 | 
|
|
Posted by Neil / wb2cir (69.162.17.9) on December 18, 2003 at 15:10:46:
In Reply to: ohm/watt/voltage ralations posted by mike from italy on December 14, 2003 at 18:26:26:
: hello friends
: i have an 200w heating element soldering iron.
: knowing is ohm measuring it with a tester how could i understand its voltage?
: could someone explain me relations between ohms,watts and volts?
: any help will be appreciated
: mike from Italy.
Thank you for doing English. Hello to Italy from Pennsylvania. You are asking the correct questions.
You did not ask about “electric charge,” but the discussion should start with this. Electric charge is determined by the density of electrons on an object. I would use the term “electron density.” Since electrons repel each other and are mobile, they want to spread out so everything has the same electron density.
But since they are mobile, they are easily moved. Did you ever get a static-electricity shock? That was probably caused by movement and friction, which causes electrons to scrape off one object, and be placed on the other object. When you lose or gain electrons, perhaps by rubbing your shoes on the carpet, your electron density becomes different than the rest of your environment. Then when you touch a door knob, for instance, your electron density suddenly re-equalizes with the rest of your environment, as the electrons flow off of you, or on to you, as required.
When the number of electrons equals the number of protons in a metal (or any type of matter), the matter is said to have a “neutral charge.” The universe is filled with electrons, and any free proton will soon be visited by an electron. Even if you do not touch the door knob, the charge on your body will slowly become neutral anyway, because extra electrons will dissipate, or electron deficits will be filled. I do not know if the universe has a total neutral charge, but your body is neutrally charged, usually.
A “battery” is another way of moving electrons. The chemical reaction pulls the electrons away from one terminal and delivers them to the other terminal. This creates a difference in electron density between the terminals.
“Voltage” is the measure of the difference in electron density between two points, and may be measured with a voltmeter. Voltage is always the "difference" in electron density between two points. There is no such thing as the “voltage of a single point.” Voltage must always be specified relative to a 2nd point. The terminals of a 9 volt battery has a greater difference in electron density between its terminals than a 1.5 volt battery. The 120 volt electricity in your house refers to the difference in electron density between the two prongs. When you develop a charge by rubbing your shoes on the rug, you develop a voltage relative to your environment. Voltage is always measured between two points.
The earth is very large. You may remove or deposit some electrons from your environment, or more specifically from the earth, but that will not cause a measurable change in earth’s electron density. In practice, the electron density of the earth does not change. Besides, if the earth’s charge became positive or negative, it would grab or shed electrons into space, as required, to become neutrally charged again.
In electronics, the charge of the earth is called “Ground,” and is the most popular reference for describing voltage. Everything on earth tends towards Ground, or zero volts. Even if a charged item is suspended in air, “leakage” will soon return its voltage to zero volts, relative to ground. If you ever hear someone refer to the voltage of a point, you have to determine if they mean the voltage relative to a different point in the circuit (such as the negative terminal of a battery), or relative to ground.
For instance, a 9 volt battery refers to the difference between the terminals. But if the battery is rubbed on the carpet, it may develop a static voltage of +1000 volts, compared to ground. The battery might be raised to +1000 volts (therefore the positive terminal will be at +1009 volts). You can place the same battery on a table, and in a few seconds, the battery will discharge to ground voltage, zero volts relative to earth, and the positive terminal will be at 9 volts relative to the other terminal AND relative to earth.
I think that’s enough for charge and voltage.
We see that electrons are mobile, and they want to spread out evenly so their density is uniform within all matter, except for the chemical reaction in a battery that will keep one terminal more positive compared to the other. But you can connect a wire to the two terminals, and the electrons will have a path available so they can balance the electron density between the two terminals. That flow of electrons is called Current; the unit of measure is Amps, and current may be measured with an Ammeter.
Instead of a piece of wire, you can use a light bulb, which has some resistance, to slow the flow of electrons. You can use a light bulb or an electronic component called a “resistor.” If you do not let the electrons flow too quickly, the battery will be able to maintain a 9 volt charge for a few hours. If you use a piece of wire, the battery’s voltage will drop immediately.
For wire or other electronic components, the unit of resistance is the Ohm, and may be measured with an Ohmmeter.
This leads to Ohms Law, I=E/R, where I=current, E=voltage, and R=resistance. You can see that if you increase the resistance (R) in the wire, the current (I) will decrease. Or, if you increase the voltage, the current will increase. If you put 12 volts across a 100 ohm resistor, 120 milliamps (mA) will flow through the resistor.
Also, you can measure the voltage across a resistor with a voltmeter, and measure the current flowing through the resistor with an ammeter. Given voltage and current, you can use Ohms law to calculate the resistance of the resistor.
When you measure the voltage across a resistor, the voltmeter is placed across (in parallel with) the resistor, to measure the difference in electron density between both sides of the resistor. Any electrons flowing into the voltmeter will the difference in electron density, so the voltmeter should have an extremely high resistance to minimize that effect.
And when you measure the current flowing through a resistor, the ammeter is placed in line (in series) with the resistor. Any current flowing through the resistor will also be flowing through the ammeter. The ammeter has some resistance that will reduce the amount of current flowing, which will interfere with the measurement. Therefore, an ammeter should have an extremely low resistance to minimize that effect.
The battery is able to maintain 9 volts across its terminals when you put a light bulb across them, because the chemical reaction continues to move electrons. However, when you touch a door knob and discharge your static charge, the current only flows for an instant, because there is nothing to maintain the current. You may have had a 5000 volt static charge on your body, relative to the door knob, but you will not be electrocuted when the static is discharged, because there is almost no current.
One practical complication. The resistance of light bulbs and soldering irons is very small when they’re cold. But when voltage is applied, their resistance increases many times when they get hot. You may use an Ohmmeter to measure their resistance when they’re cold, but any calculation of the expected operational current or power will be inaccurate because the resistance increases greatly during operation.
To find the resistance of a light bulb or soldering iron when it is in operation, you would have to measure the voltage across it and the current through it, and then use a calculator to find its resistance while it’s hot. If your 200 watt soldering iron runs on 120 volts, then you can be sure that it’s resistance is close to 1 2/3 ohms when it’s hot. When the soldering iron is cold, I’m sure its resistance is much less than 1 ohm.
Direct current is easily generated by batteries and solar panels. What about alternating current? Alternating current is easier for large power plants to generate. For instance, you can put a water wheel into a river, and use it to move a piston. At the end of the piston is a permanent magnet, which moves into and out of a coil of wire. When the magnet goes in, it pushes the electrons in the coil in one direction, and when the magnet is pulled out, it pushes the electrons in the other direction. The polarity of the voltage oscillates between negative and positive, back and forth. The electrons don’t get too far, but there is current nonetheless, which may be used to power our light bulbs. When DC is required, for instance in radio circuitry, a power supply may be used to convert the AC to DC.
Best luck,
-Neil-
|