Instructions
Total Questions: 20 | Marks: 4 each | No Negative Marking
Current Electricity – IIT JEE Notes (Set 16)
Electric Current
Definition
Electric current is the rate of flow of electric charge through a conductor.
Formula
I = Q / t
SI Unit
The SI unit of current is Ampere (A).
Direction of Current
Conventional Current
Current is assumed to flow from positive terminal to negative terminal, opposite to electron flow.
Electron Flow
Electrons move from negative terminal to positive terminal.
Ohm’s Law
Statement
At constant temperature, current flowing through a conductor is directly proportional to the potential difference across it.
Formula
V = IR
Graph Insight
For ohmic conductors, V-I graph is a straight line passing through origin.
Resistance
Definition
Resistance is the opposition offered by a conductor to the flow of electric current.
Unit
Ohm (Ω)
Factors Affecting Resistance
Resistance depends on length, area of cross-section, material, and temperature.
Resistivity
Formula
R = ρL / A
Definition
Resistivity is an intrinsic property of material that measures how strongly it opposes current flow.
Unit
Ohm-meter (Ωm)
Conductors and Insulators
Conductors
Materials with low resistance and high conductivity, such as copper and silver.
Insulators
Materials with very high resistance, such as rubber and glass.
Drift Velocity
Definition
Average velocity attained by free electrons under the influence of electric field.
Formula
I = nAeVd
Key Insight
Although electrons move randomly, drift velocity gives net movement in one direction.
Electric Power
Formula
P = VI
Alternative Forms
P = I²R
P = V² / R
Unit
Watt (W)
Electrical Energy
Formula
Electrical Energy = Pt
Commercial Unit
1 kilowatt-hour (kWh) = 3.6 × 10⁶ J
Combination of Resistors
Series Combination
Current remains same through all resistors.
Equivalent Resistance
R = R₁ + R₂ + R₃ + …
Parallel Combination
Voltage remains same across each resistor.
Equivalent Resistance
1/R = 1/R₁ + 1/R₂ + 1/R₃ + …
Kirchhoff’s Laws
Kirchhoff’s Current Law (KCL)
Sum of currents entering a junction equals sum leaving it.
Basis
Conservation of charge.
Kirchhoff’s Voltage Law (KVL)
Algebraic sum of potential differences in a closed loop is zero.
Basis
Conservation of energy.
Electromotive Force (EMF)
Definition
EMF is the energy supplied by a source per unit charge.
Unit
Volt (V)
Internal Resistance
Concept
Every cell has some resistance inside it which opposes current flow.
Terminal Voltage
V = E – Ir
Key Insight
As current increases, terminal voltage decreases.
Cells in Series and Parallel
Series Combination
EMFs add up, increasing total voltage.
Parallel Combination
Used to increase current capacity.
Potentiometer
Principle
Works on the principle that potential drop across a wire is directly proportional to its length.
Applications
Measuring EMF, comparing cells, determining internal resistance.
Ammeter and Voltmeter
Ammeter
Measures current and is connected in series. It has very low resistance.
Voltmeter
Measures voltage and is connected in parallel. It has very high resistance.
Heating Effect of Current
Joule’s Law
Heat produced H = I²Rt
Applications
Electric heaters, irons, and fuses.
Temperature Dependence of Resistance
Metals
Resistance increases with temperature.
Semiconductors
Resistance decreases with temperature.
Conceptual Insights
Key Understanding
Current is due to movement of electrons, but conventional current direction is opposite.
Common Mistakes
Students often confuse EMF with terminal voltage and misuse series-parallel formulas.
Important Exam Concepts
Conceptual Traps
Ammeter always connected in series and voltmeter in parallel. Resistance in parallel is always less than smallest resistance.
JEE Strategy
Practice numerical problems on Kirchhoff’s laws, resistors, and potentiometers. Focus on circuit simplification and conceptual understanding.