New Senior Secondary Physics Electiv Chapter 1 Electricity at Home Page 1 CHAPTER 1 ELECTRICITY 1 Energy Consumption at H Source of energy at home: 1. _________________ Form of energy conversion in 1. Lighting e.g. ______________ 2. Heating e.g. ______________ 3. As motor e.g. ______________ 4. Sound e.g. ______________ End-use energy efficiency End-use energy efficien ve C: Energy and Use of Energy Y AT HOME Home 2. ___________________ home electrical appliances: ___, _______________________ __, _______________ __, ____________________ _ ncy =
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New Senior Secondary Physics Elective C: Energy and Use Chapter 1 Electricity at Home
Page 1
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
Page 2
� 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.
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
� 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.
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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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.
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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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)
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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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.
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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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.
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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� 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 _____________.
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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� 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 ________.
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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� 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 ______________.
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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� 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.
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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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: ______________________________________________
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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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.
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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� 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.
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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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)
New Senior Secondary Physics Elective C: Energy and Use of Energy Chapter 1 Electricity at Home
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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.