




#128584  06/14/03 12:41 PM
Re: importance

Member
Registered: 06/08/03
Posts: 657
Loc: Dobbs Ferry, NY, USA

electricman03
It is only important if you want to be good at whatever it is you do in the electrical industry. Understanding the relationship of Voltage, Current, and resistance will make you better at understanding some of the code rules, why certain connections are made the way they are, being a better 'trouble shooter', ETC... and then when you learn this, you can pass it along to another. There are some very good mechanics out there, and then there are the exceptional ones who can not only build it, but know why!!
Good Luck and always keep learning,
Pierre
P.S. I read these forums, because there are some very talented people here who are more than willing to help.
_________________________
Pierre Belarge

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#128586  06/15/03 05:18 AM
Re: importance

Member
Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England

Hi Scott,
What you have are four quantities, all interrelated:
Voltage. Symbol E, measured in Volts. This is the electrical "pressure" which acts upon the electrons. You can liken it to the applied pressure in a water pipe.
Current. Symbol I, measured in Amperes (amps). This is the rate, or speed, at which electrons are flowing in the circuit.
Resistance. Symbol R, measured in Ohms. This is the degree of resistance offered by the mnaterial through which the current is flowing.
Power. Symbol P, measured in Watts. The is the actual amount of energy which is being dissipated, or used up, in the circuit. It may be energy which is radiated as heat or light, or used to create motion, as with an electric motor.
Now.....
Ohm's Law states the relationship between voltage, current, and resistance. Current is directly proportional to voltage and inversely proportional to resistance, or in mathematical terms:
I = E / R (Current = Voltage / Resistance)
Think back to waterinapipe analogy. If you increase the pressure behind the water, the rate of flow will increase. This happens in an electrical circuit too: If you increase the voltage (pressure), then the current will increase proportionally.
If you kept your water pressure constant but changed the pipe for a smaller one, the flow rate would be reduced. Similarly, in an electrical circuit if you kept the voltage the same but increased the resistance, the current would be reduced.
See where we're going with this? Let's take an example, and say you had a 12V battery connected to some device which had a resistance of 6 ohms. By Ohm's Law, the current would be:
I = E / R = 12 / 6 = 2 amps.
If you substituted a device which had a resistance of, say, 3 ohms, then the current would increase:
I = E / R = 12 / 3 = 4 amps.
With that lower reistance, the only way to get the current back down to 2 amps would be to reduce the voltage to 6V, thus:
I = E / R = 6 / 3 = 2 amps.
See how the quantities are related? You can't change one without affecting one of the others.
Now on to power. The power is proportional to both the current and the voltage:
P = I x E
Clearly then, anything which changes either voltage or current will affect the power. In the above example of a 12V battery feeding into a resistance of 6 ohms, we've seen that the current is 2 amps. The power, therefore, is:
P = I x E = 2 x 12 = 24 watts.
Now, to get to your point about how you can have high power with a low current, let's take a practical powerline example. Suppose you had an electric heater in your home running on 120V and rated at 1200W. The current would be 10 amps, because:
P = I x E = 10 x 120 = 1200W.
If you had a 1200W heater designed to run off 240V, you would get the same amount of heat (power) from it, but the current would be only half:
P = I x E = 5 x 240 = 1200W.
See how we've doubled the voltage, but halved the current, so the power stays the same?
The reason that the 240V heater only draw half as much current as the 120V version is that its resistance is much higher.
Rearranging the Ohm's Law formula, you can work out the resistance of each:
R = E / I = 120 / 10 = 12 ohms
R = E / I = 240 / 5 = 48 ohms
That fourfold difference in resistance is why the higher voltage in the second example results in a lower current, yet in both cases the power is the same.

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#128587  09/06/03 06:33 PM
Re: importance

Member
Registered: 07/31/03
Posts: 36
Loc: Cleveland, Ohio USA

[quote]i have read wattage is electrical power, but then i read its the amount of electrical energy converted to anther form?[/unquote]
That is 100 % true. Power is by definition the rate at which energy is changed from one form to another. Power, all types, not just electrical, is measured in units of watts (W). The watt (W) is defined as one joule per second (J/s). That means that when we speak of something, such as a light bulb as having a power of 100 W, we mean that the device is capable of converting energy, in the form of electrical voltage and current into another form of energy, such as light and heat at a RATE of 100 J/s.
It is the units of measure we choose that can help us understand best the properties of nature. SI units are best when needing to understand the coherancy of nature. But, some people choose or are forced to use, other units, that are not coherant with the laws of nature and tend to obscure ones understanding of how nature works.
Study the SI units, and what they mean, and you will see how nicely everything fits together.

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