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New Senior Secondary Physics Elective C: Energy and Use Chapter 1 Electricity at Home

CHAPTER 1 ELECTRICITY AT

1 Energy Consumption at Home

� Source of energy at home:

1. _________________

� Form of energy conversion in home electrical appliances:

1. Lighting

e.g. ________________, _______________________

2. Heating

e.g. _______________, _______________

3. As motor

e.g. _______________, ____________________

4. Sound

e.g. ______________

� End-use energy efficiency

End-use energy efficiency =

New Senior Secondary Physics Elective C: Energy and Use of Energy

LECTRICITY AT HOME

Energy Consumption at Home

2. ___________________ Form of energy conversion in home electrical appliances:

______, _______________________

_______________, _______________

_______________, ____________________

______________

use energy efficiency =

New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home

� Overall energy efficiency

Overall energy efficiency =

Example: Two electric kettles C and D with same capacities have end-use energy efficiency of respectively

85% and 75%. C takes 20 minutes to boil a kettle of water at o25 C . How much time will D take to do the same job? Example: The energy input and output of an electric hot plate are respectively 1.6 kW and 1.2 kW. Given that the efficiency of the power station is 35% and that of the transmission line is 95%. Find the overall energy efficiency in using the hot plate.


2 Lighting

� Types of lighting used at home:

1. ____________________ 2. _______________________

3. ____________________ 4. _______________________

2.1 Incandescent Lamp

� Consists of very thin but long _____________ wire, in the form of coil,

as the ___________.

� The bulb is filled with _________________, e.g. ___________, in

order to slow down the rate of ______________________.

� Only about _________ of the radiation emitted is in visible light.

� Halogen lamp is a modified form of incandescent lamp.

The inert gas is replaced by ______________, e.g.

____________, to further stopping the evaporation of

the filament. Hence it has a longer ____________. It

can also work at _________ temperature, so that more

_______________ can be emitted.

� However, in halogen lamp, ________ is also produced. This unwanted radiation is blocked by

the cover of the lamp.


2.2 Fluorescent Tube Lamp (FTL)

� A long tube containing inert gas at _______ pressure, with small amount of ___________.

Inner wall of the tube is coated with ____________, which glows when UV hits on it.

� Structure

Filament: _________________ when heated up.

Starter (啟輝器/士撻): A switch made with bimetallic

strips. Initially closed.

Ballast (鎮流器/牛): A coil of many turns.

� Working principles:

� When first switched on, current flows through the _________ only. Filament being heated

up to emit electrons.

� Electrons ___________ towards respective electrodes. They __________ with inert gas

molecules to produce ________, which makes the tube _____________.

� Bimetallic strip heated up and the switch becomes _______. Current now flows through the

tube only.

� Mercury in the tube also heated and __________. Upon collision with electrons, mercury

atoms will absorb energy and become __________. The excess energy will be released in

the form of ________.

� UV strikes on the inner wall and the phosphor material emits ______________.

� Function of ballast: As current increases, ionization in the tube will continue without limit

and even larger current. Ballast regulates (slow down) the increase of current by inductive

effect (Lenz law).

� Note: Fluorescent lamp works on AC circuit. It actually flashes at the same frequency of the

AC source.

� Note: Street lighting employs high intensity discharged lamps which work at similar

principle as FTL, but at much higher voltage and contain mercury or sodium.


2.3 Compact Fluorescent Lamp

� Can be regarded as a twisted form of fluorescent tube lamp.

2.4 Light Emitting Diode (LED)

� Consists of a p-n junction made by ______________.

� Semi-conducting materials can be made conducting by replacing some of its atom by other

elements. Two types of semiconductors can be formed: ________ and ________.

� n-type: charge carriers are __________.

p-type: charge carriers are ________________.

� In a p-n junction, with positive terminal connected to the p-type, electrons move across the

junction to combine with the holes. In such process, electrons move from ________ potential to

a __________ potential energy level. Light energy is emitted in such process.

� Advantage of LED: LED has ___________________ and ___________ lifetime than FTLs.

Example: The figure shows a part of the filament of an incandescent lamp. Explain why the thinner part will go on getting thinner and thinner until it breaks, when the bulb is used continuously. (3 marks)


3 Brightness and Efficacy of lamps

3.1 Sensation of Brightness by Human Eye

� Brightness: The perceived intensity of visible light stimulation.

� Even in the same physical power, human eye has

different _____________ to light at different

___________________. Human eye is most

sensitive to light at around ______nm, which is

___________________.

3.2 Physical Description of Brightness

� Power: The energy output by a light source per unit _________, may include visible or invisible

light.

� Symbol: __________ Unit: ___________

� Power is not a suitable unit for describing brightness because

________________________________________________________.

� Intensity on a surface: The energy falling per unit _________, at normal incidence.

� Symbol: __________ Unit: ___________

� Luminous flux: The visible light energy output by a light source per unit _________, after

correcting human sensitivity at different wavelengths.

� Symbol: __________ Unit: ___________

� Illuminance on a surface: The luminous flux per unit _________, at normal incidence.

� Symbol: __________ Unit: ___________

� Illuminance is a suitable unit to describe brightness because it has accounted the

sensitivity of human eye.


Example: A point light source has a luminous flux of 600 lm is placed at 0.55 m from a point P. Find the Illuminance E at P.

� Lambert’s cosine law: The illuminance must be calculated by luminous flux at normal incidence

to the surface. If the surface is inclined to the light rays at angle θ , the effective flux is _______

and the illuminance becomes ________________.

Example: An LED lamp with luminous flux of 1200 lm is placed at a distance d above a table top. Given that 1.5d = m and PQ is 1.0m, calculate the Illuminance E at P and at Q.


� Efficiency (efficacy) of a lamp: The fraction of luminous flux produced to the amount of input

energy, i.e.

Efficiency of a lamp =

� Characteristics of different types of lighting:

Incandescent Lamp Fluorescent Tube Lamp Light Emitting Diode Efficiency Low Medium to high Medium to high

Life time Short Long Very Long

Price Low High High

Operation Temperature

High Low Low

Color Continuous Depends on phosphor, mixture of discrete lights

Continuous or discrete

4 Cooking Without Fire

4.1 Electric Hotplate

� ________________ acts as heating element.

� Heat transfers to cookware mostly by ______________, a _________ surface is required to

ensure a good thermal contact.

� The power of the hotplate can be calculated by ___________________.

4.2 Induction Cooker

� Employs heating by _____________.


� Working principle:

� A coil of copper wire is placed underneath the cooking pot.

� An ____________________ is applied to this coil, which

produces _________________________.

� This magnetic field induces a __________ in an electrically

conductive pot,

which produces ________________.

� The changing magnetic field also creates magnetic __________________ in a

ferromagnetic pot, also gives heating effect.

� Advantages of Induction Cooker:

� Induction cookers are safe because there is no _________.

� The cooking surface is no hotter than the vessel, spilled food will not _________ on it.

� Induction cookers are easy to clean because the cooking surface is _______ and ________.


� The system will be shut down automatically when a pot is not present.

� Disadvantages of Induction Cooker:

� The cookware must be made of __________ materials.

� May affect persons with implanted _____________.

� Radio receivers nearby may pick up some electromagnetic interference.

4.3 Microwave Oven

� Cooks food by ___________________, in which high frequency alternating electric field

setup rapid molecular rotation in food molecules.

� The magnetron is responsible for setting up the microwave radiation.

� The transparent door is usually equipped with a metal grid with spacing ______________

than the wavelength of the microwave, preventing ___________ of the waves.

� The wave will reflect inside the chamber and form a ________________. Food at the

________ positions will not be heated, therefore a ______________ is equipped to move the

food around.

� Advantages of Microwave Ovens:

� Microwave oven heats food quickly because it causes uniform excitation in a thick outer layer

of food item, i.e. food is heated more evenly. The surface will not be ___________.

� Microwave oven must operate with a ________, it will be switched off automatically.

� Microwave ovens heat food without heating itself.

� Microwave ovens cook at low temperature. As the food is heated by its own

________________, cooked food will not be hotter than o100 C , thus preventing

formation of _______ and ________ which are ______________.


� Cookware taken out from a microwave oven is often much cooler than the food, because the

cookware is ____________ to microwaves. Traditional ovens can cause more serious

burns.

� Disadvantages of Microwave Ovens:

� Homogeneous liquids can become _____________, i.e. being hotter than its normal

boiling point without boiling. The boiling process can start explosively when the liquid is

then ___________.

� Closed containers such as eggs may __________ when heated due to high

__________________. Hence, plastic foams are also microwave unsafe.

� Any conductive object heated inside a microwave oven will act as a _______________

and causes ___________.

� Some plastics absorb microwaves and can become dangerously hot.

� A running microwave oven without food will form a __________________ inside the

chamber. Energy will accumulate and may burn the oven.

� Characteristics of different types of cookers:

� End-use energy efficiency: _____________ > _____________ > ______________

Electric hotplate Induction cooker Microwave oven Price Low High Low

Running Cost

High Low High

Safety Top plate gets hot. Spilled food may burn.

Top plate does not get hot. Spilled food will not burn.

Casing does not get hot. Closed container may explode.

Convenience Warm up time needed. No warm up time needed. No warm up time needed.

Heating Cooking by conduction, contacting side cooks better.

Cooking by conduction, contacting side cooks better.

Cooking by radiation, much more even cooking.


5 Moving Heat Around

� A device that uses heat to do work is called a ____________________. The direction of heat

flow is _____________.

� Energy conservation: ______________________________________________

� A device that requires work to force heat flows in reverse direction is called a ______________,

example: __________________.

� Energy conservation: ______________________________________________


5.1 Air-conditioner

� Two types: ______________ type and _________________ type.

� Working principle:

� The sealed coil tube consists of a coolant which is called _______________.

� Stage 1: (Cold, liquid + gas)

The refrigerant in liquid and gaseous state (coexisting), _________ than room temperature

flows through the evaporator. It absorbs heat and ______________.

� Stage 2: (Warm, gas)

The compressor does _________ on the coolant vapour. ____________ and

____________ of the vapour is increased.

� Stage 3: (Hot, gas)

Compressed vapour passes through the ___________ in the condenser, it releases heat to

the surrounding and condenses into __________.

� Stage 4 (Cold, liquid): The liquid passes through the __________________ where it

undergoes rapid expansion. It cools further before it returns to the evaporator. While the

pressure is dropped, the liquid returns into gaseous state and stage I is repeated.


� Performance indicators:

� Cooling capacity: the _________ at which heat is removed by an air-conditioner.

Cooling capacity =

� Coefficient of performance (COP): the ratio of heat removed to the work done.

COP =

Example: An air-conditioner has a cooling capacity of 2.5 kW. What is the maximum amount of heat energy can be removed by 4 hours of operation time?

Example:

A computer room is kept at o20 C by an air-conditioner. At a certain day, the air-conditioner transfer heat to outside at a rate of 4.0 kW, and its COP is 4.8, find the rate of removing heat from the room and the rate of doing work by the air-conditioner.


6 Energy Efficiency Labelling Scheme (EELS)

� Objectives:

1. to promote public _______________ on energy saving,

2. to assist consumers to ___________ energy efficient products.

� Labels are prepared by ____________.

� Two Types of label:

� 1. Recognition type:

To show the device has met the _____________ energy efficiency as specified in the

scheme.

� 2. Grading type:

Divides electrical devices into ______ grades, with grade _____ being most efficient.

� Grading determined by energy efficiency index.

Energy Efficiency Index Iε =

� An efficient device will have a ___________ Iε.

� Example on air-conditioner classification:

(Code of Practice on Energy Labelling of Products, EMSD 2010)


Example: Two window air-conditioners A and B have the following specifications: A B Grade 3 3 Annual Energy Consumption per year (kWh)

1100 900

Price $3540 $4000 The cost of electricity is $1.15 per unit. Find how long will it take to pay back the difference in price with the saving on the electricity cost.

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