Saturday, October 14, 2017

Basic Electronics on the Go - Ohms Law and Power


 Georg Ohm found that, at a constant temperature, the electrical current flowing through a fixed linear resistance is directly proportional to the voltage applied across it, and also inversely proportional to the resistance. This relationship between the Voltage, Current and Resistance forms the basis of Ohms Law and is shown below.

Any Electrical device or component that obeys “Ohms Law” that is, the current flowing through it is proportional to the voltage across it ( I α V ), such as resistors or cables, are said to be “Ohmic” in nature, and devices that do not, such as transistors or diodes, are said to be “Non-ohmic” devices.

Electrical Power in Circuits

Electrical Power, ( P ) in a circuit is the rate at which energy is absorbed or produced within a circuit. A source of energy such as a voltage will produce or deliver power while the connected load absorbs it. Light bulbs and heaters for example, absorb electrical power and convert it into either heat, or light, or both. The higher their value or rating in watts the more electrical power they are likely to consume.
The quantity symbol for power is P and is the product of voltage multiplied by the current with the unit of measurement being the WattW ). Prefixes are used to denote the various multiples or sub-multiples of a watt, such as: milliwatts (mW = 10-3W) or kilowatts (kW = 103W).
Then by using Ohm’s law and substituting for the values of V, I and R the formula for electrical power can be found as:

To find the Power (P)

[ P = V x I ]      P (watts) = V (volts) x I (amps)
[ P = V2 ÷ R ]      P (watts) = V2 (volts) ÷ R (Ω)
[ P = I2 x R ]      P (watts) = I2 (amps) x R (Ω)

Electrical Power Rating

Electrical components are given a “power rating” in watts that indicates the maximum rate at which the component converts the electrical power into other forms of energy such as heat, light or motion. For example, a 1/4W resistor, a 100W light bulb etc.
Electrical devices convert one form of power into another. So for example, an electrical motor will covert electrical energy into a mechanical force, while an electrical generator converts mechanical force into electrical energy. A light bulb converts electrical energy into both light and heat.

Electrical Energy in Circuits

 Electrical Energy is the capacity to do work, and the unit of work or energy is the jouleJ ). Electrical energy is the product of power multiplied by the length of time it was consumed. Electrical power can be defined as the rate of doing work or the transferring of energy.

The maths involved when dealing with joules, kilojoules or megajoules to express electrical energy, can end up with some big numbers and lots of zero’s, so it is much more easier to express electrical energy consumed in Kilowatt-hours.

Kilowatt-hours are the standard units of energy used by the electricity meter in our homes to calculate the amount of electrical energy we use and therefore how much we pay.

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