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WORK, POWER
AND ENERGY
By iTutor.com
T- 1-855-694-8886Email- info@iTutor.com
The Law of Conservation of MassThe Law of Conservation of Matter (or Mass) states that matter cannot be created nor destroyed in an ordinary chemical reaction. No atoms are gained or lost in a reaction; they are just rearranged.
The total mass of reactants = Total mass of products
Work is the transfer of energy through motion. In order for work to take place, a force must be exerted through a distance. The amount of work done depends on two things:
The amount of force exertedThe distance over which the force is applied.
Work
Object
Direction of Force
Direction of movement as result of Force
Work If you push a box with a force of one Newton
for a distance of one meter, you have done exactly one joule of work.
W = F x dDistance (m)
Force (N)
Work (joules)
Work (force is parallel to distance)
W = F d cos
Distance (m)
Force (N)
Work (joules) Angle
Work (force at angle to distance)
W = mghHeight object
raised (m)
Gravity (m/sec2)
Work (joules)
Mass (g)
Work done against gravity
Power Power is defined as the rate at which work is done. It
can also refer to the rate at which energy is expended or absorbed.
Power is equal to the amount of work done divided by the time it takes to do the work.
Change in workor energy (J)
Change in time (sec)
Power (W)
P E t
=
A unit of power is called a watt.
Another unit horsepower.One horsepower (the avg
power output of a horse) is equal to 746 watts.
Power
Another way to express power is as a multiple of force and it's velocity, if the velocity and force are both vectors in the same direction.
Velocity (m/sec)Force (N)
Power (W) P = F . v
Power
Efficiency Efficiency can also mean the ratio of energy output
divided by energy input.
Eo
Energy output (J)
Energy input (J)
Efficiency
Ei
e =
eff = Wout
Winwork actually done by input force
work done with no friction (often m g h)=
Copyright reserved. © 2012 The E Tutor
Efficiency is defined for a process.
A process is any activity that changes things and can be described in terms of input and output.
The efficiency of a process is the ratio of output to input.
Efficiency
Efficiency – 10%
Energy flow in systems
Energy flows almost always involve energy conversions.
To understand an energy flow:
1. Write down the forms that the energy takes.
2. Diagram the flow of energy from start to finish for all the important processes that take place in the system.
3. Try to estimate how much energy is involved and what are the efficiencies of each energy conversion.
Energy flow in human technology
1. Storage ex. batteries, springs, height, pressure
2. Conversion ex. a pump converting mechanical energy to fluid energy
3. Transmission ex. through wires, tubes, gears, levers
4. Output ex. heat, light, electricity
The energy flow in technology can usually be broken downinto four types of processes:
Energy flow
The energy flow diagram for a rechargeable electric drill shows losses to heat or friction at each step.
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