water a plant. The total water you pour is the 'Energy Input'. The water that actually reaches the plant is the 'Useful Energy Output'. The water that leaks out is 'Wasted Energy'. Efficiency is the ratio of water for the plant to the total water you poured.

The formula is:

$$ Efficiency (\eta) = \frac{\text{Useful Energy Output}}{\text{Total Energy Input}} \times 100\% $$

A higher efficiency means less energy is wasted, which saves money and is better for the environment!

Key Takeaway

Appliances convert electrical energy into other forms. End-use efficiency tells us how well an appliance does its job without wasting energy.


2. Let There Be Light: Lighting Explained

We use different types of lights at home. Let's look at how they work and which are the best.

Types of Lighting
  • Incandescent Lamps: An old-fashioned bulb. Electricity heats a tiny wire (filament) until it glows white-hot. Think of it like a toaster that gets so hot it makes light. This is very inefficient, as over 90% of the energy is wasted as heat!
  • Gas Discharge Lamps: This includes fluorescent tubes. Electricity excites gas atoms inside the tube, causing them to release ultraviolet (UV) light. A white coating inside the tube absorbs this UV light and glows, producing visible light. More efficient than incandescent bulbs.
  • Light Emitting Diodes (LEDs): These are the champions of efficiency! They use semiconductor material to convert electrical energy directly into light at an atomic level. They produce very little heat, last a long time, and are the most energy-efficient option.
Measuring Light

To compare bulbs properly, we need some specific terms. Don't worry, they're simpler than they sound!

  • Luminous Flux (Unit: lumen, lm): This is the total amount of visible light a source emits in all directions per second. Think of it as the total 'power' of the light coming out of the bulb. A higher lumen rating means a brighter bulb.
  • Illuminance (Unit: lux, lx): This is the luminous flux that falls on a unit area of a surface. It's a measure of how bright a surface appears. If Luminous Flux is the total rain from a cloud, Illuminance is how much rain falls into a small bucket on the ground.
Important Laws for Illuminance

1. The Inverse Square Law: The illuminance on a surface is inversely proportional to the square of its distance from the light source. In simple terms: the further away you are, the dimmer the light gets, and it gets dimmer very quickly!

If you double your distance from a light bulb, the light is spread over four times the area, so the illuminance is only 1/4 of what it was.

2. Lambert's Cosine Law: Illuminance is greatest when a surface is perpendicular (at 90°) to the light source. If the surface is tilted at an angle (θ), the illuminance is reduced.

Think about sunlight. The ground gets much hotter and brighter at noon (when the sun is directly overhead) than in the late afternoon (when the sun's rays hit the ground at an angle).

The formula combines these ideas: $$ Illuminance (E) = \frac{\text{Luminous Flux} \times \cos\theta}{\text{Area}} $$

For light shining directly onto a surface from a point source: $$ E = \frac{\text{Luminous Flux}}{4\pi d^2} $$

Efficacy of Electric Lights

This is the most important number for choosing a bulb! It's the ratio of luminous flux (how much light you get) to the electrical power input (how much energy you use).

$$ Efficacy = \frac{\text{Luminous Flux (lm)}}{\text{Electrical Power (W)}} $$

The unit is lumens per watt (lm/W). A higher efficacy is better!

  • Incandescent Bulb: ~15 lm/W (Poor)
  • Fluorescent Bulb: ~60 lm/W (Good)
  • LED Bulb: ~100+ lm/W (Excellent)
Key Takeaway

To choose the best light bulb, don't just look at the watts. Look at the lumens (brightness) and the efficacy (lm/W). LEDs are the most efficient choice.


3. Cooking Without Fire

Modern kitchens use electricity to cook in clever ways.

  • Electric Hotplates: A simple resistor gets hot when current flows through it (like an incandescent bulb, but for heat). It heats the pot by conduction. This is not very efficient as a lot of heat escapes into the surrounding air.
  • Induction Cookers: This is like magic! It uses a changing magnetic field to create electric currents directly inside a magnetic pot (like iron or steel). These currents heat the pot itself. The cooker's surface stays cool. It's very fast and much more efficient than a hotplate because the heat is generated exactly where it's needed.
  • Microwave Ovens: Uses microwaves (a type of electromagnetic wave) to make water molecules in food vibrate very fast. This vibration is kinetic energy, which means the food gets hot. It cooks food from the inside out!
Calculating the Cost of Running Appliances

This is a life skill! Electricity companies charge you for the amount of energy you use, measured in kilowatt-hours (kWh).

Step-by-step guide:

  1. Find the power rating of the appliance in watts (W). Example: a microwave might be 1000 W.
  2. Convert the power to kilowatts (kW) by dividing by 1000. 1000 W / 1000 = 1 kW.
  3. Determine the running time in hours (h). Let's say you use it for 15 minutes. 15 mins / 60 = 0.25 h.
  4. Calculate the energy consumed in kWh. $$ \text{Energy (kWh)} = \text{Power (kW)} \times \text{Time (h)} $$ Energy = 1 kW × 0.25 h = 0.25 kWh.
  5. Calculate the cost. Multiply the energy used by the price per kWh. If electricity costs $1.2 per kWh, then Cost = 0.25 kWh × $1.2/kWh = $0.30.
Key Takeaway

Induction cookers and microwaves are generally more energy-efficient than simple hotplates. You can calculate the running cost of any appliance using the formula Cost = Power (kW) × Time (h) × Price per kWh.


4. Moving Heat Around: Air-Conditioners

An air-conditioner doesn't "create cold". It's a heat pump – it moves heat from inside your room to the outside, against its natural direction of flow (heat naturally flows from hot to cold).

Analogy: Think of a bouncer at a club. The bouncer (A/C) uses energy to kick the unwanted "heat guests" out of the cool "room club" and into the hot "outside world".

Measuring A/C Performance
  • Cooling Capacity (Unit: kW): This is the rate at which the A/C removes heat from a room. A higher cooling capacity means it can cool a larger room or cool a room faster. Note: This is NOT the same as its electrical power consumption!
  • Coefficient of Performance (COP): This is the efficiency rating for an A/C. It's the ratio of heat removed to the electrical energy used.
$$ \text{COP} = \frac{\text{Cooling Capacity (kW)}}{\text{Electrical Power Input (kW)}} $$

Don't be confused! The COP for an A/C is often greater than 1 (usually between 2 and 4). This doesn't break the law of conservation of energy. It just means it's easier to move heat than to create it. For every 1 kW of electricity you use, the A/C might move 3 kW of heat out of your room (COP = 3).

A higher COP is better and means a more efficient A/C!

Hong Kong Energy Efficiency Labelling Scheme (EELS)

You've seen these stickers on appliances! They grade products from 1 to 5.

  • Grade 1: Most energy efficient (best!)
  • Grade 5: Least energy efficient (worst!)

Choosing a Grade 1 appliance can save you a lot of money on your electricity bill over its lifetime.

Key Takeaway

An A/C is a heat pump. Its efficiency is measured by its COP – a higher value is better. Look for the EELS label and choose Grade 1 appliances to save energy.

Section B: Energy Efficiency in Buildings and Transportation

Saving energy isn't just about appliances; it's also about the design of our buildings and how we travel.

1. Better Buildings

In a hot place like Hong Kong, a huge amount of energy is used for air-conditioning. A well-designed building can reduce this by minimising heat gain from the outside.

The main way heat gets in is through conduction via the walls, roof, and windows.

Rate of Heat Transfer by Conduction

The rate at which heat flows through a material depends on several factors:

$$ \frac{Q}{t} = \frac{\kappa A (T_{hot} - T_{cold})}{d} $$

Where:

  • $$ \frac{Q}{t} $$ is the rate of heat transfer (in Watts).
  • $$ \kappa $$ (kappa) is the thermal conductivity of the material. A good insulator like foam has a low κ. A good conductor like metal has a high κ.
  • $$ A $$ is the surface area. (Bigger walls/windows let more heat through).
  • $$ (T_{hot} - T_{cold}) $$ is the temperature difference. (The hotter it is outside, the faster heat comes in).
  • $$ d $$ is the thickness of the material. (Thicker walls slow down heat transfer).
Thermal Transmittance (U-value)

To make things simpler, we often combine conductivity (κ) and thickness (d) into one number: the U-value.

$$ U = \frac{\kappa}{d} $$

The U-value measures how easily heat can pass through a material.
A LOW U-value is GOOD! It means the material is a good insulator.

Overall Thermal Transfer Value (OTTV)

The OTTV is a measure of the average rate of heat gain for the entire building envelope (all the walls, windows, and roof). It's a more complete measure than just the U-value of the walls because it also includes heat from sunlight shining through windows.
A LOW OTTV is GOOD! It means the building is well-designed to stay cool.

Factors that affect OTTV include:

  • The U-value of the walls and windows.
  • The window-to-wall ratio.
  • Shading from the sun.
  • Using solar control window film to block heat.
Key Takeaway

To make buildings energy efficient, we want to minimise heat transfer. This is achieved by using materials with a low U-value and designing the building to have a low OTTV.


2. Smarter Transportation

Vehicles are a major source of energy consumption and pollution.

Electric Vehicles (EVs) vs. Fossil-Fuel Vehicles
  • Fossil-Fuel Vehicle: An internal combustion engine burns petrol or diesel. This process is very inefficient. Only about 20-30% of the fuel's energy is used to actually move the car; the rest is lost as waste heat!
  • Electric Vehicle (EV): Uses a battery to power an electric motor. The main components are the battery pack, an inverter, and the motor. Electric motors are extremely efficient, with over 90% of the energy from the battery being used to turn the wheels.

Hybrid vehicles use both an electric motor and a gasoline engine, combining the benefits of both.

Did you know?

Because EVs are so much more efficient, even if the electricity to charge them comes from burning coal, the overall "well-to-wheel" efficiency can still be better than a petrol car. Public transportation, like the MTR, is even more efficient because it moves many people at once.

Key Takeaway

Electric vehicles have a much higher end-use efficiency than fossil-fuel vehicles because electric motors are far more efficient than internal combustion engines.

Section C: Renewable and Non-renewable Energy Sources

Where does all our electricity come from? Let's look at the sources and their impact.

1. Types of Energy Sources

  • Non-renewable Sources: These are finite and will eventually run out. Once we use them, they're gone. They are also the major cause of air pollution and climate change.
    Examples: Fossil fuels (coal, oil, natural gas), Nuclear fuel (uranium).
  • Renewable Sources: These are naturally replenished and won't run out. They are generally much cleaner for the environment.
    Examples: Solar, wind, hydroelectric (water), geothermal (earth's heat).

2. A Closer Look at Energy Sources

Nuclear Fission

Nuclear power plants use fission. A large, unstable nucleus (like Uranium-235) is split into two smaller nuclei, releasing a huge amount of energy. This is explained by the binding energy curve, which shows that splitting a very heavy nucleus results in lighter nuclei that are more stable, and the difference in energy is released.

A fission reactor controls this chain reaction using:

  • Moderator: Slows down neutrons to make fission more likely.
  • Control Rods: Absorb neutrons to control the rate of the reaction or shut it down.
  • Coolant: Transfers the heat generated to a system that produces steam to turn turbines.
Solar Power

The solar constant is the amount of solar energy that reaches the top of Earth's atmosphere. It's about 1360 W/m².

Solar cells (photovoltaic cells) convert sunlight directly into electricity. They are the power source for satellites and are becoming increasingly common on Earth.

Wind Power

A wind turbine is like a fan working in reverse. The wind pushes the blades, turning a generator to produce electricity.

The maximum power a wind turbine can generate is given by:

$$ P_{max} = \frac{1}{2} \eta \rho A v^3 $$

Where:

  • $$ \eta $$ is the efficiency of the turbine.
  • $$ \rho $$ (rho) is the density of the air.
  • $$ A $$ is the area swept by the blades ($$\pi r^2$$).
  • $$ v $$ is the wind speed.

Important Point: Power is proportional to the cube of the wind speed ($$v^3$$)! This means that if the wind speed doubles, the potential power increases by 2³ = 8 times! This is why wind turbines are placed in very windy locations.

3. Environmental Impact

Our energy choices have consequences.

  • Fossil Fuels: Burning them releases greenhouse gases like carbon dioxide (CO₂), which trap heat in the atmosphere, leading to global warming. They also release pollutants that cause acid rain and smog.
  • Nuclear Power: It doesn't produce greenhouse gases, but it creates radioactive waste that is dangerous and difficult to store safely for thousands of years.
  • Renewables: Generally much cleaner, but they have their own challenges. Solar and wind are intermittent (they don't work if it's not sunny or windy), and large-scale projects can take up a lot of land.

In Hong Kong, our electricity is mainly generated from natural gas, nuclear energy imported from mainland China, and a smaller amount of coal.

Key Takeaway

There are two main types of energy sources: non-renewable (fossil fuels, nuclear) and renewable (solar, wind). Our heavy reliance on non-renewable sources is causing environmental problems like global warming. The future lies in transitioning to cleaner, renewable energy.