Chapter 1 - Conservation And Dissipating Of Energy Flashcards
Name the 5 ways that energy can be stored (5)
- Chemical
- Kinetic
- Gravitational potential
- Elastic
- Thermal
Name 3 different examples of chemical energy (3)
- Fuels
- Food
- Chemicals found in batteries
What do kinetic energy stores describe?
The energy an object has because it is moving
What do gravitational potential energy stores describe?
The energy an object has because of its position relative to the ground
What does elastic energy describe?
The energy an object has due to being stretched or squashed
Whats does thermal energy describe?
The energy an object has because of its temperature
How can energy be transferred?
From one store to another
Give 4 examples of energy being transferred (4)
- Heating
- Waves
- Electric current
- When a force moves an object
What energy transfers occur when an object falls and hits the ground? (5)
- Gravitational potential
- Kinetic energy
- Energy transfers to the surroundings
- Increase in thermal energy
- Sound waves
What is the name of an energy store associated with a compressed string?
Elastic
What is the name of the energy store associated with a book on a high shelf?
Gravitational potential
Energy can not be created or destroyed what is the principle?
The principle of conservation of energy
What is a closed system?
A closed system is an object or group of objects in which no energy transfers take place out of or into the energy stores of the system.
When an object falls freely what energy is being transferred and is it increasing or decreasing? (2)
- Gravitational Potential decreases
2. Kinetic energy increases
When a person stretches a elastic band what energy is being transferred and is it increasing or decreasing? (2)
- Chemical energy in muscles decrease
2. Elastic potential increases
Describe the energy stores of a pendulum swinging?(2)
- When the pendulum swings to the middle gravitational potential is decreasing and kinetic energy is increasing.
- As the pendulum moves away from the middle the kinetic energy decreases and the gravitational potential increases
In terms of energy, what happens when you turn on a torch?
The energy is transferred from the chemical energy store of the battery to the thermal energy store of the surroundings.
What occurs when force moves an object?
When force moves an object, energy is transferred to the object and WORK is done on it.
What unit is used for WORK and ENERGY?
Joules (J)
What happens when work is done to an object?
When work is done to an object, energy supplied to the object is equal to the work done in moving the object.
What is the formula for WORK DONE and what do the units stand for.
W = F s W = work done in joules F = the force in newtons s = the distance moved in the direction of the force in meters
What is friction?
Friction is the force that opposes the motion of two surfaces i contact with each other.
What is the work done on an object when a force of 600N moves it a distance of 12m?
7200J (Joules)
What is gravitational potential energy?
Gravitational Potential energy is energy associated with an object because of it’s position in the Earth’s gravitational field.
In what circumstance does gravitational potential energy increase/decrease? (2)
- Gravitational potential energy increases whenever an object is moved upwards. The increase is equal to the work done on the object by lifting force.
- Gravitational potential energy decreases whenever and object is moved downwards. This decrease is equal to the work done by the gravitational force acting on it.
When an object is lifted or lowered, the change in its gravitational potential energy is,
TriangleE = m x g x Triangleh
Describe what this equation stands for? (4)
1 .TriangleE = The change in the gravitational potential energy in joules, J.
- m = The mass of the object in kilograms, kg.
- g = The gravitational field strength in newtons per kilogram, N/kg.
- Triangleh = The change in height in metres, m.
What happens to the gravitational potential energy store of an object when the object moves downwards?
It decreases.
What is the unit of gravitational field strength?
Newtons per kilogram (N/kg)
Kinetic energy can be calculated using the equation
Ek = 1/2 x m x v squared
What do these stand for?
Ek = The kinetic energy in joules, J. m = The mass of the object in kilograms, kg. v = The speed of the object in metres per second, m/s.
An object is described as being elastic if ……………..
Finish the sentence.
It regains its original shape after being stretched or squashed.
The elastic potential energy of a spring can be calculated using the equation,
Ee = 1/2 x K x e squared
What do theses stand for?
Ee = The elastic energy in joules, J. k = The spring constant of the spring in newtons per metre, N/m. e = The extension of the spring in metres, m.
What is the elastic potential energy of a spring with a spring constant of 150 N/m when its extension is 0.2m?
3J
What is the kinetic energy of a girl of mass 40kg running at a steady speed of 5m/s?
500J
What is USEFUL ENERGY?
Useful energy is energy transferred to where it is wanted in the form that is wanted.
What is WASTED ENERGY?
Wasted energy is energy that is not usefully transferred.
Why is energy most commonly wasted?
Energy is often wasted because of friction between the moving parts of a machine.
When can friction be most useful?
Sometimes friction can be useful - for example, in the brakes of a bicycle or a car. The kinetic energy store of the vehicle decreases and the thermal energy stores of the brakes and of the surroundings increase.
What happens to the wasted energy of a lightbulb?
It is dissipated to the surroundings, it will heat up.
Why should we talk about energy being ‘wasted but not energy being ‘lost’?
Energy cannot be creator or destroyed, so it cannot be lost; but is wasted when not usefully transferred.
What is the energy supplied to a machine (device) called?
Input Energy
What is the energy that is transferred usefully by the machine called?
Output Energy
What is the principle of conservation of energy?
Input energy = Useful output energy + energy wasted.
What equation calculates the efficiency of a device?
efficiency = useful energy transferred by the device
/ total energy supplied to the device x100
What is the unit for efficiency?
There is not one, efficiency is a ratio.
Name 3 ways machines waste energy? (3)
- Friction
- Air Resistance
- Electrical Resistance
In a machine, for every 25J of energy supplied to the machine, 10J are usefully transferred. Calculate the efficiency of the machine?
0.4 or 40%
Name 1 way of reducing energy waste in machines (1)
Lubrication is used to reduce friction between moving parts.
The shape of a lorry is modified to reduce air resistance.
what happens to the efficiency of the lorry?
It Increases
Name 3 of the most common ways energy is supplied in a home? (3)
- Electricity
- Gas
- Oil
How is energy wasted by a light bulb?
Thermal Energy. By heating the surroundings.
How is energy usefully transferred by a toaster?
Thermal Energy. By heating the bread.
How is energy wasted by a toaster?
By heating the surroundings and the body of the toaster.
What is the unit of power?
1 Watt.
1 Watt is equal to the rate of transferring 1 Joule of energy in 1 second.
Power can be calculated using this equation.
P = E/t
What do they stand for?
P = The power in watts. E = The energy transferred in joules, J. t = The time taken for the transfer in seconds, s.
What is the equation for power wasted?
Power wasted = Total power supplied to device -
useful power output from device
The efficiency equation can be written in terms of power.
what is the equation?
efficiency = useful power output/ total power input (x100)
How many watts are equivalent to 2.2kW?
2200W
An electric motor transfers 36kJ of energy in 2 minutes.
What is the power output of the motor?
300W
Name the energy store of a compressed spring?
Elastic Potential Energy
Describe the changes to energy stores that take place when a child lifts a box from the floor to a high shelf?
- Chemical energy decreased in the child’s muscles.
2. Gravitational Potential increased in the box
Calculate the work done when a force of 1500N pulls a car through a distance of 60m in the direction of the force.
90000J
A Ball of Mass 0.45kg is thrown vertically upwards and reaches a height of 3.0m. The Gravitational field strength is 9.8N/kg. Calculate the change in gravitational potential energy of the ball.
0.45 x 9.8 x 3.0
= 13J
Describe the changes to energy stores that take place when a student stretches and releases a spring. (3)
- Chemical Energy in the student’s muscles decrease.
- Elastic Potential Energy increases as the spring is stretched.
- Themal energy released into the surroundings increased.
A driver pushes on a broken down car with a horizontal force, but the car does not move.
Explain why no work is being done on the car?
Work is only done when a force moves an object through a distance, because the car does not move, no work is done to it.
Calculate the kinetic energy of a toy car with a mass of 50g that is moving at a speed of 8.0cm per second?
50g = 0.05kg
8.0cm/s = 0.08m/s
kinetic energy = 1/2 x 0.05 x (0.08) squared
=1.6 x 10 to the -4
A student pushes a crate at a steady speed with a horizontal force of 200N. It takes the student 15 seconds to move the crate a distance of 12m.
Calculate the power developed by the student in pushing the crate.
power = 200N x 12m/15s
= 160W
Describe the changes to energy stores that take place when a bungee jumper jumps of a bridge and eventually comes to a stop at the end of the rope?
when the bungee jumper jumps gravitational potential is at its highest and when they are bouncing back up kinetic energy increases.
A person pushes a trolley up a slope with a constant force at a constant speed.
Explain why the work done on the trolley was less that the work done by the person?
work done on the trolley = weight of the trolley x increase in height.
work done by the person = force x distance moved along the slope.
Some of the work done by the person is wasted by doing work against friction
An electric motor is used to raise a load. When the load gains 28J of gravitational potential energy, 42J of energy is wasted by the motor.
Calculate the percentage efficiency of the motor.
total energy input = 42J + 28J
=70J
motor efficiency = 28/70 x100
=40%
A spring is stretched so it extends by 0.080m. The elastic potential energy of the stretched spring is 1.2J.
Calculate the spring constant of the spring.
K = 2E/e squared
= 2 x 1.2/e squared
= 375 N/m
