Chapter 1: Electric Current and Circuits
- Electric Current:
- Definition: Flow of electric charges inside a conductor.
- Direction: Conventional current flows from positive to negative pole.
- Charge and Current Intensity:
- Charge measured in coulombs (C).
- One coulomb = charge of approximately 6.25×1018 electrons.
- Current intensity (I) = rate of charge flow (Coulombs per second) = Amperes (A).
- Current in terms of frequency and periodic time for rotating charges.
- Electrical Resistance (R):
- Opposition to current flow in a conductor.
- Measured in ohms (Ω).
- Resistance increases with temperature due to increased vibration of conductor particles.
- Resistance depends on conductor length (L), cross-sectional area (A), and material properties.
- Formula: R = ρ L / A, where ρ is specific resistance (resistivity).
- Electric Potential Difference (Voltage, V):
- Work done to move a unit charge between two points.
- Measured in volts (V), where 1 V = 1 joule/coulomb.
- Voltage across resistors in circuits, including internal resistance of batteries.
- Electrical Energy and Power:
- Electrical energy W = QV (charge × voltage).
- Power P = W / t = VI = I²R = V² / R.
- Power measured in watts (W), 1 W = 1 joule/second.
- Power consumption in devices like lamps and heaters, efficiency considerations.
- Ohm’s Law:
- Series and Parallel Circuits:
- Series: Current is the same; voltages add; total resistance R = R₁ + R₂ + ....
- Parallel: Voltage is the same; currents add; total resistance 1/R = 1/R₁ + 1/R₂ + ....
- Effects on current, voltage, and power in each configuration.
- Advantages of parallel connections for household appliances.
- Kirchhoff’s Laws:
- Junction Rule: Sum of currents entering a junction equals sum leaving.
- Loop Rule: Sum of electromotive forces equals sum of potential drops in a closed loop.
- Application to complex circuits with multiple batteries and resistors.
- Internal Resistance of Batteries:
- Batteries have internal resistance r.
- Terminal voltage V = E - Ir, where E is emf.
- Efficiency and voltage drop considerations.
- Charging and discharging circuits.
- Measurement Instruments:
- Problem-Solving Techniques:
- Use of graphical methods to find charge from current-time graphs.
- Calculations involving resistors in various configurations.
- Use of the dot method for circuit simplification.
- Handling questions about changing wire length, cross-section, and material.