Current, Resistance, and Ohm’s Law

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A CURRENT, $I$, of electricity exist in a region when a net electric charge is transported from one point to another in that region. Suppose the charge is moving through a wire. If a charge $q$ is transported through a given cross section of the wire in a time $t$, then the current through the wire is :

$$I\,(arus)=\frac{q}{t}=\frac{charge\,that\,transported}{time\,in\,this\,transportion}$$

Here, $q$ is in $coulomb$, $C$ , $t$ is in seconds, $s$ and $I$ is in $ampere$, $A$.

$$1\,C=1\,\frac{C}{s}$$

It is mean that the current have value 1 $ampere$ if in 1 $second$ flow charge 1 $coulomb$.

Then $1\,elektron=1,6x10^{-19}\,coulomb$, so 1 $coulomb$ = $\frac{1}{1,6x10^{-19}}=6,25x10^{18}\,electrons$.

By custom the direction of the current is taken to be in the direction of flow of positive charge, thus, a flow of electrons to the right corresponds to a current to the left.

A BATERY is a source of electrical energy. If no internal energy losses occur in the battery, then the potential difference between its terminals is called the $electromotive\,force$ (emf) of the battery. Unless, otherwise stated, it will be assumed that the terminal potential difference of a battery is equal to its emf. The unit for emf is the same as the unit for potential difference, the $volt$, V.

THE RESISTANCE (R) of a wire or other object is a measure of the potential difference $V$ that must be impressed across the object to cause a current of one ampere to flow throuugh it:

$$Resistance=\frac{potential\,\,difference}{current}\,\,R=\frac{V}{I}$$

The unit of resistance is the $ohm$, for which the symbol $\Omega$ (Greek omega). 1 $\Omega$ = 1 $V/A$.

OHM’S LAW originally contained two parts. Its first part was simply the defining equation for resistance, $V$ = $I$ $R$. We often refer to this equation as being Ohm’s Law. However, Ohm also stated that $R$ is a constant independent of $V$ and $I$. This latter part of the Law is only approximately correct.

The relation $V$ = $I$ $R$ can be applied to any resistor, where $V$ is the potential difference (p.d.) the two ends of the resistor, $I$ is the current through the resistor, and $R$ is the resistance of the resistor under those conditions.

THE TERMINAL POTENTIAL DIFFERENCE ($or\,Voltage$) of a battery or generator when it delivers a current $I$ is related to its electromotive force( $emf$ or $\epsilon$) and its internal resistance, $r$.

1.      When delivering current (on discharge):

Terminal voltage = (emf) – (voltage drop in internal resistance r ) ,$V$= $\epsilon\,-\,I\,r$

2.      When recieving current (on charge):

Terminal voltage = (emf) + (voltage drop in internal resistance r ) ,$V$= $\epsilon\,+\,I\,r$

3.      When no current exists:

Terminal voltage = (emf of battery or generator),$V$= $\epsilon$

Potensial Jepit = ggl,$V$ = $\epsilon$

RESISTIVITY: The resistance  $R$ of awire of length $L$ and cross-sectional area $A$ is: $$R=\rho\frac{L}{A}$$

where $\rho$ is a constant called the $resistivity$. The resistivity is a characteristic of the material from which the wire is made. For  $L$ in meter, $A$ in $m^{2}$ and $R$ in $\Omega$, so the units of $\rho$ is $\Omega\,m$.

RESISTANCE VARIES TEMPERATURE: If a wire has a resistance  $R_{0}$ at temperature $T_{0}$, then its resistance $R$ at a temperature $T$ is $$R=R_{0}+\alpha R_{0}(T-T_{0})$$

where $\alpha$ is the $temperature\,\,coefficient\,\,of\,\,resistance$ of the material of the wire. Usually $\alpha$ varies with temperature and so this relation is applicable only over a small temperature range. The units of $\alpha$ are $K^{-1}$ or $^{\circ}C^{-1}$.

A similar relation applies to the variation of resistivity with temperature. If $\rho_{0}$ and $\rho$ are the resistivities at $T_{0}$ and $T$, respectively, then $$\rho=\rho_{0}+\alpha \rho_{0}(T-T_{0})$$

Referensi [4]

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