CURRENT ELECTRICITY — Physics STD 12 Science — Question
Gujarat BoardEnglish MediumSTD 12 SciencePhysicsCURRENT ELECTRICITY2 Marks
Question
What is electric current ? Explain about it.
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Answer
$\rightarrow$To understand electric current, imagine a small area held normal $($Perpendicular$)$ to the direction of flow of charges.
$\rightarrow$Both the positive and the negative charges may flow forward and backward across the area.
$\rightarrow$Let $q_{+}$ be the net amount of positive charge that flows in the forward direction across the area Similarly, let $q_{-}$ be the net amount of negative charge flowing across the area in the forward direction.
$\rightarrow$The net amount of charge flowing across the area in the forward direction in the time interval $t ,$ is
$q=q_{+}-q_{-}$
$\rightarrow$For steady current, electric charge is proportional to time. so,
$\therefore q \propto t$
$\therefore I =\frac{q}{t}$
$\rightarrow$ Electric current : "The amount of electric charge passing perpendicularly through the cross-section of a conductor per unit time is called electric current."
$\rightarrow$An electric current which does not change with time is called steady current.
$\rightarrow$Currents are not always steady. In that case, average value or instantaneous value of current has to be considered.
$\rightarrow$Let $\Delta Q$ be the net charge flowing across $a ^{\prime}$ cross $-$ section of a conductor during the time interval $\Delta t$, then the average current flowing through the conductor is
$< I >=\frac{\Delta Q }{\Delta t}$
$\rightarrow $ The current at time $t$ across the cross $-$ section of the conductor is defined as the value of the ratio of $\Delta Q$ to $\Delta t$ in the limit of $\Delta t$ tending to zero.
So, instantaneous current
$I =\lim _{\Delta t \rightarrow 0} \frac{\Delta Q }{\Delta t}$
$\therefore I =\frac{d Q }{d t}$
$\rightarrow$The unit of electric current is ampere $(A)$ and current is a scalar quantity.
$\rightarrow$An ampere is typically the order of magnitude of currents in domestic appliances.
$\rightarrow$An average lighting carries currents of the order of tens of thousands of amperes $\left(\sim 10^4 A\right. ).$
$\rightarrow$Currents in nerves of our body are in the order of microamperes. $\left(\sim 10^{-6} A\right)$
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