Energy transfers and stores, Work Done and such

Question 1

What is a powerful machine?

Answer 1

One that can transfer a lot of energy in a short space of time

Question 2

streching or squashing an object can transfer energy to…

Answer 2

elastic potential energy store

Question 3

A blender is 70% efficient. It has a total input power of 600 W. Calculate the useful output power.

Answer 3

efficiency = 70% = 0.7

useful power output = efficiency * total power input = 0.7 *600 = 420 W

Question 4

A car of mass 2500kg is travelling at 20m/s.

Calculate the energy in its kinetic energy store.

Answer 4

E_k = 1/2 * 2500 * 20² = 500 000J

Question 5

Elastic potential energy formula…

Answer 5

E_e (J) = 1/2 * k (N/m) * extension (m)

Question 6

What is the dissipated energy of an energy transfer between stores called?

Answer 6

wasted energy

Question 7

name two non-renewable energy sources

Answer 7

fossil fuels and nuclear power

Question 8

Name an example of a frictional force

Answer 8

air resistance

Question 9

air resistance can transfer and object’s ____ energy store to its ______ energy store

Fill in the blanks

Answer 9

1) kinetic
2) thermal

Question 10

What is the formula for power?

Answer 10

Power (W) = energy transferred (J) / time (s)

Power (W) = Work Done (J) / Time (s)

Question 11

Where does all energy eventually end up?

Answer 11

thermal energy stores

Question 12

name the three main types of energy sources

Answer 12

coal

oil

gas

Question 13

What make a machine faster than another?

Answer 13

the faster one transferring the same amount of energy over less time

Question 14

lifting object in a gravitational field requires…

Answer 14

work, which causes a tranfer of energy to the gravitational potential energy store of the raised object. The higher the object is lifted, the more energy is transferred to this store.

Question 15

What things can you do to prevent energy losses through heating?

Answer 15

have thick walls with low thmeral conductivity

Use thermal insulation

Question 16

What is the formula for efficiency?

Answer 16

useful power output

total power input

Question 17

What makes something more efficient?

Answer 17

the less energy wasted and the more energy used usefully

Question 18

A 2.0 kg object is dropped from a height of 10 m. Calculate the speed of the object after it has fallen 5.0 m, assuming there is no air resistance. Give your answer to 2 significant figures.

g = 9.8N/kg

Answer 18

E_p = mgh = 2.0 * 9.8 * 5.0 = 98 J

or

98 m²/s²

Question 19

if there is no air resistance…

Answer 19

energy lost from the g.p.e store = energy gained in the kinetic energy store

however, in real life, there is air resistance, and it acts against all falling objects. It causes some energy to be transferred to other sources. e.g thermal energy stores of the object and surroundings.

Question 20

What is one Watt equal to?

Answer 20

1 joule of energy transferred per second

Question 21

Where is waste energy usually transferred to?

Answer 21

thermal energy store

Question 22

is energy always transferred usefully?

Answer 22

No. When it is transferred between stores it always has some energy that is dissipated, unless it is a closed system.

Question 23

what is power measured in?

Answer 23

Watts

Question 24

is any device 100% efficient?

Answer 24

no, other than an electric heater as all the energy is transferred to useful thermal energy stores

Question 25

air resistance acts against all falling objects, causing some energy to be transferred to other energy stores, such as…

Answer 25

thermal energy stores via friction between the object and surroundings

Question 26

Explaaaaiinn how cavity wall insulation reduces the amount of energy transferred out of a house.

Answer 26

Cavity wall insulation reduces energy transfer by convection. also reduces energy transfer by conduction because insulating foam has a low thermal conductivity and so has a low rate of energy transfer from thermal energy stores.

Question 27

describe energy transfers for a closed system

Answer 27

energy that occurs within the system, but no energy leaves, so none is wasted (the net change is zero)

Question 28

kinetic energy formula

Answer 28

E_k (J) = 1/2 * mass (kg) * velocity² (m/s)²

Question 29

what is thermal insulation?

Answer 29

the reduction of heat transfer (i.e. the transfer of thermal energy between objects of differing temperature) between objects in thermalcontact or in range of radiative influence. The insulating capability of a material is measured as this inverse of thermal conductivity (k).

Question 30

Falling objects transfer…

Answer 30

Gravitational potential energy store to kinetic energy store

Question 31

It takes 8000 J of work to life a stunt performer to the top of a building. Motor A can lift the stunt performer to the correct height in 50 s. Motor B can do it in 300 s to life the performer to the smae height. Which motor is more powerful?

(use Power formula)

Answer 31

Motor A as it can do the same amount of work faster.

P = 8000 / 50 = 160

Question 32

what is power?

Answer 32

the rate of energy transfer, or the rate of doing work.

Question 33

Gravitational potential energy formula

Answer 33

E_p (J) = mass (kg) * gravitational field strength (N/kg) * height (m)

Question 34

Name two examples of ways you can reduce the amount of energy that is wasted

Answer 34

lubrication and thermal insulation

Question 35

what is the conservation of energy principle?

Answer 35

that energy is always conserved/transferred, never destroyed or created

it can be transferred usefully, stored or dissipated

Question 36

Hi, it’s your sister, what did I want to tell you?

Answer 36

I love you