Topic 8- Forces And Energy Flashcards
What is the principle of the conservation of energy ?
The principle of the conservation of energy says that energy cannot be created or destroyed.
It can only be transformed from one store of energy into another.
What are the 4 ways energy can be transformed ?
Waves
Electrical work
Mechanical work
Heating
How does an electrical current transfer energy ?
An electrical current transfers energy from one object to another
such as a battery powering a torch
How do waves tranfer energy ?
Light and other types of waves can transfer energy from one object to another.
How does mechanical work transfer energy ?
Energy transferred from one object to another via a force is an example of mechanical work.
When an object falls from a building, the weight does mechanical work to transfer energy from the gravitational potential store to the kinetic store.
When you push a wheelchair, energy is transferred to the kinetic store of the wheelchair.
How does heating transfer energy ?
Heating an object transfers energy to its internal store (the sum of the energy in the kinetic and chemical stores of its particles).
Which principle tells us that energy cannot be created or destroyed ?
Conservation of energy
What is the work done by a force equal to ?
The work done by a force is equal to the energy transferred to the object.
Fact
More work is done if you:
Push with the same force for a longer distance.
Push with a larger force for the same distance.
Work done = ?
force x distance (moved along the line of action of the force)
What is transferred to an object that has had work done on it?
Energy
What quantitie is considered in the equation for work done ?
Force
The work done by a force is equal to what ?
The work done by a force is equal to the change in energy of the object (the energy transferred to the object).
How is work done measured ?
The work done (measured in joules)
What is work done equal to (detail) ?
is equal to the force (in newtons) multiplied by the distance moved in the direction of this force (in metres).
A book is pushed 60cm along a desk by applying a force of 12N. What is the work done on the book?
A book is pushed 60cm along a desk by applying a force of 12N. What is the work done on the book? W = F × d 60 cm = 0.6 m W = 12 N × 0.6 m W = 7.2 J
Energy = ? x ? x ?
Energy = potential difference x current x time
What is the electrical energy transferred to an component when a current of 3A and a potential difference of 2V act on it for 4s?
E = V I t
E = 2 x 3 x 4
E = 24 J
Energy = power x ?
Energy = power x time
What is power ?
Power is the rate of energy transfer (or work done).
What is power equal to ?
The power (measured in watts) is equal to the work done (in joules) divided by the time taken (in seconds).
Whats teh equation for power ?
Power = worl done / time
When one store of energy is transferred to another, some energy is wasted (transferred into unwanted stores of energy).
What happens ro this ?
This dissipates (spreads out) to anything nearby.
In almost every energy transfer, friction creates what energy and what hppens to this energy ?
In almost every energy transfer, friction creates heat energy and this energy is often wasted.
Car engines make a lot of noise and get hot when they are being used to move you about.
What is the useful energy store and wasted energy store ?
The useful energy store here is the kinetic store because you want the car to move you from one place to another.
The heat and sound energy are wasted energy because they are of no use to you.
When wires are used in an electrical circuit, they heat up this is often not useful energy.
So what can we say about the heat energy ?
and so it is wasted
The efficiency of an energy transfer can never be more than what ?
100
What is the equation for the efficiency of (energy in joules) ?
Efficiency = (useful energy output/total energy input) x 100.
The useful energy output and total energy input are measured in joules (J).
What is the equation for the efficiency of power in watts ?
Efficiency (power in watts)
Efficiency = (useful power output/total power input) x 100.
The useful power output and total power input are measured in watts (W).
What diagrams can show energy transfers ?
Sankey diagrams
How much energy is wasted by a 10% efficient filament bulb with 5 J of input?
4.5 J
Perpetual machines can continue moving forever without ever stopping. Why are perpetual motion machines impossible?
In every process some energy is waisted
This waste of energy means that the efficiency of energy transfer is less than 100%.
This means that the machine would eventually stop
How is efficiency most often expressed ?
As a percentage
