How Things Work
Electricity, Magnetism, Light and Sound
Overview
"How Things Work" is a core physics theme in the PSTET Paper II Science section, covering four foundational topics that explain everyday phenomena: electricity, magnetism, light and sound. These concepts form the backbone of upper-primary science (Classes VI-VIII) and are tested both for content knowledge and pedagogical understanding.
For PSTET, expect questions on basic definitions, simple circuits, magnetic properties, reflection/refraction of light, and characteristics of sound. The examiner often frames questions around practical applications—why a fuse blows, how an electromagnet works, why we see rainbows, or how sound travels through different media. Mastering these topics requires understanding the underlying principles rather than rote memorisation.
This section bridges the gap between abstract physics and the child's daily experience. A strong grasp here also supports pedagogy questions on activity-based learning and the use of simple experiments in classrooms.
Key Concepts
Electricity
- Electric current is the flow of electric charge (electrons) through a conductor; measured in amperes (A).
- A complete circuit requires a source (cell/battery), conducting wire, load (bulb/appliance) and a switch; any break stops current flow.
- Conductors allow current to pass (metals like copper, iron); insulators resist current (rubber, plastic, wood).
- Heating effect of current: when current flows through a resistance, heat is produced (used in heaters, fuses, electric iron).
Magnetism
- A magnet has two poles—north and south; like poles repel, unlike poles attract.
- Magnetic field: the region around a magnet where its influence can be felt; represented by field lines running from north to south pole externally.
- Electromagnet: a coil of wire wound around an iron core becomes a magnet when current flows; strength increases with more turns or higher current.
- Earth behaves as a giant magnet with magnetic north and south poles.
Light
- Light travels in straight lines (rectilinear propagation); this explains shadows and eclipses.
- Reflection: bouncing of light from a surface; angle of incidence = angle of reflection.
- Refraction: bending of light when it passes from one medium to another (e.g., air to water); causes objects to appear bent in water.
- Dispersion: splitting of white light into seven colours (VIBGYOR) by a prism.
Sound
- Sound is produced by vibrating objects and requires a material medium (solid, liquid or gas) to travel; it cannot travel through vacuum.
- Frequency determines pitch (high frequency = high pitch); amplitude determines loudness.
- Speed of sound is fastest in solids, slower in liquids, slowest in gases.
- Echo is reflected sound heard after a delay; requires minimum distance of about 17 metres from the reflecting surface.
Formulas / Key Facts
| Concept | Formula / Fact |
|---|---|
| Ohm's Law | V = I × R (Voltage = Current × Resistance) |
| Resistance unit | Ohm (Ω); Current unit: Ampere (A); Voltage unit: Volt (V) |
| Electric power | P = V × I (measured in Watts) |
| Law of reflection | Angle of incidence (i) = Angle of reflection (r) |
| Speed of light in vacuum | 3 × 10⁸ m/s (approximately) |
| Speed of sound in air (at 20°C) | Approximately 343 m/s |
| Minimum distance for echo | About 17 m (sound must travel 34 m total in 0.1 s) |
| Magnetic poles | Like poles repel; unlike poles attract |
| VIBGYOR sequence | Violet, Indigo, Blue, Green, Yellow, Orange, Red |
Worked Examples
Example 1: Simple Circuit Problem
Question: A circuit has a 12 V battery and a bulb of resistance 6 Ω. Find the current flowing through the bulb.
Solution: Using Ohm's Law: V = I × R Rearranging: I = V / R I = 12 / 6 = 2 A
The current flowing through the bulb is 2 amperes.
Example 2: Reflection of Light
Question: A light ray strikes a plane mirror at an angle of 35° to the normal. What is the angle of reflection?
Solution: By the law of reflection, angle of incidence = angle of reflection. Angle of incidence = 35° Therefore, angle of reflection = 35°.
Example 3: Echo Calculation
Question: A person claps near a cliff and hears the echo after 0.4 seconds. If the speed of sound is 340 m/s, how far is the cliff?
Solution: Total distance travelled by sound = Speed × Time = 340 × 0.4 = 136 m Since sound travels to the cliff and back, distance to cliff = 136 / 2 = 68 metres.
Common Mistakes
| Wrong Thinking | Correct Understanding |
|---|---|
| "Current gets used up in a bulb, so less current returns to the battery." | Current remains the same throughout a series circuit; energy is transferred, not current consumed. |
| "Magnets attract all metals." | Magnets attract only ferromagnetic materials (iron, cobalt, nickel); they do not attract aluminium, copper or gold. |
| "Light bends towards the normal when going from glass to air." | Light bends away from the normal when moving from a denser to a rarer medium (glass to air). |
| "Sound travels fastest in air because air is everywhere." | Sound travels fastest in solids (particles are closest), then liquids, then gases. |
| "An echo can be heard at any distance from a wall." | An echo requires a minimum distance (~17 m) so that the reflected sound reaches the ear after 0.1 s (persistence of hearing). |
Quick Reference
- Ohm's Law: V = I × R — the fundamental relationship in electricity.
- Fuse works on heating effect: excess current melts the fuse wire, breaking the circuit.
- Electromagnet strength increases with more coil turns, higher current, or soft iron core.
- Angle of incidence = Angle of reflection — always measured from the normal.
- VIBGYOR: order of colours in spectrum; red bends least, violet bends most.
- Sound needs a medium: no sound in vacuum; astronauts use radios in space.
- Echo needs 17 m minimum distance for distinct hearing.