Work. Energy and Power By: Listiya Pawitra Grade: 11 Science
Work• Work is done when a force move a point at which it acts in the direction of the force.• Work done = force x distance moved by force- When a force moves its point in the direction of the force, the force works and the work done is said to be positive.• Work done is said to be negative, when the work is done on force, which means that the direction of the force is opposite to the direction of the movement.
Work done• Has no direction• Only has size• It is a scalar quantity• Unit measurement is Joule (J)
• Initial position Final position of the foece Direction of movement of force• This diagram shows work si done by force.
• Final position Initial position Direction of force• This is the diagram of work is done on the force.
Displacement• It is a vector quantity• It is another name of distance moves ina particular direction.• Work done for displacement x = F cos y x x
Work done by an expanding gas• When gas expands it works breaking down a concrete building.• Work done of gas = pressure x change in volume• When gas expands work is done by gas• When gas contracts, work done is done on gas
Energy• The ability to do work is called energy• Its unit measurement is in Joule ( J )• There are 2 types of energy:- Potential Energy- Kinetic Energy
Potential Energy• Potential energy is the ability of an object to do work as a result of its position or shape.• The energy can be called elastic.• It is a form of energy that is stored in objects which have had their shape changed elastically. Ex: a wound up spring, twisted elastic bands.
Electrical Potential energy• It is the law of charges like charges repel and attract. This means that repelling an attracting need work.• Electrical potential energy increases when 2 positive charges move towards each other and work is done.
Gravitational Potential Energy• When 2 masses are pulled apart, it causes work o be done this is gravitational potential energy.• If masses move closer together they lose gravitational potential energy.Potential energy = mg h
Kinetic Energy• This energy is due to motion.• When an object moves it gains in kinetic energy but loses its potential energy.Kinetic energy = 1/2mv2
Energy conversion and conservation• All the energy changes happening on earth is governed by the law of conservation of energy.• The law states that:Energy can’t be created or destroyed. It can only be converted from one from to another.
Efficiency• In energy changes some energy are lost as heat energy.• Efficiency gives a measure of how much the total energy can be used and is not lost.Efficiency = useful work done / total energy inputIt can be given in ratio or percentage.* Efficiency can never be > than 100% because it can’t be created.
Deformation of Solids• Deformation occurs when you pull a spring over its elastic limit.• Tensile deformation is when there is force that can stretch it, make it longer, squash it and make it shorter. The example is a metal cylinder.
Hooke’s Law• It states that, provided the elastic limit is not exceeded, the extension of a body is proportional to the applied force.• Extension is the increase in length of a spring.• The load attached to the spring is called the load.• Elastic change is when a body or object returns to its original shape and size when the load is removed.
• Elastic limit is when it is the maximum elasticity it can reach in order for it to return to its original size and length.• Beyond the elastic limit the spring is said to be plastic. F=k x• Unit measurement is Nm-1
Strain Energy• Strained is when an object changed its shape n=my forces acting on it.• Strain energy is the energy stored in a body because of change of shape. Strain energy W = 1/2k(x)2
Young Modulus• By using the young modulus we can find the extensions knowing the constant and dimensions of the object.• Strain Energy = extension / original length• The strain produced within an object is caused by stress.• Stress = force / area of cross section• Unit measurement of stress is Nm-2
• Young Modulus E = Stress / Strain• This has no units because it only shows the ratio.
Behaviour of different materials under tensile stress X X is the maximum force and this is called as ultimate tensile stress. Ultimate stress gives a sign of the maximum force that the wire can afford.
Graph of brittle materials Brittle means that it is delicate and can break easily. Brittle materials obeys the Hooke’s law . There is very little plastic deformation that’s why glass can shatter into many pieces.
Graph of polymeric material showing hysteresisEnergy dissipated Elastic hysteresis is when a cord is not permanently deformed but, when stretched it just doesn’t return to it’s original length. This graph shows that the strain energy needed to deform the rubber, area under the graph is greater than the work done for the material to return to it’s original length.
Specific Heat Capacity• Q = m.c. t• Q is the energy• m is the mass ( in kilograms )• C is the specific heat capacity t is the change of temperature• The SI unit of specific heat capacity is J kg-1K-1
Thermal heat Capacity• This is to find the change in heat energy.• The thermal heat capacity of a body is the quantity of heat energy needed to increase the temperature of the whole body by one degree• Q=C t The SI unit of heat capacity is J K-1
Specific latent heat• There are 2 types:- Specific latent heat of fusion- Specific latent heat of vaporization
Specific latent heat of Fusion• It is the quantity of heat energy required to change the mass of solid to liquid without any change of temperature.• Q = m . Lf• Unit measurement is J kg-1
Specific latent heat of vaporization• It is the quantity of heat energy to convert the mass of liquid to gas with no change in temperature.• Q = m.lv• Unit measurement is J kg-1
Power• Power is the rate of doing work.• Power = work done / time• The SI unit of power is watt (W)• Power is a scalar quantity• Power = force x speed
Kilowatt-hour• One kilowatt hour is the energy used when work is done at the rate of 1 kilowatt in 1 hour.• It is the unit measurement in electricity meters at home.
Moment of a force• When there is ruler put in horizontal nad there is a weight attached to it, we can feel a turning effect on the ruler. This turning effect is called moment of the force.• Moment of force depends on the magnitude and the distance of the forcefrom the pivot
• So, the moment of a force is the product of the force and the perpendicular distance of the line of action from the pivot.• Moment of force = F x l.cosx
Couples• Consist of 2 forces• They have equal magnitude but opposite direction.• When you turn a screw driver, It produces a turning effect. Since there are 2 forces acting on it, the turning effect is called a torque.
The Principle of moments Hanging a weight on the ruler will make it the ruler to rotate around the pivot. Hanging one weight on one side of the ruler will make the ruler to rotate the opposite direction.Pivot Hanging 2 weights on both sides of the ruler will make the ruler to rotate clockwise or anti clockwise.
• Rotational equilibrium is when a body has no tendency to change its rotation speed.• The sum of the clockwise moments at any point must be equal to the sum of anticlockwise moments at that same point.• Centre of gravity is a point which the whole weight of the object looks balance.
Equilibrium• For an object to be at an equilibrium state:- The total value of the forces in any directions must be zero.- The sum of the moments of the forces around any point must be zero.