Iist all scalar quantities of electricity chapter of class 10ncert
Answers
Answer: Imagine a small area held normal to the direction of flow of charges. Both
the positive and the negative charges may flow forward and backward
across the area. In a given time interval t, let q+
be the net amount (i.e.,
forward minus backward) 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. The net amount
of charge flowing across the area in the forward direction in the time
interval t, then, is q = q+
– q–
. This is proportional to t for steady current
Chapter Three
CURRENT
ELECTRICITY
2015-16(20/01/2015)
Physics
94
and the quotient
q
I
t
= (3.1)
is defined to be the current across the area in the forward direction. (If it
turn out to be a negative number, it implies a current in the backward
direction.)
Currents are not always steady and hence more generally, we define
the current as follows. Let ∆Q be the net charge flowing across a crosssection of a conductor during the time interval ∆t [i.e., between times t
and (t + ∆t)]. Then, the current at time t across the cross-section of the
conductor is defined as the value of the ratio of ∆Q to ∆t in the limit of ∆t
tending to zero,
( )
0
limt
Q
I t
t
∆ →
∆
≡
∆
(3.2)
In SI units, the unit of current is ampere. An ampere is defined
through magnetic effects of currents that we will study in the following
chapter. An ampere is typically the order of magnitude of currents in
domestic appliances. An average lightning carries currents of the order
of tens of thousands of amperes and at the other extreme, currents in
our nerves are in microamperes.