potentiometer

電位器(Potentiometer、Variable resistance)，又稱為可變電阻，可以透過銅箔與印刷膜接觸旋轉或滑動產生於輸出、輸入端的不同電阻，而調整光源或聲源，所以又有稱為調光器。

　　電位器，顧名思義，就是可以調整電阻的大小。電路接在該電阻的中間時，電阻只有原來的一半，接到最邊緣時，則是該電阻的原來大小。看需要來選擇接的地方，就是可變電阻。電位器<可變電阻>為電阻值可以調整改變的電阻。在類比電路中，為符合所謂設計值規格的調整作業非常麻煩。但為考慮精確度，必須對各定數的偏差作局部限制，而在這調整作業中就必須用到可變電阻。 小型電位器又稱為半固定電阻器，為隨著年代而漸漸小型化的一種可變電阻。

　　電位器<可變電阻>在使用面的部份，例如1.5V的馬達要將轉速降低需加裝可變電阻，但可變電阻上有三向接點標示1，2，3，那電池電源正負極要如何接上可變電阻呢？電阻沒有極性因此不用擔心接反,可變電阻上之1,3之間之電阻是工廠製造時之該顆電阻之最大值(不會改變),可變部分在1,2或2,3,因此只要將電源正負任一端串聯馬達之一端再從馬達另一端接到可變電阻2端,再從1端或3端拉到電源另一端就可以了,如果馬達逆轉再將電源正負反接就可以轉正了,也可以調整可變電阻來改變馬達之轉速。


可變電阻在電阻調整大小時,電壓及電流變化說明：

　　第一 : 再串聯電路裡可變電阻其電阻調整越大時(有串聯其他電阻 , 而且是一個迴路)是可變電阻上的電壓降越大(指電壓會越大的意思)但是電流不變

　　第二 : 再並聯電路裡可變電阻其電阻調整越大時是可變電阻上的電流越小但是電壓不變

以上可以用歐姆定律來證明

　　第一 :串聯電路同一條路線上是電流不變如果把上一題代入就是V=IR , I是電流不變但R可變電阻調整越大則V電壓越降大

　　第二 :並聯電路剛好相反也就是說再分枝電路是電壓不變同樣代入第二題目I=V/R則V是電壓不變但R可變電阻調整越大則電流越小

What is the input resistance of a potentiometer?

This depends on the material you use as the potentiometer wire. For a 1-metre wire, a resistance of about 1~2 k-ohms will be sufficient.

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How is the terminal voltage of a bettery affected by its internal resistance?

The larger the internal resistance of a battery, the lower will the terminal voltage be.

This can be described by the equation:
V = emf - I.r ------------------- (1)
where V is the terminal voltage
emf is the electromotive force of the battery
I is the current giving out by the battery
r is the internal resistance of the battery

It can be seen that, for a given current, a higher value of r gives a lower value for V.

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Why might the terminal voltage of a battery depend on whether a voltmeter
or a potentiometer is used to measure the voltage?

A voltmeter can only be used to measure the terminal voltage of a battery because the voltmeter has to draw in some current, no matter how small it is.
Thus, according to equation (1), there is a drop of voltage across the internal resistance r. The voltage across the voltmeter (which is what the voltmeter actually measures) is V(= emf - I.r)

On the other hand, when the balance point is obtained on the potnetiometer wire, there is no current giving out by the battery (as during balance, the voltage on the potentiometer wire exactly opposing the emf of the battery).

From equation (1), when I=0, then V = emf. The voltage recorded on the potneiometer wire therefore gives the emf of the battery