聆听与学习科学/能量
< 聆听与学习科学
In physics, a Force is said to do work, when it acts on a body, and there is a displacement, in the direction of the applied Force. In other words, when a Force is applied, and the body moves, work is done. A Force of 'F' Newtons, is applied to an object. The object moves, by a distance, 'd'. The work done is Force, multiplied by, distance. Work is equal to 'F', multiplied by 'd'. If the Force 'F', is equal to 1 Newton, Distance moved, is equal to 1 metre, Work done is equal to, 1 Newton metre. This is called as, a 'Joule'. One Joule is equal to, one Newton metre.
The unit of Energy is the same as, the unit of work. The unit of Energy is, 'Joule', One Joule, is equal to, 1 Newton metre. The Joule, is the S I unit, for Energy and work.
Power is nothing but, work done per unit time. Since unit of work, is same as Energy, Power is the Energy, per unit of time. 'Joule' is the unit, for work and Energy. The unit of Power, is Joule per second. One Joule per second is called, a 'Watt'. One Watt is equal, to one 'Joule' per second.
We are all, used to the unit, of 'Watt'. For example, a bulb in our house could, have a rating, of 40 Watts. This means, that the bulb consumes 40 Watts, of Energy. That is, the bulb consumes 40 Joules per second, of Energy.
A Kilo Watt is equal to, 1000 Watts. A Kilo Watt is represented, as 'KW'. A water heater, in our house could, consume about 1 KW. That means, it consumes, 1000 Watts, per second.
Normally, the total Energy consumed is measured, as Kilo Watt hour. That is one Kilo Watt, consumed in one hour, is called as 1 Kilo Watt hour. A forty Watt bulb, if used for 100 hours, Will consume a total Energy, of 40 multiplied by 100, Watt hours This is equal to, 4000 Watt hours. This is equal to, 4 Kilo Watt hour. One Kilo Watt hour, is represented as, 'K' 'W' 'H'. Electricity companies, call one 'K', 'W', 'H', as one unit of Energy.
Our electricity company charges, for the electrical Energy consumed, The charges are, per Kilo Watt hour. The charges may vary, Let us take an example, where the charges, by our electricity company, is 4 rupees per 'K', 'W', 'H', or unit. So, a 40 Watt bulb used for 100 hours, will use 4 Kilo Watt hours, or four 'K', 'W', 'H', or units. This will cost us, at the rate of 4 rupees per unit, a total of 16 rupees.
Let us take the water heater, Its rating is 1 KW. If we use it for, two hours, every day, for 30 days, We will consume, 2 multiplied by 1, multiplied by 30, Equal to, sixty 'K', 'W', 'H' or 60 units. This will incur a charge, of 60 multiplied by 4 rupees, equal to 240 rupees.
The term 'horse Power', has an interesting history. There was an 18th-century, Scottish engineer called, James Watt. He invented a steam engine. In those days, they used to have a mill, to grind grain. This was done by a horse, which will rotate the mill, by going around it. It used to do it, for hours together. Farmers were, very familiar with this form, of animal Energy. James Watt, tried to market his steam engine, He found it very difficult, to convince the farmers, of the Power of his engine. So, he calculated the Energy, the horse spends, in grinding the grain. He compared this with, the Power of his engine. So, he started rating his engines, as equivalent to number of horses. Now, it was easy to communicate to the farmer, that his engine was equal to 3, or 4 horses. This he called it as, 'horse Power' or ‘H P’. Amazingly, even today, modern cars are rated, in horse Power. More interesting, the unit of Watt, is the official S I unit, for Power. The name of the unit was chosen, in honour of James Watt, who coined the phrase horde power. The scientific and S I unit, for Power is a Watt, But, do not be surprised, if you come across horse Power, as a popular unit of Power. So, we might as well know that, one horse Power or H P is equal to, 746 Watts. We can also state it as, one horsePower or H P is equal to, 0.746 KW.
A Compact car, might have a rating, of forty H P. It is like driving a chariot, pulled by 40 horses.
In our daily life, we use many appliances, which consume Energy. Let us discuss a few examples, of Energy consumption. The Power of the Electric bulb can be rated, at about 40 Watts. This is equivalent to saying that, it consumes 40 Watts of Energy, A ceiling fan can have a rating, of about 90 Watts. A computer might have a rating, of about 250 Watts. A television set could be rated, at 300 Watts. A refrigerator could be rated, at 380 Watts. A washing machine could be rated, at 500 Watts. An electric iron could be rated, at 1 KW. A hot plate could be rated, as 1.5 KW. A water pump could be rated, at 3 KW.
You will note that many, of the appliances, that we use, consume Energy. Most of them also, use electrical Energy. Electrical Energy, is a very convenient way, to produce, and transmit Energy. It is the most widely, used form of man made Energy.
The Power requirement, of a metro city could be about, 2500 MW. In modern life, almost everyone uses electrical Energy. A farmer uses electrical Energy, for running an irrigation pump.
Where does, all this electrical Energy, come from? Electricity is generated, in Power plants. Power plants could be thermal, hydro electric, or nuclear. Energy can also be generated, from solar Power plants, and wind mills. Capacity of the Power plants, to produce Power, could vary widely. Typically a Power plant, could produce, around 100 MW, to about 1000 MW. This Power is transmitted, to industries, and households. The electrical Energy consumption, per person is called, per capita, electrical Power consumption. The per capita electrical Energy, consumption in India, is about 2 KW hours, per day. This is equivalent to saying that, every person in India, on an average, Consumes 2 KW hour, or 2 units of electrical Energy, every day. In comparison, the per capita, electrical Power consumption, in the United States, is about 40 KW Hour.
There is a very interesting, aspect to Energy. Though we talk about, Power 'generation' plants, Energy can never be created, or destroyed. The total Energy, in the system is always, conserved. We can only transform Energy, from one form, to another form.
Potential Energy, is one form of Energy. Let us say, there is a ball, of mass equal to, 'm' kg. Let this ball be suspended, at a height of, 'h' metres. The ball is stationery. It appears, that the ball has, no Energy. But, this ball does have, some Energy. This is due to the Force, of gravity, acting on it. This Energy is called, potential Energy. The potential Energy, is measured as, 'm' 'g' 'h'. 'm' is the mass, of the ball, in Newtons. 'g' is the acceleration, caused by the Force, of gravity. 'h' is the height, in metres. The potential Energy, of the ball, is 'm' 'g' 'h' Joules.
Let us take an example. Mass of the ball, 'm' is 1 kg. The gravitational acceleration, 'g' is equal to 9.8 m/sec. 'h' is equal to, 10 m. The potential Energy of the ball, is 'm' 'g' 'h', = 1 multiplied by 9.8, multiplied by 10, = 98 Joules.
Now we understand, that any object which is at a height, will have potential Energy. For example, when we sit at top of the slide, in the play ground, we have potential Energy. The water that is stored, in the over head tank, has potential Energy. A diver standing on a diving board, in the swimming pool, has potential Energy. When we are top of the bridge, on our bicycle, we have potential Energy.
A body moving, with a velocity, has kinetic Energy. Let us take the ball, which was suspended, at a height of, 'H', metres. If the ball is released, it will come down, and reach the earth, at a velocity, of 'V', metres per second. When the ball was suspended, it had a potential Energy of, 'M', 'G', 'H', We know that Energy, cannot be created, or destroyed. The potential Energy, equal to, 'M', 'G', 'H', has to be conserved. The kinetic Energy of the ball, when it reaches the ground, Should be equal, to the initial potential Energy.
We can work this out. We know that initial velocity, of the ball, 'U', is equal to, 0. The final velocity, of the ball, is equal to, 'V'. The distance travelled, is equal to, 'H'. 'V', squared, is equal to 'U', squared, + 2, 'A', 'S'. Acceleration 'A', is equal to, ‘g’. Distance 'S', is equal to height, 'H'. So, 'V', squared, is equal to, 0 +, 2, ‘g’, 'H'. 'V', squared, is equal to, 2, ‘g’, 'H'. Transposing, 'H', is equal to 'V', squared, divided by 2 'G'. Potential Energy is equal to, 'M', ‘g’, 'H'. Let us substitute, the value of 'H', equal to 'V', squared, divided by, 2 ‘g’. The Energy now, is equal to, 'M', ‘g’, multiplied by 'V', squared, divided by 2 ‘g’. ‘g’, gets cancelled out. Kinetic Energy is equal to half, multiplied by, 'M', 'V' squared.
Kinetic Energy is equal to half, 'M', 'V' squared Joules, This is the general formula, for kinetic Energy. A body of mass 'M', Moving with a velocity of 'V', will have a kinetic Energy, of half 'M', 'V' squared.
In this example, of the suspended ball, at a height, of 'H' metres, Let us calculate the final velocity, on reaching the ground. 'V', squared, is equal to 'U', squared, + 2, 'A', 'S'. 'U', is equal to 0. ‘g’, is equal to, 9.8 m/sec, And 'S', is equal to, 10 m. So, 'V', squared, is equal to 0 + 2, multiplied by 9.8, multiplied by 10 . 'V', squared, is equal to 2, multiplied by 9.8, multiplied by 10. 'V', squared, is equal to 196. 'V', is equal to root, of 196. 'V', is equal to, 14 m/sec. So, when the ball, reaches the ground, it has a final velocity, of 14 m/sec. We know that kinetic Energy, is equal to half, 'M', 'V' squared. Kinetic Energy is equal to half, multiplied by 1, multiplied by 14 squared, Is equal to half, multiplied by 196, Is equal to, 98 Joules.
So, the ball which had a potential Energy, of 98 Joules, when suspended, Has the same Energy of 98 Joules of kinetic Energy, on reaching the ground. Energy has been conserved. Potential Energy of 98 Joules has been, converted to kinetic Energy of 98 Joules.
Taking the case of the child, which was sitting on, top of the slide, It has potential Energy, When it slides down, the potential Energy is converted, to kinetic Energy.
If we take a child, playing in a swing, When it swings up, It gains potential Energy, When it swings down, Potential Energy is transformed, to kinetic Energy. This kinetic Energy helps, to swing up, on the other side, and so on. While playing on the swing, the child is constantly, Converting kinetic Energy, into potential Energy, and vice versa.
In a water tank, The stored water, has potential Energy, When we open the tap, the water flows down, Potential Energy is converted, into kinetic Energy.
All the rivers flow, into the sea, Land is always above sea level. So, the river water, has potential Energy, Which it uses, to flow, into the sea.
When we build, a dam across the river, The stored water, in the dam has potential Energy. When we open the gates, at the bottom of the dam, Water flows out with, kinetic Energy.
When a diver, in the swimming pool, Dives, from the diving board, Potential Energy is converted, into kinetic Energy.
When we come down the bridge, on our bicycle, We don't have to pedal, We can just glide down, By converting Potential Energy, into Kinetic Energy.
We know that Energy, can neither be created, nor destroyed. Energy can exist, in many different forms. It can be transformed, from one form, to another form. Like we just discussed, potential Energy can be converted, into kinetic Energy, And kinetic Energy can be converted, to potential Energy.
Energy can also, exists in many other forms. Some examples are, Elastic potential Energy, Chemical Energy, Electrical Energy, Thermal Energy, Nuclear Fission Energy, Light Energy, Solar Energy, Wind Energy, Food Energy,
All industries require Energy. A farmer requires Energy. Most appliances, in our home, require Energy. We can even say, for economic progress, we require Energy. Mankind spends a lot of effort, in generating Energy, which can be supplied to industries, farmers and households. Electrical Energy is the best way, to deliver Energy, To a wide variety, of users spread, over a large area. Electrical Energy can be generated, in one place, And used in another place, thousands of Kilo metres away. So, generation of electricity has become crucial, in the modern world. The electricity thus generated, can be put to a wide, variety of uses. This of course requires transformation, of one type of Energy, to another. There are many other types of Energy, which are transformed.
We will discuss a few examples.
When we elongate, a rubber band, It has elastic potential Energy. We can use this, to fire a paper bullet. The elastic potential Energy gets converted, into kinetic Energy.
When we elongate a bow, it has elastic potential Energy. This is used, to fire an arrow, It converts the potential Energy, to kinetic Energy. A catapult, uses the same principle.
A spring when compressed, has potential elastic Energy. This principle is used, to make shock absorbers, for automobiles. The shock of the bump, on the road, is absorbed by the spring.
The rubber soles in our shoes, Acts as a mini, shock absorber.
In the battery, chemical Energy is stored, This is converted, to electrical Energy, The electrical Energy, in the torch is converted, to light Energy.
Thermal Power plants widely use, coal as a fuel. The coal is burnt, and thermal Energy is created. The thermal Energy is used, to heat water and convert, it to steam. The steam drives a turbine, and rotates it. This is converted, into electrical Energy. Electrical Energy can be easily, transported through cables, to where ever we want. The capacity, of a thermal Power plant, can vary widely. A medium size plant could, have a capacity, of about 1000 MW. One of the disadvantages, of Thermal Power plant is that, it releases polluting gases, into the atmosphere. Also, we should remember, that coal is a non renewable fossil fuel.
A dam stores water, which has potential Energy, When the water is released, it has kinetic Energy. This is used to rotate, a turbine. The turbine generates electricity. A medium size, hydro electric plant, will have a capacity, of 1000 MW.
When a radioactive material, like Uranium is split, Energy is released, This is used to generate steam, This drives a turbine, which generates electrical Energy. A medium size, nuclear Power plant can have, a capacity of about, 1000 MW.
We receive solar Energy, in the form of light Energy. Solar Energy is the largest source, of natural Energy, on earth. Plants receive solar Energy, and use it to convert, nutrients into food. This process, of using solar light Energy, is called photosynthesis. Energy is stored in food, as food Energy. All living being depend, on plants for food. So indirectly, the sun is the source of Energy, for production of food, for all living beings. We could even say the sun, is the source of Energy, for life itself.
Solar Energy can also be directly used, to convert to other forms, of Energy. For example, a solar water heater, converts solar light Energy, to heat Energy. A solar panel is able to receive, light Energy and convert, to electrical Energy. So, we can produce electricity, directly from sunlight.
You will be pleasantly surprised to know, That more solar Energy falls, on earth in one hour, Then all the man made Energy, the whole of humanity consumes, In one whole year. Many countries, already source a significant portion, of their Energy requirements, from solar Energy. Germany is one such country. India also has an ambitious plan, to use more solar Energy. Energy generated from solar Energy, is clean Energy. It creates no pollutants. Also solar Energy, is a renewal resource. The sun in all its benevolence, blesses us, with solar Energy, day in and day out.
In earlier days, wind mills were used to convert, wind Energy to mechanical Energy. This was used, to mill grains, grind flour, etc., Wind Energy can be converted, to electric Energy. Special wind mills are used for this. Historically, Wind Energy has always been used to propel ships. Till the industrial revolution, all ships were sailing ships, using wind Energy. Even today fishermen use wind Energy to propel their boats. Today, high technology wind mills, can produce more than, 7 MW of Power. Many countries, use wind Energy as a source of Power. Denmark for example, derives 25% of its Energy, from wind Energy. India also uses wind Energy.
Most appliances that we use, convert electrical Energy, Into some other form of Energy, useful to us. We discussed some examples like, Ceiling fan, Computer, Electric iron, Refrigerator, Water pump, Television, Washing machine, Hot plate, etc. Do all these appliances completely, convert the electrical Energy, to other forms of Energy? No, we often hear, of Energy losses, For example, when an electric bulb burns, it creates light Energy, But, some of the Energy, is 'lost'. We discussed, that Energy can neither, be created nor destroyed. Then, how can the bulb lose, Energy? The fact is, when a bulb burns, some heat Energy is created. This is an unwanted Energy. So, we call it, 'lost' Energy. In reality, the bulb is creating light Energy, that we want, And heat Energy, that we do not want. There is a trend to design appliances, where the wasted Energy, is minimised. This is called Energy efficiency. A L E D bulb is much more efficient, than the incandescent bulb. Many common appliances are rated, according to their, Energy efficiency. Higher the efficiency, lesser the wastage. One kind of rating, uses stars. A five star rated appliance, is much more energy efficient, than a 3 star appliance.
We all need Energy, to live. All living beings require Energy, to live. Energy, for living organisms is produced, in the organic cell. The mitochondria, is like the Power plant, of the cell. It is the source of Energy, for the cell. It uses organic compounds, derived from nutrients, to produce A T P. A T P, stands for, adenosine triphosphate. The mitochondria, is the place, where oxygen is combined, with food molecules. In the process of this chemical reaction, the Energy contained in the food, is released. There might be, more than one, mitochondria in a cell. This depends upon, the amount of Energy needs, from the cell. For example, a muscle cell, which expends a lot of Energy, contains thousands of mitochondria.
A human being, just to live, requires 80 Watts of Energy. This Energy is required, for the basic metabolism, to function. When we are engaged in physical activity, more Energy is required. For highly energetic physical activity, we can generate thousands of Watts, of Energy. For example, we can do this, when running a 100 m race. But, we can sustain this activity, only for a few seconds. For a task, lasting a few minutes, we can generate around, 1000 Watts. For example, Running a 3000 m race. To engage in an activity, for an hour, we can generate around, 300 Watts. For example, running in a 10 km race. This explains, why we run slower, in a long distance race, But, able to run much faster, in a short distance sprint. If we have to engage in an activity, for a whole day, we can at most, generate 150 Watts of Energy. For example, if we are going, for an all day trek, 150 Watts, is the kind of Energy, we can spend.
The Energy, that we require to live, comes from the food, we eat. This food is converted to Energy, in the cells of our body. The main types, of food that we eat are, Carbohydrates, Proteins, Fats.
We need about, six to eight mega Joules, of Energy per day. This means, we have to consume food, which will give us, 6 to 8 mega Joules of Energy, every day. Packaged food normally, contains information about the food, in the label. The Energy content, of the food is typically mentioned, in the label. Though Joule, is the scientific, S I unit, an older unit called 'Calorie', is still used to measure, Energy content. A food Calorie is a Kilo cal, or a food Calorie, with capital 'C'. We can remember that, one food capital 'C' calorie, Is equal to, one small 'c' Kilo calorie, Is equal to, 4.2 Kilo Joule.
Let us get an idea, of how much of Energy, is contained in the various types, of food that we eat. Fats contain about, 9 food Calories per gram. Proteins contain about, 4 food Calories per gram. Carbohydrates contain about, 4 food Calories per gram. On the whole, we require 1500 to 2000, food Calories, per day.
In the modern world, machines do, most of the physical work, for us. So, we do not need as much Energy, as a typical working person, would have required, a century ago. It is a healthy habit, to expend physical Energy. The body is built to do work, and expend Energy. Many illness are attributed, to our modern, sedentary life style. So to compensate, for this sedentary life style, we do exercise in a gym. It is useful to remember, that physical activity like running, playing, and dancing, is good, for good physical health. In other words, we must consume, enough Energy, through food, And also, expend enough Energy, through healthy physical activity.