please be descriptive in these answers 1. a) Before turning on the voltage to a

Question

please be descriptive in these answers
1. a) Before turning on the voltage to a circuit containing an ammeter or voltmeter with a range selection switch, what range should be selected and why?
b) Why is an ammeter always connected in series in a circuit?
c) Why is a voltmeter always connected in parallel in a circuit?
d) Is the scale of an ohmmeter linear? Why or why not?
e) At which end of the ohmmeter scale can you read resistance more precisely?
f) An ohmmeter is never used to measure resistance in a live or powered circuit. Why?
g) An ohmmeter should never be stored with its probes connected together. Why?
h) Why is a given ammeter likely to provide a more accurate current measurement in a high resistance circuit than a low resistance circuit?
i) Why is a voltmeter with a large internal resistance more desirable than one with a low internal resistance?

j) Briefly summarize how a golvanometer is combined with resistors to become a multimeter.

Explanation / Answer

1. a) Before turning on the voltage to a circuit containing an ammeter or voltmeter with a range selection switch, what range should be selected and why? The largest range should be selected, so that the current does not damage the ammeter/voltmeter. b) Why is an ammeter always connected in series in a circuit? It is in series because current flows in parallel branches of a circuit. The ammeter has to be a series element in order for all the current to go through it. c) Why is a voltmeter always connected in parallel in a circuit? Voltmeters measure the voltage difference between two points. If it is connected in series, then it would be measuring the potential difference between the same 'node', thus it would read nothing. It needs to 'bridge' an element(s) that cause a potential difference in order to read any values. It needs to compare two different points (nodes). d) Is the scale of an ohmmeter linear? Why or why not? If it is a passive ohmeter, then the ohms scale is non-linear. This is because the resistance is measured by applying a fixed voltage to the resistor and measuring the current. Since I = V/R, the current/resistance relation is non-linear. Electronic ohmeters can have linear scales since they use a current source to send a small current through the resistor and measure the voltage. Sine V = I*R, the scale is linear. e) At which end of the ohmmeter scale can you read resistance more precisely? Let us suppose that on a X1 scale we usually have around 10 or 15 Ohms in the center of the scale. As we can see it is logarithmic-the entire scale is compressed all the way to infinity.Usually,the closer the reading is to zero, the more elongated the divisions are,therefore it is easier to see must be more precise. Try finding test resistors at 1,10,100,1k,10k etc.at the highest tolerance and compare the results with a digital meter that is working good. Batteries in all meters must be fresh for best results. f) An ohmmeter is never used to measure resistance in a live or powered circuit. Why? You first need to know the principle on which an ohmmeter works. It makes a random value of current to flow through the resistor, and measures the voltage drop across it. Now imagine, what would happen to the voltage drop when both the already flowing current in the circuit and your ohmmeter's own current superimposed... This will severely throw off the readings. g) An ohmmeter should never be stored with its probes connected together. Why? When you put together both the probes, the effective resistance is zero, and it short circuits your ohmmeter. It tries to measure that (theoratically) zero resistance, a (theoratically) infinite current flows through your ohmmeter, and burns it/damages it. h) Why is a given ammeter likely to provide a more accurate current measurement in a high resistance circuit than a low resistance circuit? With very low resistance circuit, the internal resistance of the ammeter has a significant impact on measurement accuracy. For larger resistance circuits, there will still be a small amount of measurement error due to the "impact" of the meter, but not enough to cause serious disagreement with calculated values. i) Why is a voltmeter with a large internal resistance more desirable than one with a low internal resistance? Voltmeters are not supposed to disturb the circuit. Since voltmeters are connected in parallel, if they had a low resistance, a lot of the current that should have been flowing through the circuit element(s) will instead flow through the voltmeter. Therefore they should have a HIGH internal resistance, this way they 'discourage' the current from flowing through it, and aim to disrupt the circuit as little as possible. j) Briefly summarize how a golvanometer is combined with resistors to become a multimeter. Analog versions of ammeters and voltmeters typically utilize a galvanometer, which relies on magnetic effect. A needle is deflected by an amount proportional to the current passes through the device (coil). A good ammeter should have a resistance that is small compared to other resistance in the circuit. A good voltmeter should have a large resistance. If a galvanometer was connected in serial with a 198kohm resistor, it would become a 10V voltage meter. Because total resistance becomes 200k, it need 10V between the two ends to produce a 50 uA current. This current flow will be proportional to voltage across the device. If a smaller resistor were connected in parallel to galvanometer, it could be used to detect more current than 50uA.

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