Applications Of Ohm's Law | Work Energy & Power:
Ohm's law is applicable in determining the power dissipation in resistive elements.
Energy:
Energy and work have the same units, that is joules. Energy is defined as the ability to do work. There are many forms of energy, like electrical, mechanical, kinetic thermal and potential. Electricity is a form of energy. Energy can change from one form to another will result in heat, which is also an important form of energy, when you study electronics and electrical engineering. The change of energy from one form to another is called energy transformation. Heat changes the electrical properties of materials. A battery is a source of electrical energy and can do work.
Energy = Power*Time
Work:
Work is defined as an expenditure of energy. The amount of work done is the amount of energy transformed.
Power:
Power is the rate of change of energy or rate of energy transformation or the rate at which work is done. When studying electricity, work is done whenever current flows through the circuit. The greater the flow of current, the more work is done and more power is consumed.
When current flows through a certain element, electrical energy is converted into heat energy or some other form of energy. The rate at which energy is converted depends on the voltage across and current through that element.
Power = Current*Voltage
P = I*V
When current flows through the resistor, it becomes hot. And hence electrical energy is converted into heat energy. If the current flows through the fan, it means electrical energy is converted into mechanical energy and heat energy. And the rate at which energy is transformed into another form is termed Power and measured in watts.
Power Ratings:
As a technology enthusiast or student of electrical and electronics engineering, you must see the power ratings printed on the appliances. The maximum power an appliance can tolerate without being damaged is its power rating.
Now understand in simple terms. If we apply electrical power to a device that is less than its rated power then its efficiency is less than the rated or expected. Or if we apply electrical power to a device that is higher than its rated power then its efficiency would be higher than the rated efficiency and the device may be damaged.
Energy consumption = power ratings * time
E = P*t
An Introduction to Kilowatts Hours:
“The kilowatt-hour (symbolized kW⋅h as per SI) is a composite unit of energy equivalent to one kilowatt (1 kW) of power sustained for one hour. One watt is equal to 1 J/s. One kilowatt-hour is 3.6 megajoules, which is the amount of energy converted if work is done at an average rate of one thousand watts for one hour.”
The higher the power the more energy is converted at a given amount of time. If you run an appliance for a longer period then it consumes more energy (electricity). Let's have a look at different appliances and their power ratings and energy consumption.
Electrical Appliances
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Operating Voltage
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Time
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Power Ratings
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Energy Consumption
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Kilowatt hours (kwh)
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Philips Hair Dryer
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220 - 240 V
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30 min
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1000 - 1200 W
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E = 1000*30*60 = 1,800,000 J
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E = 1kw * 0.5 hours = 0.5 kwh
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Philips Hair Straightener
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220 V
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15 min
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65W
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E = 65*15”60 = 58,500J
|
--
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Black & Decker Iron
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220 V
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10 min
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1500W
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E = 1500*10*60 = 900,000J
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E = 1.5 kw * ⅙ hours = 0.25 kwh
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Look at Philips hairdryer ratings. When the operating voltage is 220 V then the power rating is 1000W. And when operating voltage 240 V then the power rating is 1200 W.
Change In Power:
According to Ohm’s law, if the voltage applied to the circuit changes, the current flowing through the circuit changes in the same proportion (as the voltage is directly proportional to current). In the same manner, if the resistance in a circuit changes, the current drawn by the circuit changes, as long as the voltage across the circuit remains the same. Similarly, if voltage or current through the circuit changes, the power consumption of the circuit also changes.
V = IR Eq 1
P = IV Eq 2
With the help of simple algebra, we can evaluate the following equations.
P = I2R Eq 3
P = V2/R Eq 4
From the above equation, we can conclude that power dissipation in resistors is a non-linear function of current and voltage.
Example: Calculate the power taken by the circuit. V = 20V, I = 0.1A.
Calculate power taken by the circuit if the voltage applied to the circuit doubles.
P = IV
P = 20*0.1
P = 2W
Now applied voltage doubles, then
P = IV
P = 40*0.1
P = 4 W
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