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physics > Energy, Work And Power > Flashcards

Energy, Work And Power Flashcards

1
Q

A snowball hitting a wall
Explain which store to which store energy is transferred

A

Kinetic energy into thermal energy of surroundings

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2
Q

A car driver braking in an emergency

A

Kinetic energy of moving car
To thermal energy of surroundings

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3
Q

Name the 8 energy stores

A

-kinetic energy
-chemical energy
-magnetic energy
-thermal/ internal energy
-nuclear energy
-electrostatic energy
-gravitational potential energy
-elastic potential energy

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4
Q

Name four ways that energy can be transferred

A

-mechanically
- electrically
-through heating
-through radiation

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5
Q

How much GPE does an object have before work is done on it?

A

0

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6
Q

What is the formula for calculating GPE

A

GPE= mass(kg) x gravitational field strength(N/Kg) x CHANGE in height(m)

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7
Q

What kind of energy do our muscles store

A

Chemical energy

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8
Q

Thermal energy is transferred from… to …

A

Higher temperatures
Lower tempertaure

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9
Q

Why is conduction more rapid in metals

A

Because they have delocalized electrons which are able to flow easily throughout the structure

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10
Q

Explain how heat is transferred through a kebab stick in terms of energy stores and energy transfers

A

Energy from thermal energy store of fire is transferred to kinetic energy store of particles in kebab
Kinetic Energy from most energy particles is transferred to other particles mechanically

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11
Q

Is copper a better conductor of heat of steel

A

Copper

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12
Q

Is air an water a conductor or insulator of heat

A

Insulator

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13
Q

Explain how woolen clothing allows you to keep warm

A

Air, an insulating substance gets trapped in fibers
Where body heats the air and stops you losing heat

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14
Q

Explain how an offshore breeze works at night

A

-water has a higher specific heat capacity than water so stays longer for warmer
- thus air above sea warms faster as particles above it gain kinetic energy
-so they begin to expand and rise, as cool air from the land replaces it

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15
Q

When a bathroom heater is turned on, name two reasons the heat reaches you so quickly

A

The infrared waves are traveling in a straight line
The reflector is specially shaped and is very shiny

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16
Q

How is it possible that we can receive light from the sun

A

Because Infared waves can travel through a vacuum

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17
Q

Explain how a heat sink prevents a transistor from over heating (or electrical device)

A

Transistor is connected to heat sink, where thermal energy is transferred through conduction to the heat sink.
The heat sink is Matt black and this is a good radiator of heat
It is also designed in such a way that air convection currents can carry the hot air away

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18
Q

Do shiny objects reflect or radiate heat

A

Reflect

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19
Q

Explain why a reflector(shiny material) emits less thermal energy than a radiator(black material)

A

Because reflectors reflect heat away whereas radiators absorb and emit heat energy

20
Q

Complete the blanks
At lower temperatures, most thermal energy is radiated through …… waves which is….to human eye
But at higher temperatures, most thermal energy is radiated through….. waves which is …. To human eye

A

Infrared, invisible
Shorter wave lengths, visible(because object starts glowing)

21
Q

Explain how aluminum foil prevents marathon runners from losing heat

A

Inner side is reflective and so reflects any heat back to their body
Outerside is a poor radiator so does no emit any heat back to surroundings

22
Q

How can wind cause body temperatures to fall

A

Wind molecules come very close to face and warm molecules from face begin to be swept into atmosphere through forces convection

23
Q

Why are copper kettles kept polished

A

So that they remain good reflectors and thus reduce heat loss

24
Q

What is the statement that answers what energy does

A

Energy is the ability to do work(where work is equal to energy transfer)

25
Q

Formula for work done

A

Work(joules) = force (N) x distance (m)

26
Q

when an object falls, explain the three energy stores that energy is transferred through

A

Gravitational potential energy —) kinetic energy—-) heat/sound

27
Q

GPE= mgh is the same version of what equation?

A

W=FD
(Because the force acting on a mass is the same as the gravitational field strength, and the distance is the same as the height)

28
Q

Formula for calculating kinetic energy

A

KE= 1/2 mv^2

29
Q

Explain how energy is transferred when an object falls and then when it lands, describing the GPE and KE energy stores

A

When object falls, gravitational potential energy decreases because it’s height decreases
But kinetic energy increases because it gains speed
When it reaches the ground, all of the GPE from original stationary object is transferred to KE

30
Q

Fill in the blanks
Energy is conserved so, … of GPE = … of KE

A

Loss
Gain

31
Q

In a roller coaster, the truck falls through a height of 17m. Calculate the trucks speed at the bottom of this fall

A

GPE=KE
mgh=1/2mv^2
gh=1/2v^2(because m cancels out)
10 x 17= 1/2v^2
V= 18.44 m/s

32
Q

Even if you calculate the speed at which a roller coaster ride falls to the bottom, explain why the real speed will be less

A

Because some GPE will be transferred to heat or sound

33
Q

Explain how you can investigate your power input

A

1) weigh yourself
2)measure height of steps
3)measure time taken to climb stairs
4) power= work done /time taken so multiply your weight by distance, and divide by time

34
Q

Formula for power

A

Power (w) = work done(j) / time (s)

35
Q

What does it mean if a lamp is 20% efficient

A

20% of the energy input
Is transferred usefully as light

36
Q

Jim is doing an experiment to investigate heat loss in water , by changing the surface area of different containers. He plans to use the same volume of water for each. Explain why this must be controlled

A

Could affect reliability of results
Because different volumes of water could affect the amount of heat lost
So would be unable to tell which variable mattered in the experiment

37
Q

Suggest why when a ball is hit on the moon is travels further than on the earth(2)

A

Moon less gravitational field strength so their is less air resistance

38
Q

Attached to a motor is a string and a mass. Suggest why the GPE gained by the mass is larger than the electrical energy gained

A

Electrical energy could have been lost by heat or sound
The mass of the string is ignored

39
Q

Describe the energy transfer that takes place in lamps when they are connected to a power supply

A

Chemical to electrical to light energy

40
Q

Explain how a cavity in windows reduces heat loss from convection and from conduction

A

Heat can not be lost through convection as there is not enough space for convection current to circulate

And heat can not be lost through conduction because air is trapped inside which is a bad conductor of heat

41
Q

Energy transferred can also be written as….

A

Work done

42
Q

Polar bears have long hair that are hollow and contain air.
Explain how this reduces the amount of thermal energy lost by them

A

Can trap air which is a good insulator
So body heat produced will be trapped and will be near to the skin

43
Q

Explain how microwave sheets food

A

Infrared radiation from microwave heats outer surface of food
Then particles gain kinetic energy and through conduction the rest of the food is heated

44
Q

Explain how a potato in water is heated by conduction

A

When thermal energy is supplied, the particles in the metal begin to gain kinetic energy and start vibrating

These particles then collide with particles of the water which also gain kinetic energy

This energy is then transferred to the potatoes

45
Q

Explain 3 ways how a woolen blanket helps keep you warm

A

Fibers in the blanket trap air which is a good insulator and so traps heat

Because air is trapped in the fibers, there is little air movement and so convection currents are reduced

the blanket prevents sweat from evaporating and so the cooling effect of sweat is reduced

46
Q

Ok

A

Sure

physics (10 decks)

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