Current Electricity – Stage 0 (Foundations)
Page 3: Electric Current, Drift Velocity & Microscopic View
1️⃣ What is Electric Current?
Electric Current is defined as the rate of flow of electric charge through any cross-section of a conductor.
Mathematical Definition:
I = Q / t
- I = Electric current (Ampere)
- Q = Charge flowing (Coulomb)
- t = Time (second)
SI unit: Ampere (A)
1 Ampere = 1 Coulomb of charge flowing per second.
2️⃣ Direction of Electric Current
By convention, electric current is assumed to flow from:
- Positive terminal → Negative terminal
⚠️ Important Concept:
Actual motion of charge carriers (electrons) is in the opposite direction
(from negative to positive).
3️⃣ Microscopic Picture of Current in Conductors
In metallic conductors:
- Free electrons move randomly due to thermal motion
- Random motion gives zero net current
- Applying an electric field produces a small net drift
This slow average motion of electrons is called Drift Velocity.
4️⃣ Drift Velocity (vd)
Drift velocity is the average velocity with which free electrons move under the influence of an electric field.
vd ∝ Electric Field
Even though electrons move very slowly, electric current appears almost instantaneously because the electric field propagates rapidly through the conductor.
5️⃣ Relation Between Current and Drift Velocity
Consider a conductor with:
- n = number of free electrons per unit volume
- q = charge of one electron
- A = cross-sectional area
- vd = drift velocity
Fundamental Relation:
I = nqA vd
This equation is extremely important for JEE conceptual questions.
6️⃣ Key Observations for JEE
- Higher current does not mean faster electrons
- Drift velocity is typically very small (~10⁻⁴ m/s)
- Current depends on number density and area
- Microscopic formulas connect Physics + Mathematics
7️⃣ Common Mistakes to Avoid
- Confusing direction of current with electron flow
- Assuming electrons move at speed of light
- Ignoring cross-sectional area in current calculation
8️⃣ JEE Foundation Takeaway
Think like IIT:
Current is not just Q/t — it is a collective microscopic effect
of billions of drifting electrons.
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