Volt Meter
How does putting a volt meter in circuit affect total voltage?
When you attach a voltmeter to measure voltage, it has internal resistance which affects the total voltage, right? Does the voltage go higher than the source voltage or lower? And why?
…and what about current and resistance?
Eric is right that the voltmeter should not change the circuit much, but it can change the voltage if you are measuring across a high-value resistor in series with another one. Most digital meters have about 10 megohms resistance on all ranges. Analog meters have a resistance that increases with the range: a typical value is 10,000 ohms/volt, which would give a resistance of 10 megohms on the 1000 volts scale. If the voltmeter is changing the resistance of the circuit, you will get a lower reading than you should. Of course, the voltmeter is reading correctly, but it is changing the voltage in the circuit.
When you are on the current range, the resistance of the meter is very low, and you should be measuring in series with the load. Most digital meters will have a voltage across the meter of about 100 millivolts. Here again, if you are measuring high currents, the meter’s resistance will be enough to reduce the current in the circuit and give you a lower reading than you would have without the meter.
There is no loading problem when you are measuring resistance, but make sure that power is not applied to the circuit, or your readings will not mean anything, and you may damage the ohmmeter.
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Battery Experiments: Voltaic Pile
If you want to learn about the electrochemical reactions used to create batteries, it is easy to do experiments at home to try out different combinations. To do these experiments accurately, you will want to purchase an inexpensive ($10 to $20) volt-ohm meter at the local electronics or hardware store. Make sure that the meter can read low voltages (in the 1-volt range) and low currents (in the 5- to 10-milliamp range). This way, you will be able to see exactly what your battery is doing.
You can create your own voltaic pile using coins and paper towels. Mix salt with water (as much salt as the water will hold) and soak the paper towel in this brine. Then create a pile by alternating pennies and nickels. See what kind of voltage and current the pile produces. Try a different number of layers and see what effect it has on voltage. Then try alternating pennies and dimes and see what happens. Also try dimes and nickels. Other metals to try include aluminum foil and steel. Each metallic combination should produce a slightly different voltage.
Another simple experiment you can try involves a baby food jar (if you don’t have a baby around the house, just purchase a few jars of baby food at the market and empty them out), a dilute acid, wire and nails. Fill the jar with lemon juice or vinegar (dilute acids) and place a nail and a piece of copper wire in the jar so that they are not touching. Try zinc-coated (galvanized) nails and plain iron nails. Then measure the voltage and current by attaching your volt meter to the two pieces of metal. Replace the lemon juice with salt water, and try different coins and metals as well to see the effect on voltage and current.
Probably the simplest battery you can create is called a zinc/carbon battery. By understanding the chemical reaction going on inside this battery, you can understand how batteries work in general.
Imagine that you have a jar of sulfuric acid (H2SO4). Stick a zinc rod in it, and the acid will immediately start to eat away at the zinc. You will see hydrogen gas bubbles forming on the zinc, and the rod and acid will start to heat up. Here’s what is happening:
- The acid molecules break up into three ions: two H+ (hydrogen) ions and one SO4– (sulfate) ion.
- The zinc atoms on the surface of the zinc rod lose two electrons (2e-) to become Zn++ ions.
- The Zn++ ions combine with the SO4– ion to create ZnSO4 (zinc sulfate), which dissolves in the acid.
- The electrons from the zinc atoms combine with the hydrogen ions in the acid to create H2 molecules (hydrogen gas). We see the hydrogen gas as bubbles forming on the zinc rod.
If you now stick a carbon rod in the acid, the acid does nothing to it. But if you connect a wire between the zinc rod and the carbon rod, two things change:
- The electrons flow through the wire and combine with hydrogen on the carbon rod, so hydrogen gas begins bubbling off the carbon rod.
- There is less heat. You can power a light bulb or similar load using the electrons flowing through the wire, and you can measure a voltage and current in the wire. Some of the heat energy is turned into electron motion.
The electrons go to the trouble to move to the carbon rod because they find it easier to combine with hydrogen there. There is a characteristic voltage in the cell of 0.76 volts. Eventually, the zinc rod dissolves completely or the hydrogen ions in the acid get used up and the battery “dies.”
In the next section, we’ll look at exactly what is happening with the battery’s chemistry.
More:http://www.b2c-battery.com.au/
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