Energy levels 7-9 Flashcards
Prove mathematically that the final velocity of a falling object can be found from this equation
Explain why your hands get hotter when you rub them together
When a force acts over a distance it transfers energy. This can be found from the equation
W = Fxd
W = work (Joules)
F = force (Newtons)
d = distance (m)
when you rub your hands together the friction force acts over a distance. This causes the transfer of energy from the chemical potential energy store into the thermal energy store, this makes your hands heat up
Explain how you would find the maximum height that a slingshot could shoot an object upwards
for example: a slingshot with a spring constant of 3 N/m is stretched back by 10cm. How heigh would it shoot a marble of 5g mass? The strength of gravity on earth is 10 N/kg
To answer this question you need to use conservation of energy, at the heighest point all of the energy in the elastic potential energy store has been transferred into gravitational potential energy. this means that you need to find the total energy at the start, which is also the total energy at the end
Explain why the maximum height at one side of the swing of a pendulum is equal to the maximum height on the other side of the swing.
Why is the fastest part of the swing when it is directly under the pivot?
This is because of the law of conservation of energy. At the heighest point on one side the energy is stored as gravitational potential energy. As it swings downwards it transfers energy into the kinetic energy store. The most kinetic energy is when it is at its lowest point (directly below the pivot)
As it swings back up on the other side the the energy is transferred back from the kinetic energy store into the gravitational potential energy store. The total energy is conserved, and as GPE = mgh (with g and m constant) the height at the end must be the same as the height at the start
Explain how you can use a ramp, a marble and some light gates to prove the law of conservation of energy
To prove the law of conservation of energy, set up the ramp and the light gate as shown in this diagram.
Measure the starting height of the marble and then measure the velocity of the marble when it has reached the lowest point using the light gates. then you measure the mass of the marble and use the equations KE = 1/2mv2 and GPE = mgh to prove that the energy at the start and the end is the same
Explain how you use the equation for gravitational potential energy to calculate the gravitational field strength of a planet or moon
For example: A 400kg satellite orbiting at a height of 2km from the surface of the moon has a gravitational potential energy of 1 280 000J. What is the gravitational field strength of the moon
To answer this question you need to rearrange the equation for gravitational potential energy to give
Explain why a roller coaster cannot roll up the track to the same height as the peak that it has just come from
This is due to energy dissipation as thermal energy into the environment. When the roller coaster rolls down the track some of the energy is dissipated out of the system as thermal energy. This means that the total energy in the system has reduced, reducing the maximum gravitational potential energy it can gain.
As GPE = mgh, with mass and gravitational field strength remaining constant, if the total energy in the system reduces, then the maximum height reduces as well
Explain how you can increase the efficiency of the energy transfers of a bicycle
To improve the efficiency you can use a lubricant such as oil on moving parts. This reduces the size of the friction force that is acting. As W = f x d, reducing the size of the force will reduce the ammount of energy dissipated out of the system as thermal energy
Wearing a streamlined helmet will also make it more efficient becuase it reduces the size of the air resistance force acting. This in turn reduces thermal energy dissipated into the environment
Explain how you find the total energy you need to put into a process if you know the outcome that you require and the efficiency of a system.
Example: how much energy do you need to supply to a motor which is 60% efficient, to lift a 5kg mass by 3m?
To do this you need to first find the energy output requred, then rearrange the efficiency equation to find the required energy input
Explain how to find the power output of a device if you know the potential difference, current and the efficiency
Example calculation: What is the power output of a 15% efficient bulb that is plugged into the mains electricity supply and has a current of 0.2A passing though it
To find the output power, first you need to find the input power. This can be done using the equation P = IV
P = 0.2 x 230
P = 46W
Then you find the output power by using the efficiency equation
How does thermal energy conduct through a solid
Explain why glass conducts heat better than air and copper conducts heat better than both of them
Thermal energy conducts through a solid becuase the atoms that are heated up start to vibrate. These atoms then bump into the next atoms along, and they start to vibrate as well.
This happens in a domnino effect until they are all vibrating
glass conducts thermal energy better than air becuase the particles in glass are much closer together than the particles in air. This means that it is easier gor the glass to pass on the thermal energy
Gold conducts thermal energy better than both of them becuase it contains free electrons. These free electrons can carry thermal energy from one place to another, increasing the rate of thermal conduction
How does specific heat capacity relate to the time that it takes for an object to cool down
The higher the specific heat capacity, the longer it takes for something to cool down. This is becuase a higher specific heat capacity means that something needs to loose more energy to drop in temperature by each degree.
As it has to loose more energy to cool down it means that it takes longer for it to happen
Explain if installing cavity wall insulation which costs £200 and saves £50 per year is better or worse for a householder than installing solar panels that cost £10,000 and save £800 per year if
(a) they will be moving in a short time
(b) they are staying in the house long term
To answer this question you need to work out the payback time
payback time for wall insulation = total cost/savings per year
= 200/50 = 4 years
payback time for solar panels = 10000/800 = 12.5 years
(a) in this situation the insulation is better becuase they will quickly pay back the cost and start to make savings of £50 per year before they move house
(b) in this situation it is better to use the solar panels becuase although it takes 12.5 years to pay it back, they will then make much greater savings, which amounts to a greater total saving in the long term
What are the pros and cons of fossil fuel power stations
Pros
- Cheap
- Short start up time
- Does not explode if there is a problem
Cons
- Supplies will run out in near future
- Causes global warming and acid rain
- Fuel comes from unstable countries
What are the pros and cons of nuclear fission power stations?
Pros
- No global warming.
- We have more than 200 years supply.
- Mined in areas with stable governments
Cons
- Waste can last for 100,000 years
- Waste has to be guarded as it can be turned into nuclear bombs
- Waste is radioactive – needs safe storage
