Essential Physics Concepts: Circuits, Waves, and Mechanics
1. Ohm’s Law, Series, and Parallel Circuits
Q: State Ohm’s Law and derive formulas for series and parallel combinations of resistors.
Answer:
Ohm’s Law:
Ohm’s Law states that the current through a conductor is directly proportional to the potential difference across its ends, provided the temperature remains constant.
Formula: V = IR
Series Combination
In a series circuit, resistors are connected end-to-end.
- The same current flows through all resistors.
- Total voltage is the sum of individual voltages: V = V₁ + V₂ + V₃.
Using Ohm’s Law (IR = IR₁ + IR₂ + IR₃), the result is: R = R₁ + R₂ + R₃.
Parallel Combination
In a parallel circuit, resistors are connected side-by-side.
- The same voltage exists across each resistor.
- Total current is the sum of individual currents: I = I₁ + I₂ + I₃.
Using Ohm’s Law (V/R = V/R₁ + V/R₂ + V/R₃), the result is: 1/R = 1/R₁ + 1/R₂ + 1/R₃.
2. Total Internal Reflection and Optical Fibres
Q: Explain total internal reflection and its applications.
Answer:
Total Internal Reflection:
When light travels from a denser medium to a rarer medium and the angle of incidence is greater than the critical angle, the light is completely reflected back into the denser medium.
Conditions for Reflection
- Light must travel from a denser to a rarer medium.
- The angle of incidence must be greater than the critical angle.
Critical Angle
The angle of incidence for which the angle of refraction is 90°.
Optical Fibre
A thin glass or plastic fibre that transmits light via total internal reflection.
Common Uses
- Internet communication
- Medical endoscopy
- Surgery
3. Measuring the Speed of Sound via Echo
Q: Describe the method to measure the speed of sound using an echo.
Answer:
A person stands at a known distance (d) from a wall, claps, and measures the time (t) taken to hear the echo. Since sound travels to the wall and back, the total distance is 2d.
Formula: v = 2d / t
Factors Affecting Speed
- Temperature
- Density
- Humidity
4. DC Motor Construction and Working
Q: Explain the construction and working of a DC motor.
Answer:
Definition: A DC motor converts electrical energy into mechanical energy.
Construction
- Coil (armature)
- Magnet
- Split ring (commutator)
- Carbon brushes
Working Principle
- Current flows through the coil.
- The magnetic field applies a force (F = BIL).
- The coil rotates.
- The split ring reverses the current to maintain continuous rotation.
5. Transverse vs. Longitudinal Waves
Q: Differentiate between transverse and longitudinal waves.
Answer:
- Transverse Waves: Vibrations are perpendicular (⟂) to the direction of travel (e.g., light).
- Longitudinal Waves: Vibrations are parallel (∥) to the direction of travel (e.g., sound).
Comparison Table
| Property | Transverse | Longitudinal |
|---|---|---|
| Motion | Perpendicular | Parallel |
| Example | Light | Sound |
| Parts | Crest/Trough | Compression/Rarefaction |
6. Simple Harmonic Motion and Pendulums
Q: Define SHM and explain it using a simple pendulum.
Answer:
Definition: Simple Harmonic Motion (SHM) is motion where a body moves to and fro about a mean position, and the restoring force is proportional to the displacement and directed toward the mean position.
Simple Pendulum
Consists of a bob attached to a string fixed at a point. When displaced and released, it oscillates about its mean position.
Conditions of SHM
- Motion must be oscillatory.
- Restoring force must act toward the mean position.
- Force must be proportional to displacement.
Formula: T = 2π√(L/g)