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Chapter 6:Energy(Physics) - Coggle Diagram
Chapter 6:Energy(Physics)
Work is said to be done on an object when a constant force is applied to the object makes it move a certain distance
Definition of work done
Work done by a constant force is given by the product of the force and the distance moved in the direction of the force
When a man is carrying a box, no work is done as the force applied by his hands is in an upward force but he is walking in a straight line.The force of direction the man is going and the force of direction applied to the box is not the same
Models/approaches to understand energy
Energy store models
Energy in the kinetic store
Energy in the gravitational potential store
Energy in the thermal store
Energy is transferred from 1 store to another
Energy can be defined as the capacity of a body to do work
SI unit of energy is joules
Energy transfer
Energy is a non-physical substance that can be transferred from 1 energy store to another
The total amount of energy remains constant
Energy transfer processes
Mechanically(by a force acting over a distance
Electrically(by an electric current)
By heating(due to a temperature difference
BY propagation of waves(both electromagnetic and mechanical)
The principle conservation of energy
Energy cannot be created or destroyed.It can be transferred from 1 store to another.The total energy of an isolated system is constant
Kinetic energy
If 2 objects are moving but one has greater mass, they exert more kinetic energy even though they go at the same speed
Ek=1/2 x m x s square
m=mass
Ek=Joules
s=metre per second
If an object is stationary, no kinetic energy is possesed by the object
Eg: Kinetic energy posessed by a car of mass 500kg travelling at a constant speed of 20m/s Ek=1/2x500x(20) square=100000J or 100KJ
Eg: car of mass 500kg accelerates from rest under a constant acceleration 0f 4m/s square.WWhat is the kinetic energy after 2 seconds a=v-u/t 4=v-0/2 v=8m/s Ek=1/2x500x8 square=16000J
Eg:A package is lifed vertically through a distance of 10m at a constant speed.Taking the acceleration due to gravity to be 10m/s square. Gain in GPE=5x10x10=500J. Work done in lifting the package. Work done=fxd=50Nx10m=500J
Eg:A 50kg girl is standing on the 10th step of a staircase.She starts to walk up 40 steps,each 15cm high to reach the 50th step. Tkae gravitational potential energy at the bottom of the flight of steps to be zero and g to be 10N kg.Ep=50x10x7.5=3750J 15cm=0.15m 0.15mx50=7.5m
Ball of mass 5kg is dropped from a height of 10m.Calculate the velocity of the ball before it hits the ground.Loss in Ep=Gain in Ek(Total amount of energy remains constant).mgh=1/2 x m x v square 10x10=1/2 v square 200= v square v=14.1m/s
Parcel of mass 4kg slides down a smooth curved ramp. What it the speed of the parcel when it reaches the bottom? Ek= 1/2xmxv square=4x5x10(GPE) 1/2x4xv square=200 v square=100 v= 10m/s square
Which has greater kineti energy, a motorbike of mass 100kg at a speed of 50km/h or a bullet 0.02kg travelling at a speed of 1200m/s. motorbike=1/2x100x(2500/3) square=34722222.22 bullet=1/2x0.02x1200 square=14400
Power is defined as the rate of energy transfer
P=W/t or E/t
P=Power(in watts)
E=energy transferred(in joules)
W=work done(in joules)
t=time taken(in seconds)
Eg:If each step has a height of 0.20m and there are 10 steps altogether, what is her body power if she runs up the flight of steps in 5s given her weight is 450N. Power=work done/time=(fxd)/t=(440x0.2x10)/5=180W
Eg: If an electric heater is rated 250W. Calculate the quantity of thermal energy generated in 10min: Power=energy x time E=pxt=250x600s=150000J
In practice, the efficiency of a filament lamp is much lower than 100% because most of the energy is transferred electrically to the interal stores of other components and surrounding air
In the context of power, efficiency means the ratio of useful pwoer output to total power input.A device with high efficiency means that it is using its input power effectively while a device with low efficiency means that its using its input power ineffectively
However, in reality, the useful energy output if a machien is always less than the energy input
Some energy will be dissipated during energy transfer, largely due to friction.This energy usually takes in the form of heat and sound energy