


Simple Electric Circuit

 Click the switch to close or open it
 Select an appropriate electric component (resistor, wire or meter) from
the lower portion, click and drag it to a vacant site in the circuit.
The component will fill into the vacant site when the mouse button is released.
 To remove an inserted component in the circuit, just drag it out and release.
 The resistance of the rheostat can be varied over the range 0 Ω  100 Ω.
 Ideal ammeters and voltemters have zero and infinite internal resistances respectively..
However, this assumption can be lifted, by unchecking the box "Ideal meters", then
their internal resistances become 20 Ω and 10 kΩ respectively.
 The resistance of the red resistor R is not shown (unless "Show Value of R" is pressed); it could
be used, e.g., in a circuit of finding an unknown resistor.
 Press "Change R" to reassign a value between 10 Ω and 10 kΩ to R randomly.
 Press " Show Value of R" to show the value of R. Press the label again to
hide the value.







Resistors in Parallel


 In parallel, the voltage across each resistor is the same while the current passing through each resistor is inversely proportional to its resistance.
 Equivalent R: 1/R = 1/R_{1} + 1/R_{2} + 1/R_{3} + …
 The equivalent resistance R is smaller than the smallest among those in parallel.
 The equivalent resistance of n identical resistors of each R is R/n.

Resistors in Series


 In series, the current passing through each resistor is the same while the voltage across each resistor is proportional to its resistance.
 Equivalent resistance: R = R_{1} + R_{2} + R_{3} + …
 The equivalent resistance R is larger than the largest among those in series.
 The equivalent resistance of n identical resistors of each R is nR.

Measuring Unknown Resistance by VoltmeterAmmeter Method


 R is measured by the formula R = voltmeter reading / ammeter reading ……(*)
 Using (*), the above two circuits give two different values of R unless both the voltmeter and ammeter are ideal,
i.e., the former has infinite internal resistance and the latter has zero internal resistance.
 In Circuit (i), the ratio “voltmeter reading / ammeter reading” is the total resistance of the parallelcombination
of R and the voltmeter. But if R << internal resistance of voltmeter, their parallelcombinedresistance is about the same as R.
In other words, Circuit (i) is good for measuring an unknown R of small resistance.
 In Circuit (ii), the ratio “voltmeter reading / ammeter reading” is the total resistance of the seriescombination
of R and the ammeter. But if R >> internal resistance of ammeter, their seriescombinedresistance is about the same as R.
In other words, Circuit (ii) is good for measuring an unknown R of large resistance.





