# current and resistance

1.是不是一定要有electric current才會有resistance? Why?

2. There are three series circuits and each of them is connected with an ammeter.

Close all circuits. The readings of the ammeter are 0A, 0.5A and 1.5A respectively.

Which of the circuit has a lowest value of resistance? Why?

Thx!!!

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1. Of course not. "Resistance" is a physical property of a substance, just like mass or volume. The resistance of a substance always exists irrespective there is current passing through it or not.

2. Provided that voltages applied to the three circuits are the same, the one with lowest resistance will draw the largest current from the source. This follows Ohm's Law. Hence, the circuit with 1.5A current has the lowest resistance.

Mathematically, if V is the applied voltage to the circuit, then the current I flowing through it is given by,

I = V/R (Ohm's Law)

or R = V/I

For the three circuits, the respective resistances R1, R2 and R3 are thus,

R1 = V/0 (this indicates infinite resistance, or open circuit)

R2 = V/0.5 = 2V

R3 = V/1.5 = 2V/3

Clearly, the value of R3 is the lowest.

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