work, energy, power and efficiency Flashcards
what is energy needed for?
Energy is needed to make a stationary object move, to change its shape, or to warm it up
examples of energy stores;
gravitational potential stores
kinetic energy stores
thermal energy stores
elastic energy stores
what are different ways energy can be transferred
by radiation (light), electrically, mechanically
what is energy measured in ?
joules
1 ____ is equal to the ______ needed to raise a 1 _____ weight through a ______ height of 1 ______
Energy is measured in joules (J). One joule is equal to the energy needed to raise a 1 N weight through a vertical height of 1 m.
what happens when energy is transferred?
Whenever energy is transferred, the total amount of energy after the transfer is always equal to the total amount of energy before the transfer. The total amount of energy is unchanged.
what is the principle of conservation of energy?
energy cant be created or destroyed
what does it mean when work is done?
work is done on an object when a force acting on it makes a move - energy is transferred to the object
what does the amount of work done depend on?
The amount of work done depends on the force and the distance the object moved.
greater the _____ or further the ______, ______ the work done
The greater the force or the further the distance, the greater the work done.
equation for work done
force x distance moved in direction of force (parallel)
unit of work done
joule
equation for work done involving an angle?
W = F s cos theta
-s is distance travelled
-theta is angle between direction of the force and direction of motion
what does area under force displacement graph represent?
work done
how to work out area under graph?
constant/straight line = area of shapes such as triangle/square
variable-counting squares
what is kinetic energy
energy an object has due to its motion
faster an object moves the more kinetic energy it has
equation for kinetic energy?
1/2mv^2
what is potential energy?
energy an object has due to its position
equation for graviatational potential energy?
Mgh
mass x gfs x change in height
how does the work done affect gravitational potential energy?
work done on the object increases gravitational potential energy
what can you do with KE equation and PE equation if air resistance is negligible
Consider an object of mass m released above the ground.
If air resistance is negligible, the object gains speed as it falls.
Its potential energy therefore decreases and its kinetic energy increases.
After falling through a vertical height ∆𝒉, its kinetic energy is equal to its loss of potential energy:
𝟏 𝒎𝒗𝟐 = 𝒎𝒈∆𝒉
what are the ways of transferring energy from one object to another?
work done ( work done by a force due to one object making other object move)
heat transfer ( heat transfer from a hot object to cold object - conduction, convection, radiation)
electromagnetic waves
definition of power
rate of transfer of energy
unit for power
watt
equal to energy transfer rate of 1 joule per second
equation for power
energy / time or work done/time
what is the output power of an engine called?
motive power
what happens when a powered object moves at a constant velocity at a constant height?
When a powered object moves at a constant velocity at a constant height, the resistive forces (e.g., friction, air resistance, drag) are equal and opposite to the motive force.
where is the work done in an engine transferred to
The work done by the engine is transferred into the internal energy of the surroundings by the resistive forces.
equation for work done for a powered vehicle
P = F V
what is useful energy?
energy transferred for a purpose
what is wasted energy
not all energy supplied is transferred for intended purpose
equation for efficiency
useful energy transferred by machine / energy supplied to machine
or
work done by machine/energy supplied to machine
how can we express efficiency
output / input
how can efficiency be improved?
how efficient a device/machine is dependent on how much useful energy/power it provides compares to the total input
rest of the energy is dissipated as heat/sound - this can be made more efficient
