Download advantage academic centre class –xii(notes)

ADVANTAGE ACADEMIC CENTRE
CLASS –XII(NOTES)
Current electricity
1. Electric current and current density :
The time rite of flow of change through any cross section is called current. If ∆q change
passes through a cross-section in time ∆t
∆𝑞
I an = ∆𝑡
𝑙𝑡
Instantaneous current, I= ∆𝑡 → 0

∆𝑞
∆𝑡
𝑑𝑞
= 𝑑𝑡

Current is assumed to be fundamental quantify in physics with S.I unit Ampere and
dimension [A]
The commotional direction of current is taken to be the direction of flew of +ve change.
Through current has a direction, it is not a vector.
Current is also defined as flues of current density I=∫ 𝐽⃗⃗⃗⃗⃗⃗
𝑑𝑠 where J is the current density


which is treated as a vector .
Current through a conductor doer not change with cross section .
Conductor remains unchanged when current flews through it.


Current density J : Current density at a point is defined as a vector heving magnitude
equal to current per unit area surrounding that point that point and normal to the
direction of change flew.
The cross sectional area normal to current is ds cosθ.
𝑑𝑡
J= 𝑑𝐴 𝑐𝑜𝑠𝜃
=>dt=J dscosθ
⃗⃗⃗⃗⃗
=>dt = 𝑗⃗ .𝑑𝐴
𝐼 = ∫ ⃗⃗⃗
𝑗 . ⃗⃗⃗⃗⃗
𝑑𝐴
2. Drift velocity 𝒗𝒅 and its relation with I and E :
The free electrons is a conductor move randomly in a with lattice following a zig-zag
path due to collisions with positive ions. As there are a large number of electrons
crossing an area ∆s from one side very nearly equals the number crossing from the
other side in a given interval of time. The electric current through the area is therefor
zero. [ neam free path = 𝑣𝑑 𝑡
When there is electric field inside the conductor, a fprce acts on each election in the
direction opposite to the field . Th electrons get biased in favour of the force and dirft
slowly election. New there is a net transfer of elections across any cross section
resulting in current.
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