Energy Topic Flashcards
What is Kinetic Energy?
The energy stored in moving objects.
- Stationary objects have no kinetic energy.
- Kinetic Energy (J)= 0.5 x M x v2
Mass (KG) Speed (m/s)
Example Questions:
- A Car with the mass of 700kg is moving with the speed of 20 m/s work out the the kinetic energy of the car.
Ek= 0.5 x M x v2 Ek= 0.5 x 700 x 20(Squared) 140,000 J /1000 = 140 kJ.
A Cyclist and a Bike have a total mass of 100 kg
and a speed of 15 m/s. Calculate the Kinetic Energy.
Ek= 0.5 x 100 x 15(squared)= 11250 J
kJ= 11.25
A Tennis Ball is travelling at 50 m/s and has a kinetic energy of 75 J. Calculate the mass of the tennis ball.
75= 0.5 x m x 50 (squared)
= 0.06 kg.
What is Elastic Potential energy?
- Putting energy in to stretch the string . The stretched strings is storing this energy and we call this the Elastic Potential Energy.
- When we apply a force on a string and the string stretches is called the extension.
- The extension is directly proportional to the force applied.
- If we apply to great of a force on the string the extension is no longer directly proportional.
- At this point the string has been stretched beyond its limit of proportionality.
- It will not return to its original length when we take back the force.
How do we calculate this?
Ee= 0.5 x K x e2
(J) (Spring constant N/m) (m)
A Spring has an extenstion of 20cm. Calculate the elastic potential energy stored in the spring (K = 100N/m).
Ee= 0.5 x 100 x 0.2 (squared)
100cm= 1m
= 2 J
What is gravitational Potential energy?
- Gravitational Potential Energy is the energy stored in an object due to its position above the Earths surface.
- ## This is due to the force of Gravity acting upon the object.
How do we calculate the gravitational potential energy?
- Ep= m x g x h
J) (Mass kg) (gravitational field strength (N/kg) (Height m
Example:
- A Crane lifts a 75 kg mass a height of 8m.
- Calculate the gravitational potential energy gained by the mass.
- g(= 9.8 N/kg)
Ep= m x g x h
Ep= 75 x 9.8 x 8
= 5580 J.
Pratice:
- A ball with the mass of 500g is lifted onto a shelf which is 1.5 m above the ground. Calculate the Gravitational Energy Potential gained by the ball. (g= 9.8 N/kg).
Ep= m x g x h
Ep= 0.5 x 9.8 x 1.5= 7.35 J.
1000 g = 1kg
What is Specific Heat Capacity?
- The Specific Heat Capacity of a substance is the amount of energy required to raise the temperature of 1kg of the substance by 1 celcius.
How do we work the equation of this out? (Specific Heat Capacity).
Change in Thermal Energy (J)
= Mass (Kg) x Specific Heat Capacity (J/kg Celsius) x Temperature Change (Celsius).
Example Question:
Calculate the energy required to increase the temperature of 2kg of water from 20 Celsius to 100 Celsius .
The specific Heat Capacity of the water is 4200 J/kg celcius.
Change In thermal Energy:
2 x 4200 x 80= 672000 J
Kj= 672 Kj.
Practice Question: 1
An Iron has an aluminium plate with the mass of 1.5 kg. Calculate the thermal energy stored in the plate when the temperature rises from 20 Celsius to 200 Celsius.
The Specific Heat capacity of the aluminium is 913 J/kg Celsius.
Change in Thermal Energy= Mass x Specific Heat Capacity x Temperature change.
Change In thermal Temperature=
1.5 x 913 x 180= 246510 J.
Kj= 246.51
Practice Question 2:
A hot water bottle cools down from 80 Celsius to 20 Celsius releasing 756000 J of Thermal Energy. Calculate the mass of water in the hot water bottle. The Specific Heat capacity of water is 4200 J/kg Celsius.
756000= M x 4200 x 60
756000/252000= 3 kg.
What is the law of conservation of energy?
- ## Energy Can not be created or destroyed it simply transfers from one store to another.
What is a system?
- A system is a group of objects or just an singular object.
- What is a closed system?
- In a closed system no energy can leave or enter.
Energy transferred in a Pendulum:
- At the furthest top the pendulum travels to this is the maximum gravitational potential energy.
- As it swings this is transferred to a connected energy store.
- The mass has a maximum kinetic energy store when its at the bottom (This is where its moving at the fastest speed).
- As the mass swings up the kinetic energy transfers to the gravitational potential energy store again.
Where on a Pendulum is friction?
- fixed point.
- pendulum passes through the air particles.
- Friction causes energy to be transferred to thermal energy.
- The fixed point and the air around the pendulum gradually gets warmer.
- these stores of thermal energy are less useful and therefore energy has been wasted.
- ## This will cause the pendulum to swing with less energy gradually with less energy and eventually stop.
How do we reduce unwanted energy transfers?
- We can reduce unwanted energy transfers by reducing friction.
- We can do this by using an lubricant such as oil on the fixed point.
Energy Transfers: Bungee Jump.
- 1) At the start of the jump, all the energy of the system is stored in the gravitational potential energy store.
2) Jumper Falls; Energy transferred from the GPE store the the KE store.
3) When Bungee Rope starts to tighten: Kinetic energy store is at its maximun.
4) Rope is fully extended: KE is 0 as the jumper is not moving, all energy has been transferred to the elastic potential store.
5) Bungee Rope recoils: Energy is transferred from elastic potential store to KE energy store.
6) Ascent: Energy transfers from KE back to GPE.
7) Top ascent; All energy is in GPE store.
Jumper never returns to their original position as energy is dwasted as thermal energy due to friction of air particles as the jumper passes and the strecthing of the bungee which isnt fully elastic.
Work Done by a force.
- Work is done whenever energy is transferred from one store to another.
Mechanical Work Done: Using a force to move an object.
Electrical Work done: Current transferring energy,
Work done= Force (N) x Distance (m).
What is power?
- Power is the rate at which energy is transferred or the rate at which work is done.
Power (W)= Energy Transferred (J)/ Time (S).
Power (W)= Work Done (J)/ Time (s).
1 Watt is an energy transfer (or work done) of 1 J per second.
Effienency:
- Efficiency= Useful Output Transfer/Total input energy transfer.
- Efficency= Useful power input/ total power input.
Factors of Coolings of a building:
- Thermal conductivity:
- Higher thermal conductivity of a material= higher rate of energy transfer by conduction across that material.
Modern Houses:
- Built with 2 layers. Eternal Brick and internal breezeblock.
- Between the walls is a cavity.
- Thermal conductivity is high.
- Alot of thermal energy can transfer out of the house.
- Builders pack a lot of cavity with an insulating material which has a very low thermal conducitvity.
- Reduces overall thermal cinducity, so less enegry passes trhough the walls and escapes from the house.
Glass Factors and Roof:
- Single Glazed:
- High thermal conductivity.
- most houses have double glazed windows.
- Lower thermal conducvitity, so less thermal energy can pass through and leave the house interior.
Roof:
- Loft insulation.
- Low thermal conducitvity.
- Reducing tj rate thermal enery passes trhough.
Summary of how we can reduce the thermal energy transfer of a house:
- Construct the buildings using materials with a low thermal confuctivity.
- ## Build the house with thick walls as this reduces the rate of thermal energy transfer.
