How Things Work
Electricity, Magnetism, Light and Sound
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Overview
"How Things Work" covers the fundamental physical phenomena that explain everyday devices and natural events—electric circuits, magnetic effects, behaviour of light, and properties of sound. For UTET Paper II, this topic bridges Class VI–VIII NCERT Science and tests both conceptual clarity and the ability to connect theory with classroom demonstrations.
Expect questions on circuit components, Ohm's law basics, magnetic field patterns, reflection/refraction, and characteristics of sound. The pedagogy angle often asks how to use simple activities (making an electromagnet, pin-hole camera, string telephone) to build scientific understanding. Mastering this unit also helps with the "Natural Phenomena" and "Moving Things" sections, so invest time here.
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Key Concepts
- **Electric current** is the flow of electric charge (electrons in metals); measured in amperes (A). A complete, closed path is essential for current to flow—this path is called a **circuit**.
- **Conductors** (metals, graphite, salt solution) allow current; **insulators** (rubber, plastic, wood) do not. This distinction underpins electrical safety.
- **Ohm's law** (V = I × R) relates voltage (V, in volts), current (I, in amperes) and resistance (R, in ohms). Higher resistance means less current for the same voltage.
- A **magnet** has two poles (north and south); like poles repel, unlike poles attract. The region around a magnet where its influence is felt is the **magnetic field**, visualised by iron-filing patterns.
- An **electromagnet** is a temporary magnet made by passing current through a coil wound on an iron core; its strength depends on number of turns and current.
- **Light travels in straight lines** (rectilinear propagation). It can be **reflected** (bouncing off surfaces) or **refracted** (bending when entering another medium).
- **Sound is a mechanical wave** requiring a medium; it cannot travel through vacuum. It is produced by vibration and perceived when vibrations reach the ear.
- Sound has measurable properties: **pitch** (frequency—high or low), **loudness** (amplitude—soft or loud), and **quality/timbre** (distinguishes voices/instruments).
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Formulas / Key Facts
| Quantity | Formula / Fact | |----------|----------------| | Ohm's Law | V = I × R | | Resistance in series | R_total = R₁ + R₂ + … | | Resistance in parallel | 1/R_total = 1/R₁ + 1/R₂ + … | | Law of reflection | Angle of incidence = Angle of reflection (∠i = ∠r) | | Refractive index (basic) | n = speed of light in vacuum / speed in medium | | Speed of sound in air (at 20 °C) | ≈ 343 m/s | | Speed of light in vacuum | 3 × 10⁸ m/s | | Audible frequency range (humans) | 20 Hz – 20,000 Hz | | Infrasound | < 20 Hz | | Ultrasound | > 20,000 Hz |