WHAT IS WATT HOUR - UNIT OF ENERGY BASIC DEFINITION AND TUTORIALS

There is an important difference between energy and power. You’ve probably heard the two terms used interchangeably, as if they mean the same thing. But they don’t. Energy is power dissipated over a length of time. Power is the rate at which energy is expended.

Physicists measure energy in joules. One joule is the equivalent of one watt of power, dissipated for one second of time. In electricity, you’ll more often encounter the watt hour or the kilowatt hour. As their names imply, a watt hour, abbreviated Wh, is the equivalent of 1 W dissipated for an hour (1 h), and 1 kilowatt hour (kWh) is the equivalent of 1 kW of power dissipated for 1 h.

An energy of 1 Wh can be dissipated in an infinite number of different ways. A 60-watt bulb will burn 60 Wh in an hour, or 1 Wh per minute. A 100-W bulb would burn 1 Wh in 1/100 hour, or 36 seconds. A 6-watt Christmas tree bulb would require 10 minutes (1/6 hour) to burn 1 Wh. And the rate of power dissipation need not be constant; it could be constantly changing.

Figure 2-6 illustrates two hypothetical devices that burn up 1 Wh of energy. Device

A uses its power at a constant rate of 60 watts, so it consumes 1 Wh in a minute. The power consumption rate of device B varies, starting at zero and ending up at quite a lot more than 60 W. How do you know that this second device really burns up 1 Wh of energy?

You determine the area under the graph. This example has been chosen because figuring out this area is rather easy. Remember that the area of a triangle is equal to half the product of the base length and the height. This second device is on for 72 seconds, or 1.2 minute; this is 1.2/60 0.02 hour. Then the area under the “curve” is 1/2 100  0.02 1 Wh.


When calculating energy values, you must always remember the units you’re using. In this case the unit is the watt hour, so you must multiply watts by hours. If you multiply watts by minutes, or watts by seconds, you’ll get the wrong kind of units in your answer. That means a wrong answer!

Sometimes, the curves in graphs like these are complicated. In fact, they usually are. Consider the graph of power consumption in your home, versus time, for a whole day.

But there is another way to determine the total energy burned by your household in a day, or in a week, or most often, in a month. That is by means of the electric meter.

It measures electrical energy in kilowatt hours. Every month, without fail, the power company sends its representative to read that meter. This person takes down the number of kilowatt hours displayed, subtracts the number from the previous month, and a few days later you get a bill.

This meter automatically keeps track of total consumed energy, without anybody having to do sophisticated integral calculus to find the areas under irregular curves

ENERGY TRANSFORMATION EFFECTS BASIC INFORMATION AND TUTORIALS

Most electrical energy is generated by electromagnetic induction. However, electricity can be produced by other means. Batteries use electrochemistry to produce low voltages.

An electrolyte is a solution of chemicals in water such that the chemical separates into positively and negatively charged ions when dissolved. The charged ions react with the conducting electrodes and release energy, as well as give up their charge

A fixed electrode potential is associated with the reaction at each electrode; the difference between the two electrode potentials drives a current around an external circuit. The electrolyte must be sealed into a safe container to make a suitable battery.

‘Dry’ cells use an electrolyte in the form of a gel or thick paste. A primary cell releases electricity as the chemicals react, and the cell is discarded once all the active chemicals have been used up, or the electrodes have become contaminated. A secondary cell uses a reversible chemical reaction, so that it can be recharged to regenerate the active chemicals.

The fuel cell is a primary cell which is constructed so that the active chemicals (fuel) pass through the cell, and the cell can be used for long periods by replenishing the chemicals. Large batteries consist of cells connected in series or parallel to increase the output voltage or current.

Electricity can be generated directly from heat. When two different materials are used in an electrical circuit, a small electrochemical voltage (contact potential) is generated at the junction. In most circuits these contact potentials cancel out around the circuit and no current flows.

However, the junction potential varies with temperature, so that if one junction is at a different temperature from the others, the contact potentials will not cancel out and the net circuit voltage causes current to flow (Seebeck effect). The available voltage is very small, but can be made more useful by connecting many pairs of hot and cold junctions in series.

The thermocouple is used mostly for measurement of temperature by this effect, rather than for the generation of electrical power. The efficiency of energy conversion is greater with semiconductor junctions, but metal junctions have a more consistent coefficient and are preferred for accurate measurements.

The effect can be reversed with suitable materials, so that passing an electric current around the circuit makes one junction hotter and the other colder (Peltier effect). Such miniature heat pumps are used for cooling small components.

Certain crystalline chemicals are made from charged ions of different sizes. When a voltage is applied across the crystal, the charged ions move slightly towards the side of opposite polarity, causing a small distortion of the crystal. Conversely, applying a force so as to distort the crystal moves the charged ions and generates a voltage.

This piezoelectric effect is used to generate high voltages from a small mechanical force, but very little current is available. Ferromagnetic materials also distort slightly in a magnetic field. The magnetostrictive effect produces low frequency vibration (hum) in ac machines and transformers.

Electricity can be produced directly from light. The photovoltaic effect occurs when light falls on suitable materials, releasing electrons from the material and generating electricity. The magnitude of the effect is greater with short wavelength light (blue) than long wavelength light (red), and stops altogether beyond a wavelength threshold.

Light falling on small photovoltaic cells is used for light measurement, communications and for proximity sensors. On a larger scale, semiconductor solar cells are being made with usable efficiency for power generation.

Light is produced from electricity in incandescent filament bulbs, by heating a wire to a sufficiently high temperature that it glows. Fluorescent lights produce an electrical discharge through a low pressure gas. The discharge emits ultraviolet radiation, which causes a fluorescent coating on the inside of the tube to glow.