How Things Work
Electricity, Magnetism, Light and Sound
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.
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).
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 |
Worked Examples
Example 1 — Ohm's Law Calculation
Problem: A torch bulb has resistance 5 Ω and is connected to a 3 V battery. Find the current through the bulb.
Solution: Using V = I × R I = V / R = 3 / 5 = 0.6 A
Example 2 — Law of Reflection
Problem: A ray of light strikes a plane mirror at an angle of 35° to the mirror surface. What is the angle of reflection?
Solution: Angle of incidence is measured from the normal (perpendicular to surface). If ray makes 35° with mirror surface, angle of incidence = 90° − 35° = 55°. By law of reflection, angle of reflection = 55°.
Example 3 — Sound Characteristics
Problem: Two sounds have the same pitch but one is louder. Which property differs?
Solution: Pitch depends on frequency; loudness depends on amplitude. Since pitch is equal, frequency is the same. The differing property is amplitude (and hence energy carried by the wave).
Common Mistakes
| Wrong Thinking | Correct Fix |
|---|---|
| "Current gets used up as it flows through a bulb." | Current is the same throughout a series circuit; energy is transferred, not current. |
| "A magnet has magnetism only at its poles." | The magnetic field exists around the entire magnet; it is strongest at the poles. |
| "Light slows down and bends away from the normal when entering glass from air." | Light slows down in glass (denser) and bends towards the normal, not away. |
| "Loudness and pitch are the same thing." | Loudness relates to amplitude; pitch relates to frequency—independent properties. |
| "Sound travels fastest in air because air is lightest." | Sound actually travels fastest in solids (particles closer), slowest in gases. |
Quick Reference
- Circuit rule: No gap → current flows; any break → circuit open, no current.
- Ohm's law trio: V = I × R — remember voltage drives current against resistance.
- Magnetic poles: Like repels, unlike attracts; field lines exit north, enter south.
- Electromagnet strength: More turns + more current = stronger magnet.
- Reflection law: ∠i = ∠r, both measured from normal.
- Sound needs medium: Vacuum = silence; solids transmit fastest.
- Pitch ↔ Frequency, Loudness ↔ Amplitude — keep them distinct.
Revise with simple home activities: build a lemon cell, trace magnetic field lines with iron filings, observe a spoon in water for refraction, and use a tuning fork to show vibration-produces-sound. These demonstrations are pedagogically valuable and often appear in method-based questions.