Observational experiment: Determine a relationship between current and voltage D
ID: 1422751 • Letter: O
Question
Observational experiment: Determine a relationship between current and voltage Design an experiment to determine a mathematical relationship between the current through a resistor and the voltage across the resistor. Available equipment: Voltage source (such as a variable power supply), resistor, light bulb, voltmeter and ammeter, and connecting wires. Remember, to measure the current through the resistor, you need to let this current pass through the ammeter. Thus the resistor and the ammeter need to be connected like this: To measure the voltage (potential difference) across the resistor, you need to connect the voltmeter so it measures the potential before and after the resistor: If you ACCIDENTALLY connect the ammeter like a voltmeter, you will burn the fuse, so please be careful with connections.Explanation / Answer
This experiment is designed to investigate the relationship between current and potential in simple resistor circuits . In addition the Ohm’s Law, the proportionality between voltage and current, is true for many things that conduct current but not for everything. Conductors, which do have resistance, always yield the same V/ I ratio no matter what voltage you apply to it. Then it is possible to say that V I = R is the resistance, because the ratio is always the same. In this experiment we will also investigate the applicability of Ohm’s law to circuit elements.
To measure the electrical potential difference across an element in a circuit, a voltmeter is put across its terminals. In other words, the voltmeter is connected in parallel with the circuit element. The internal resistance of the voltmeter is huge compared to the resistance of the circuit element being measured and since elements in parallel have the same potential differences a small amount of current goes to the meter, enough for the meter to make a measurement. The remaining (larger amount) of current passes through the circuit element. To measure the current flowing through a circuit element an ammeter must be inserted in series with the element where the current must flow through the ammeter in order to be measured. The ammeter has a negligible resistance and thus does not affect the total circuit resistance. However, one should be careful to never insert the ammeter in parallel around the circuit element. In parallel the ammeter will have essentially the entire current pass through it blowing the fuse in the meter. Again, since elements in parallel have the same potential differences, the lower resistance element will get the greater share of the current. In this case it will be the ammeter. On a similar note, one should take care not to wire the voltmeter in series with the circuit element, not because you will hurt the meter, but because the resistance of the meter is so large in voltmeter setting that a negligible current will be drawn from the power supply.
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