Tuesday, 20 September 2011
Power questions
PFY, p. 120
f) Work, watts, joules, time
g) joule per second
13) P= Wd/t= 1000/5= 200 watts
14) Wd= F*d= 300*2
P= Wd/t= 600/6= 100 watts
16)Wd= F*d= 3000*10= 30000
P= Wd/t = 30000/4= 7500 watts= 7.5 kilowatts
4.14
· 4.14 describe power as the rate of transfer of energy or the rate of doing work
"Rate" just means "divided by time" (see Entrance Activity)
So
Power = Energy / Time
or
Power = Work done / Time
4.14 Entrance Activity Answers
· You can run up the stairs in the Science block in 30 seconds.
· Mr. Roff can run up the stairs in the Science block in 20 seconds.
· Who is the most powerful? Why?
· Mr Roff. He does more work running up the stairs (because has a higher weight) in a shorted time so he is more powerful
· It takes 2 minutes to fill the 20 litre sinks in this room with water.
· How fast is the sink filling?
· 10 litres/min
· At what rate is the sink filling?
· 10 litres/min
· How long would it take to fill a 10 litre sink?
· 1 min
Wednesday, 14 September 2011
CoE questions
19. a) W= mg= 50*10= 500N
b) GPE= wh= 500*4= 2000 J
c) GPE=KE=2000 J 3. a) GPE= mgh= 30*35*10=10500 J
b) KE= GPE
10500= 0.5mv2
v2= 600
v=24.5 m/s c) Not all of the GPE is transferred to KE because some energy is wasted due to friction.
b) GPE= wh= 500*4= 2000 J
c) GPE=KE=2000 J 3. a) GPE= mgh= 30*35*10=10500 J
b) KE= GPE
10500= 0.5mv2
v2= 600
v=24.5 m/s c) Not all of the GPE is transferred to KE because some energy is wasted due to friction.
4.13
· 4.13 understand how conservation of energy produces a link between gravitational potential energy, kinetic energy and work
Wednesday, 7 September 2011
4.10
· 4.10 understand that work done is equal to energy transferred
PFY p.112
Collins p. 91
1. a) joules, force, distance
b) newton, meter, force
d) energy
2. Wd= F*d
Wd= 5*2=10J
4.a) Wd= F*d
Wd= 500*40= 2 kJ
b) Chemical energy from food
GPE as she walks up
1. a) W= m*g= 400*10= 4000N
Wd= F*d
50000= 4000N*d
d= 12.5m
Tuesday, 6 September 2011
Sunday, 4 September 2011
Thursday, 1 September 2011
Questions
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4.8
4.8 describe how insulation is used to reduce energy transfers from buildings and the human body
Task 1
· Use this interactive animations to investigate how different insulation affects the rate of cooling of hot water
<<Cooling of hot water with insulation - interactive.swf>>
Task 2
· Use these interactive animations to find out about the energy transfers in a house
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| <<Types of energy transfers in a house - interactive.swf>> | <<Insulation in a house - % heat losses and savings.swf>> |
Task 3
· Test your knowledge with this quiz
<<Quiz - Types of energy transfers in a house.swf>>
4.7
4.7 describe the role of convection in everyday phenomena
Task 1
· Try the animation
· Extension: what is happening to the particles in the air?
<<Convection in a room with a radiator.swf>>
Task 2
· Why is Lee tired after cycling to the Sea in the morning?
· Predict what will happen when Lee cycles home in the evening
<<Lee cycles towards the sea ... and back worksheet.pdf>>
4.7
1. What is the most common heat transfer mechanism in solids?
Conduction
2. What is the most common heat transfer mechanism in liquids and gases?
Convection
3. What is the only heat transfer mechanism in a vacuum?
(Infra-red) Radiation
4. Why can't conduction happen in a vacuum?
There are no particles in a vacuum to transfer the vibrations
5. What happens to the space between the particles in a gas when you heat it?
The space between the particles increases
6. What happens to the density of a gas when you heat it?
It decreases because the space between the particles increases
7. Why can't convection happen in a solid?
In a solid the particles are fixed in position and are not free to move
Objective 4.6: Notes on Conduction, Convection and Radiation from Collins
Energy will always flow from areas at high temperatures to areas at low temperatures. This is called thermal transfer. Thermal energy can be transferred in three main ways: conduction, convection and radiation. CONDUCTION:
Materials that allow thermal energy to transfer through them quickly are called thermal conductors. Those that do not are called thermal insulators. If one end of a conductor is heated, the atoms that make up its structure start to vibrate more vigorously. As the atoms in a solid are linked together by chemical bonds, the increased vibration can be passed on to other atoms. The energy of movement passes through the whole material. Conduction cannot occur when there are no particles present, so a vacuum is a perfect insulator. CONVECTION:
Convection occurs in liquids and gases because these materials flow. The particles in a fluid move all the time. When a fluid is heated, energy is transferred to the particles, causing them to move faster and further apart. This makes the heated fluid less dense than the unheated fluid. The less dense warm fluid will rise above the more dense colder fluid, causing the fluid to circulate. This convection current is how the thermal energy is transferred.
If a fluid's movement is restricted, then energy cannot be transferred. That is why many insulators, such as ceiling tiles, contain trapped air pockets. Wall cavities in houses are filled with fibre to prevent air from circulating and transferring thermal energy by convection. RADIATION:
Radiation, unlike convection and conduction, does not need particles at all. Radiation can travel through a vacuum. This is clearly shown by the radiation that arrives from the sun. Radiated heat energy is carried mainly by infra-red radiation, which is part of the electromagnetic spectrum.
All objects take in and give out infra-red radiation all the time. Hot objects radiate more infra-red than cold objects. The amount of radiation given out or absorbed by and object depends on its temperature and on its surface.
Materials that allow thermal energy to transfer through them quickly are called thermal conductors. Those that do not are called thermal insulators. If one end of a conductor is heated, the atoms that make up its structure start to vibrate more vigorously. As the atoms in a solid are linked together by chemical bonds, the increased vibration can be passed on to other atoms. The energy of movement passes through the whole material. Conduction cannot occur when there are no particles present, so a vacuum is a perfect insulator. CONVECTION:
Convection occurs in liquids and gases because these materials flow. The particles in a fluid move all the time. When a fluid is heated, energy is transferred to the particles, causing them to move faster and further apart. This makes the heated fluid less dense than the unheated fluid. The less dense warm fluid will rise above the more dense colder fluid, causing the fluid to circulate. This convection current is how the thermal energy is transferred.
If a fluid's movement is restricted, then energy cannot be transferred. That is why many insulators, such as ceiling tiles, contain trapped air pockets. Wall cavities in houses are filled with fibre to prevent air from circulating and transferring thermal energy by convection. RADIATION:
Radiation, unlike convection and conduction, does not need particles at all. Radiation can travel through a vacuum. This is clearly shown by the radiation that arrives from the sun. Radiated heat energy is carried mainly by infra-red radiation, which is part of the electromagnetic spectrum.
All objects take in and give out infra-red radiation all the time. Hot objects radiate more infra-red than cold objects. The amount of radiation given out or absorbed by and object depends on its temperature and on its surface.
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