【DSE Physics E3】Energy 能量｜Lighting｜AC｜Transportation

Energy 能量

Lighting
Cooking and Air-Conditioning
Buildings and Transportation
Different Sources of Energy

Chapter 1 Lighting

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Domestic electricity

Lighting
Air conditioning
Cookware

Energy conversion and efficiency

An electrical appliance converts electrical energy into other forms of useful energy.

$$\eta=\frac{E_\mathrm{out}}{E_\mathrm{in}}\times100\%$$

$$\eta=\frac{P_\mathrm{out}}{P_\mathrm{in}}\times100\%$$

Saving energy at home

Save energy by

choosing more energy-efficient appliances
using energy saving devices

Energy efficiency labels

Part of Energy Efficiency Labelling Scheme (EELS)
Tell us which models are more energy efficient

Energy saving devices

Turn on/off automatically to reduce electricity used

Example：

thermostat
timer
motion sensor

Light and power

Brightness of light source depends on

amount of light energy emitted by the source per unit time (power output of source)
response of human eye to wavelengths of light emitted

Human eyes sensitivity

Sense lights of different wavelengths differently
For same power output, green light source looks brighter

Luminous flux Φ

Measures brightness of a source
Include sensitivity of human eye to different wavelengths
Unit: lumen (lm)

Luminous efficacy

Measures how efficiently a lighting device converts electricity into light.

$$\text{efficacy}=\frac{\text{luminous flux }\Phi}{\text{input power }P_{\mathrm{in}}}$$

Unit: Im W^-1

Light emission

這裏和atomic 的 Chapter 2 Atomic Models and Spectra 非常相似，DSE Source 也有一篇文章講解。

Types of lighting

Different types of lighting make use of different processes to excite atoms.
All emit visible light when they return to stable states.

Incandescent lamp

Lighting element

Hot filament

Structure

Sealed glass globe and coiled filament

Working principle

Current heats up filament (to about 2000 °C).
Electrons in hot filament gain energy and repeatedly jump up to different energy levels.
When electrons return to lower energy levels, EM waves of various wavelengths are emitted.
A continuous colour spectrum is produced.

Gas discharge lamp

Lighting element

Ionized gas
Phosphor (only for FTL)

*FTL = florescent tube lamp

Structure (for FTL)

Sealed glass tube filled with mercury vapour
Coating of phosphor on internal side of tube to produce visible light

Working principle

When an FTL is switched on, an electric field
is set up in the tube.
Electrons are emitted from one of the heated filaments and accelerate towards the other.

⇒ A current is formed.

The mercury atoms gain energy when bombarded by accelerating electrons.
When they return to a lower energy level, UV is mainly emitted.
Phosphor atoms gain energy when they absorb UV.
They return to lower energy levels by emitting
visible light.
Different phosphors are used to emit
different coloured lights.

⇒ White light is produced.

LED lamp

Consists of light emitting diodes (LEDs)

Lighting element

Semiconductors

Structure

Two types of semiconductors (n-type and p-type) sandwich together to form a p–n junction.

n-type has negative charge carriers (electrons).
p-type has positive charge carriers (holes).

Working principle

Electric field is applied and points from p-type towards n-type semiconductor.
Holes and electrons move towards the junction in opposite directions.
When an electron falls into a hole, it jumps down to a lower energy
Energy is released as visible light.

Choosing suitable lighting

Lamp Type	Incandescent Lamp	Gas Discharge Lamp	LED Lamp
Size	Medium	Large	Small
Light Source	Hot Filament	Fluorescent Coating	2 Layers of Semiconductor
Working Temperature	Low	Medium	High
Efficacy	Low	Medium	High
Heavy-Metal Content	High	Medium	Low
Price-Efficacy Balance	Low	Medium	High
Mercury Content	High	Medium	Low

Measuring illumination

Luminous flux falling on a surface per unit area
Measures how much light is incident on a surface

$$E=\frac\Phi A$$

Unit: lux (lx) & 1 lx = 1 lm m^-2

Lambert’s cosine law

Consider a parallel light beam shining on a surface.
Compare the areas being shone.

$$E=E_0\cos\theta $$

Inverse-square law

Point light source

Illuminance E on spherical surface

$$E=\frac\Phi{A_0}=\frac\Phi{4\pi r^2}$$

$$E\propto\frac1{r^2}$$

Combine inverse-square law & Lambert’s cosine law

Chapter 2 Cooking and Air-Conditioning

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Cooking without fire

Cooking without fire has higher efficiency of energy transfer than cooking with fire.

Appliance	Microwave Oven	Induction Cooker	Electric Hotplate
Working Principle	Water molecules being flipped up and down by microwaves	EM induction heating effect of eddy current	Heating effect of current
Heating Efficiency	Medium	High	Medium
Major Advantage	Cook food evenly throughout	High energy efficiency	Cheap
Major Disadvantage	Only for food of rich water content	cooking utensils must be electrically conducting	Cooking surface is very hot

Electric hot plate

Use heating elements to heat up utensils
When a current I flows through a heating element of resistance R, power of heat generation is P = I²R

Working principle

Electric hot plate cooker

A cooking utensil placed on hot heating elements
Heat up food by conduction

Electric oven

Heat up food by convection and radiation

Advantage

Cheap

Efficiency

~70%

Major energy loss

By convection and radiation

Induction cooker

Use EM induction & heating effect of eddy current to heat up utensils

Working principle

A high frequency ac current (~25 kHz) passes through solenoid inside an induction cooker
A rapidly alternating magnetic field is produced
Eddy currents induced in utensil above
Utensil is heated up, food inside is cooked

Advantage

High energy efficiency

Safer (cooking surface does not produce any heat)

Efficiency

>80%

Major energy loss

Heat in circuit inside cooker

Microwave oven

Use microwave to cook food of rich water content

Working principle

A water molecule is electrically polar
Centres of +ve & −ve charges are slightly separated
Microwaves (2.45 GHz) consists of oscillating E-field
and B-field
E-field exerts force on water molecules
Oscillating field flip molecules up and down violently

Advantage

No heating up utensil or
open flame
Cook food more evenly

Efficiency

~60%

Major energy loss

Heat in circuit when producing microwaves

Moving heat around

Air conditioners are used to remove heat from a cold indoor area and dispose of the heat in a warm outdoor area.

How heat flows

Naturally heat flows from a hot region to a cold region
Need heat pump (e.g. air conditioner) to go against this natural flow

Heat pump

Heat pump has to do work W to remove heat Q_C from a cold region.
Total amount of heat disposed of to the hot region Q_H

$$Q_{\mathrm{H}}=Q_{\mathrm{C}}+W$$

Air conditioner

Cooling capacity: heat removed per unit time

$$\text{cooling capacity}=\frac{Q_\mathrm{C}}t$$

Coefficient of performance (COP): efficiency of air conditioner

$$\mathrm{COP}=\frac{Q_\mathrm{C}}W$$

$$\mathrm{COP}=\frac{\mathrm{cooling~capacity}}{P_{\mathrm{in}}}$$

COP can be larger than 1

Refrigeration cycle

An air conditioner transfers heat (from colder to hotter) by pumping refrigerant (a fluid) through the system.
Refrigerant evaporates ⇒ absorbs latent heat from indoor air
Refrigerant condenses ⇒ releases heat to outdoor environment

Evaporator

When refrigerant flows through evaporator (coiled tubes), it absorbs ℓ_v from indoor air and evaporates.

Compressor

Compressor compresses vaporized refrigerant to a high temperature and high pressure.

Condenser

At condenser, refrigerant releases ℓ_v to outdoor air and changes to a liquid.

Expansion valve

At expansion valve, liquid refrigerant expands.

Central air conditioning system

Two separate water circulatory loops

Air handling unit (AHU) + chiller
Chiller + cooling tower

Use water as a coolant

AHU to chiller

AHU cools indoor air and distribute cooled air to different indoor regions through air ducts.
In 1st circulatory loop, heat is transferred from AHU to chiller

Chiller to cooling tower

In 2nd circulatory loop, heat is further transferred from chiller to cooling tower

⇒ Heat removed from indoor is disposed of to outdoor environment

Heat recovery

To save energy , wasted heat can be reclaimed for

preheating water for bathing
reheating air during dehumidification in AHUs

Energy labels for air conditioner

Chapter 3 Buildings and Transportation

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Energy transfer in buildings

Outermost components (roofs, walls and windows) are collectively called building envelope
Heat can enter or leave a building through its building envelope mainly in two ways

Conduction

e.g. If you live on top floor of a building, your home can be hotter than others
Also through other parts of envelope (walls and windows)

Radiation

e.g. If your room has a large window facing west, it will become hotter than others in late afternoon

U-value and OTTV

Heat is conducted through a layer when there is a temperature difference between two sides
Rate of conduction depends on layer materials, surface area and thickness of layers

Law of conduction

The rate of conduction for a layer depends on

temperature difference between two sides of layer
(ΔT = T_hot − T_cold)
area of layer A
thickness of layer d
materials that make up the layer

Rate of heat conduction P obeys the law of conduction:

$$P=\frac Qt=\kappa\frac{A\Delta T}d$$

where
κ: thermal conductivity (describing how well a material conducts heat)

Unit of κ: W m^-1 K^-1

Thermal conductivity

The higher the value of κ, the better the material conducts heat

	substance	κ / W m−1 K−1
metals	copper	400
	aluminium	235
	steel	46.6
non-metals	glass	1
	concrete	0.8
	wood	0.14
	asbestos	0.126
	glass wool	0.04
	aerogel	0.004
gases	air	0.0259
	argon	0.0177
	sulphur hexafluoride	0.012

U-value

Or thermal transmittance
Measures how well a layer of a building envelope (rather than a material only) conducts heat
Defined as

$$U=\frac\kappa d$$

Unit: W m^-2 K^-1

U-value depends on properties of layer but not temperature difference
Law of conduction in terms of U-value:

$$P=\frac Qt=UA\Delta T$$

A layer with a larger U-value conducts heat better
The lower the U-value, the better the energy performance of layer
⇒ less heat is gained or lost through this layer

OTTV

Energy consumption for air-conditioning can be huge in subtropical areas like Hong Kong
Overall Thermal Transfer Value (OTTV) is used to quantify energy performance of a building
OTTV measures rate of heat gain per unit area through the building envelope, averaged over one year:

$$\mathrm{OTTV}=\frac{P_{\mathrm{tot}}}{A_{\mathrm{tot}}}=\frac{P_1+P_2+…+P_N}{A_1+A_2+…+A_N}$$

P_tot : total heat transfer through building envelope
A_tot : total area of envelope

The lower the OTTV, the slower the heat flows into building
⇒ need for air conditioning can be reduced

Improving energy efficiency

Walls and roofs

Add layers of heat-insulating materials to walls and roof
Such materials have many air spaces and low κ
⇒ help slow down conduction of heat

Windows

Double-glazed windows: two glass panes with a layer of gas (air or gases with lower κ, e.g. argon) trapped in between
Can effectively cut down on heat conducted into a building

To reduce heat radiation entering the building

Apply Low-emissivity (low-e) coatings on windows
(made of semiconductors or metal oxides)
⇒ reflect most IR while letting in most visible light

Solar control films

Similar to low-e coatings, but less costly and easy to remove

Shades

Reduce the heat radiation that enters a building
Can apply controls to keep shades facing sunlight

Building designs

Proper building orientations to reduce heat gain

e.g. set up windows facing south rather than facing west

⇒ allow abundant sunlight to warm building up in winter

Vegetation

Vegetation helps cool the surroundings by evaporation of water from their leaves

Energy efficiency in transportation

Most vehicles are still powered by internal combustion engines (ICE) that burn fossil fuels
In ICE, chemical energy stored inside fuel is converted to mechanical energy by engines
Large amounts of pollutant is produced
⇒ environmentally friendly vehicles have been employed

Electric vehicles

Powered by electric motors (driven by batteries)
No emissions and pollution on area it is being used

Battery

Mass ≈ hundreds of kg, energy storage ≈ 20 kW h
Can be charged up within very short time using a quick charger (with high V and large I)
Energy stored per kilogram inside a battery is much less than what fossil fuels can store

⇒ An electric car usually has a shorter mileage than a traditional car

Motor

Convert electrical energy into mechanical energy
In general, energy conversion process in an electric car is more efficient than in an ICE car

Regenerative braking

When an electric car brakes, its motor can act as a generator, converting part of the KE into electrical energy

Hybrid electric vehicles

A hybrid electric vehicle (HEV) cuts down on fuel consumption by combining an engine with a motor powered by a battery
Depending on power need, computer inside the car will choose whether motor, engine or both should be used
As engine is assisted by motor, smaller engine can be used ⇒ Less energy is wasted in moving parts and doing work against friction within engine

Vehicles Comparison

	traditional vehicle	electric vehicle	HEV
type of energy storage	fossil fuel	battery	fossil fuel + battery
mass of energy storage	light	heavy	light
energy converter	internal combustion engine	motor	internal combustion engine + motor
energy refill time	short	long	short
mileage	long	short	medium
regenerative braking system	no	yes	yes
main advantage	long mileage	almost zero emissions	energy efficiency
main disadvantage	pollute air where it travels use fossil fuels (not environmentally friendly)	long charging time short mileage more expensive	more expensive more complex designs

Mass transportation

More energy efficient than private cars

e.g. carry 125 people with a double-decker instead of cars

1 bus = 25-30 cars

⇒ much energy can be saved

⇒ emissions & traffic congestion reduced

Use of electric mass transportation (such as MTR trains)

further relieve traffic congestion
further improve air quality (no emissions by themselves)
In view of energy conversion, power plants are
more efficient than petrol or diesel vehicles

Chapter 4 Different Sources of Energy

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Electricity generation

Mains electricity comes from

⇒ electrical energy from generators in power plants

⇒ mechanical energy in turbine

⇒ different sources of energy
(e.g. chemical, nuclear, solar)

Renewable and non-renewable

Renewable: can be regenerated in a short time
⇒ from the sun, natural physical or biological processes

Non-renewable: can be depleted and cannot be regenerated in a short time
⇒ e.g. fossil fuels, nuclear power

Energy sources：Fossil fuel

Fossil fuels were formed from plants and animals buried about 100 million years ago
Non-renewable
∵ their formation takes a very long time

Energy sources：Nuclear power

In Hong Kong, about 1/4 of electricity we use comes from nuclear power

Binding energy

In an atom, although nucleus contains positively-charged protons, it does not break up

∵ nucleons are bound by strong short-range attractive forces

⇒ binding energy E_b:
work to be done to pull nucleus apart

Mass–energy relation

$$E=\Delta\text{mс}^2$$

As values for binding energy are very small, use electronvolts (eV) instead of joules
⇒ use 931 MeV for mass defect of 1 u

1 u of mass is equivalent to 931 MeV of energy

Uranium as fuel

Two factors for sustaining chain reactions:

Enough concentration of U-235
⇒ natural uranium has to be enriched (so that % U-235 becomes 3–4%)
Neutrons should be relatively slow so that they can be captured by U-235 nuclides
⇒ moderators are needed to slow down neutrons

Fission reactor

Fission processes are under control in fission reactor
Structure of a typical pressurized water reactor:

⇒ fuel rods contain uranium fuel

⇒ control rods control power output by absorbing excess neutrons

⇒ pressurized water carries heat from fuel rod to boiler

Generator

Three water circulation systems:

Safety

Safety measures are to ensure

nuclear reactions are under control
no radiation leaks
nuclear waste is properly handled

Examples:

Fission reactors can be shut down in more than one way
e.g. insert control rods completely, or inject a boric acid solution into reactor
Physical conditions in reactors are carefully monitored by multiple independent devices
A reinforced building is used to isolate a fission reactor from environment
Environmental radiation is monitored regularly
Spent fuel rods are stored in power plant until their activity is low enough to be moved away
Nuclear waste is permanently sealed and buried deep underground

Pros of nuclear power

No emissions, and only waste of small volume (though radioactive)
Produce a lot of energy from a small amount of fuel
Controllable output
Independent of weather

Cons of nuclear power

Costly to dispose of radioactive wastes with long half-lives
Expensive and time consuming to end service of a nuclear power plant
If accidents happen, radiation can spread over a wide area and have a great impact on environment and living things

Wind power

Costly to dispose of radioactive wastes with long half-lives
Expensive and time consuming to end service of a nuclear power plant
If accidents happen, radiation can spread over a wide area and have a great impact on environment and living things

Power calculation

How to work

Blades around rotor are blown by wind
⇒ rotor drives generator to produce electricity

Estimate maximum power generated

KE of air swept by blades:

$$\mathrm{KE}=\frac12(\rho A\nu t)\nu^2$$

If KE of air is completely captured by wind turbine, maximum generated power would be

$$P=\frac{\mathrm{KE}}t=\frac12\left(\frac mt\right)v^2\\=\frac12(\rho A\nu)v^2=\frac12\rho Av^3$$

Pros of wind power

No harmful by-products
One of the cheapest renewable energy technologies available

Cons of wind power

A large amount of space is needed to build a farm of wind turbines
Depend on weather and location
May threaten birds and bats flying past
Noise pollution to living things nearby

Hydroelectric power

Make use of gravitational potential energy (PE) of water

Hydroelectricity power plant:

A dam + a water reservoir at a high position

Energy conversion

Water in upper reservoir has higher PE
Part of PE changes to electrical energy

Pros of hydroelectric power

No harmful by-products
Low operating cost (but building the dam iscostly)
Stable power output

Cons of hydroelectric power

A power plant alter natural water flow and disrupt aquatic habitats
Electricity generation and electricity price can be affected during droughts
A large area has to be flooded (to make a reservoir)
If dam fails, a large area can be destroyed

Solar power

The sun produces a huge amount of energy each second (~10²⁶ W)

Solar constant

Earth receives radiation from the sun all the time
Solar radiation power per unit area received at top of Earth’s atmosphere is about 1360 W m^-2
⇒ solar constant

Solar heating

In the form of solar water heater

Cold water flows into tubes and is heated by the sun
Hot water rises and flows out (due to natural convection)
Each tube usually has a blackened surface and is enveloped in a transparent flask
⇒ help water absorb radiation from the sun
Simple, inexpensive and efficient (> 60%)

Solar electricity

Generate electricity from solar energy

directly using photovoltaic cells (solar cells)
indirectly by using solar heating
Photovoltaic cell:
structure very similar to an LED, but works in reverse way

Using solar heating to produce electricity

Curved mirrors can concentrate sunlight on tube in which oil flows
⇒ Oil is heated to about 400 °C
Heated oil is used for boiling water to steam
⇒ Steam can drive turbines and power generators
Typical efficiency ~40 to 60%

Pros of solar power

In general:

No harmful by-products
High potential as sunlight is abundant

Solar cells:

Silent during operation and simple maintenance

Cons of solar power

In general:

Depend on weather
Need a large area

Solar cells:

Manufacturing solar cells may cause pollution
Expensive and have relatively low efficiency
Dc current generated has to be converted to ac to fit existing electrical facilities

Environmental impact

Air pollution

Emissions can be released in:

Extracting, refining and transporting fuels
Building power plants
Converting energy sources to electricity (e.g. burning fossil fuels) and maintaining parts (e.g. lubrication)
Dismantling power plants

Other waste disposal

Other wastes produced:

Radioactive waste from nuclear plants
Heat disposed of to coolant water in power plant

Global warming

Earth keeps absorbing energy (from the Sun) and radiating energy every time
With an atmosphere, Earth’s surface temperature becomes much warmer because of greenhouse effect

Natural greenhouse effect

Without atmosphere,

Earth’s surface absorbs and emit radiation at same rate
(100 unit absorbed, 100 unit emitted)

With atmosphere,

Earth’s surface gets warmed due to direct radiation from the sun and radiation from atmosphere
Atmosphere absorbs EM radiation
and re-emits radiation of longer λ

⇒ more energy is trapped by Earth and atmosphere

Energy in Hong Kong

How to save up energy use?

Use energy saving appliances
Improve efficiency of buildings and vehicles
Look for alternative energy sources
Avoid creating too much waste

Physics 選修課題 E1

Physics 選修課題 E2

Physics 選修課題 E3

Physics 選修課題 E4

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