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.
The voltages across resistors connected in parallel are the same, but their currents are in the same ratio of the reciprocals of their resistances.
Equivalent R: 1/R = 1/R1 + 1/R2 + 1/R3 + …
The equivalent resistance R is smaller than the smallest among the resistors connected 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 voltages across them are in the same ratio of their resistances.
Equivalent resistance: R = R1 + R2 + R3 + …
The equivalent resistance R is larger than the largest among the resistors connected in series.
The equivalent resistance of n identical resistors of each R is nR.
Measuring Unknown Resistance by Voltmeter-Ammeter 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 result of “voltmeter reading ÷ ammeter reading” is the total resistance of the parallel-combination
of R and the voltmeter. But if R << internal resistance of voltmeter, their parallel-combined-resistance 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 result of “voltmeter reading ÷ ammeter reading” is the total resistance of the series-combination
of R and the ammeter. But if R >> internal resistance of ammeter, their series-combined-resistance is about the same as R.
In other words, Circuit (ii) is good for measuring an unknown R of large resistance.
The author (Chiu-king Ng) has the copyright on all the simulations in this website.
Email phyAA@phy.hk, where AA is the prime number following 7.
