Current Electricity (Unit 07) - Study Guide
Exam secrets from Ohm's Law to precise measurements in the Potentiometer.
Resource Book Core Concepts
- • Ohm's Law (V = IR): Under constant physical conditions, the current flowing through a conductor is directly proportional to the potential difference across its terminals.
- • Kirchhoff's 2nd Law: The algebraic sum of potential differences around any closed circuit loop is zero (ΣV = 0). This is the law of conservation of energy.
- • Potentiometer: The most accurate method to measure potential difference. Since it draws no current at balance, the true EMF of a cell can be measured.
Paper Marking Secrets (Exam Traps)
📘 Resource Book Summary: Current Electricity
Key Definitions:
- Electric Current (I): The rate of flow of charge through a cross-section of a conductor.
- Drift Velocity (v): The average velocity with which free electrons drift inside a conductor due to the flow of current.
Formulas and Equations:
- I = Q / t
- I = nAev (n = number of free electrons per unit volume, A = area, e = electron charge, v = drift velocity)
- Ohm's Law: V = IR
Key Points:
- Ohm's Law is valid only as long as the temperature and other physical conditions of the conductor remain constant.
⚠️ Special Notes (Exam Notes/Traps):
According to the equation I = nAev, when the cross-sectional area (A) of a wire decreases, the drift velocity (v) increases.
Key Definitions:
- Resistivity (ρ): The resistance of a portion of a material of unit length and unit cross-sectional area.
Formulas and Equations:
- R = ρ * l / A
- Resistors in series: R = R₁ + R₂ + R₃
- Resistors in parallel: 1/R = 1/R₁ + 1/R₂ + 1/R₃
Key Points:
- The resistance of metals increases with temperature [R_θ = R_0(1 + αθ)]. The resistance of semiconductors decreases as temperature increases.
⚠️ Special Notes (Exam Notes/Traps):
If a metal wire is stretched to double its original length (2l), its cross-sectional area becomes half (A/2) since volume is conserved. Thus, its resistance increases by 4 times.
Key Definitions:
- Electromotive Force (EMF - E): The energy supplied by a cell per unit positive charge passing through it.
Formulas and Equations:
- Potential difference: V = E - Ir (during discharging)
- Kirchhoff's First Law: ΣI = 0
- Kirchhoff's Second Law: ΣE = ΣIR
Key Points:
- Kirchhoff's first law represents conservation of charge, while the second law represents conservation of energy.
⚠️ Special Notes (Exam Notes/Traps):
When applying Kirchhoff's second law to a closed loop, the directions (+) or (-) of cells and currents must be substituted consistently along a chosen direction (clockwise or counter-clockwise).
Key Definitions:
- Potentiometer: An instrument used to measure potential difference or EMF very accurately without drawing any current from the circuit (null-deflection method).
Formulas and Equations:
- Meter Bridge (Wheatstone Bridge): P / Q = l₁ / l₂
- Potentiometer: V ∝ l
Key Points:
- By obtaining the balance point near the middle of the wire, the percentage error in the reading can be minimized.
⚠️ Special Notes (Exam Notes/Traps):
For a potentiometer experiment to succeed, the EMF of the cell in the primary circuit must be greater than the EMF of the cell being measured (E > Ex). Otherwise, no balance point can be found on the wire.
⚡ Ohm's Law Simulator
Change voltage and resistance to observe their effect on the current in real time.
📏 Potentiometer Experiment (Null Point)
Slide the jockey along the wire to find the position where the galvanometer reading becomes zero (balance length).
Galvanometer Needle