Pariksha SaathiLesson 4 of 8 · Science and its Pedagogy (Paper II)
How Things Work: Electric Current and Its Effects, Magnets, Light and Sound
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Cells and circuits
An electric cell has a positive terminal (the metal cap) and a negative terminal (the metal disc), and the chemicals inside it produce electricity until they are used up. A bulb has a thin filament joined to two terminals; it glows only in a closed circuit, in which current flows from the positive terminal through the bulb to the negative terminal, and a fused bulb has a broken filament. A switch opens or closes the circuit. Conductors such as metals, graphite and the human body let current pass; insulators such as rubber, plastic, wood and glass do not. Two or more cells joined positive to negative make a battery. Children experiment only with cells, never with the mains supply.
Heating and magnetic effects
A wire carrying current gets hot, an effect used in room heaters, electric irons, immersion heaters and geysers. The filament of an incandescent bulb glows but wastes much energy as heat, so CFLs and LEDs save electricity. An electric fuse is a wire of special material that melts and breaks the circuit when too much current flows because of a short circuit or overloading; MCBs switch off on their own. In 1820 Hans Christian Oersted found that a wire carrying current deflects a compass needle, showing that current has a magnetic effect. A coil of insulated wire around an iron piece becomes an electromagnet only while current flows; electromagnets lift scrap iron in cranes and work the electric bell.
Magnets
Magnetite is a natural magnet; artificial magnets are made as bars, horseshoes, cylinders and rings. Iron, nickel and cobalt are magnetic materials; wood, plastic, rubber, glass, copper and aluminium are not. Attraction is strongest at the poles, and a freely suspended bar magnet comes to rest in the north-south direction, the principle of the compass. Like poles repel and unlike poles attract; repulsion is the sure test of a magnet, since an unmagnetised iron bar is also attracted. Magnets lose their magnetism when heated, hammered or dropped, so they are stored in pairs with unlike poles on the same side, a piece of wood between them and soft iron keepers across the ends.
Light
Light travels in straight lines, which explains shadows and the inverted image in a pinhole camera; a shadow needs a source of light, an opaque object and a screen. By the laws of reflection the angle of incidence equals the angle of reflection, and the incident ray, the normal and the reflected ray lie in one plane. A plane mirror gives a virtual, erect image of the same size, as far behind the mirror as the object is in front, and laterally inverted, which is why AMBULANCE is written in reverse. A concave mirror gives a real, inverted image of distant objects and an enlarged, erect image of a very near one, so dentists, torches and headlights use it; a convex mirror always gives an erect, diminished image of a wide area, so vehicles use it as a rear-view mirror. A convex lens converges light and magnifies; a concave lens diverges it. A prism splits white light into seven colours, as raindrops do in a rainbow.
Seeing
We see an object when light from it enters our eyes. The iris controls the size of the pupil, and the lens focuses light on the retina, whose rods respond to dim light and cones to bright light and colour; the blind spot has no light-sensitive cells. An image persists on the retina for about 1/16 of a second, which makes motion pictures possible. Louis Braille's raised-dot system lets visually challenged people read by touch.
Sound
Sound is produced by vibrating objects; our voice comes from the vocal cords in the larynx, which are about 20 mm long in men and about 5 mm shorter in women. Sound needs a medium and travels through solids, liquids and gases but not through a vacuum. The number of vibrations per second, the frequency in hertz (Hz), decides the pitch, and the amplitude decides the loudness, measured in decibels (dB). Humans hear sounds of about 20 Hz to 20,000 Hz, and dogs can hear higher frequencies. Sound above 80 dB becomes physically painful, and noise pollution can cause sleeplessness, hypertension and hearing loss.
Chemical effects of current
Tap water, salt solution and lemon juice conduct electricity, while distilled water is a poor conductor until salt is added; most conducting liquids are solutions of acids, bases and salts. A tester with an LED, which glows even with a weak current, shows conduction. Current through a solution can produce gas bubbles at the electrodes, deposit metal or change colour; in 1800 William Nicholson showed that current through water gives oxygen bubbles at the electrode joined to the positive terminal and hydrogen at the other. In electroplating with copper sulphate solution, copper from the plate joined to the positive terminal is deposited on the object joined to the negative terminal. Chromium plating gives taps, car parts and cycle handlebars a shiny, scratch-resistant surface, and tin-plated iron is used for food cans because tin is less reactive than iron.
Key facts
- Closed circuit - the bulb glows; open circuit or fused (broken) filament - no current; conductors: metals, graphite, human body; insulators: rubber, plastic, wood, glass.
- Heating effect: heater, iron, geyser, fuse; an MCB switches off by itself on a short circuit or overload; CFLs and LEDs waste less energy as heat than incandescent bulbs.
- Oersted (1820): a wire carrying current deflects a compass needle; an electromagnet is a magnet only while current flows; used in cranes and electric bells.
- Magnetic materials: iron, nickel, cobalt; like poles repel; repulsion is the sure test of a magnet; store magnets in pairs with keepers.
- Plane mirror image: virtual, erect, same size, laterally inverted, as far behind the mirror as the object is in front.
- Concave mirror: dentists, torches, headlights; convex mirror: rear-view mirror (erect, diminished, wide view); convex lens: magnifying glass.
- Persistence of vision: about 1/16 of a second; rods - dim light, cones - colour; the blind spot has no light-sensitive cells.
- Sound needs a medium and cannot travel through a vacuum; pitch depends on frequency (Hz), loudness on amplitude (dB); human hearing about 20 Hz to 20,000 Hz; above 80 dB is painful.
- Electroplating: object at the negative terminal, plating metal at the positive; chromium on taps and handlebars; tin on iron for food cans; William Nicholson (1800) split water with current.
Quick revision
Cell: positive metal cap, negative metal disc; a closed circuit lights the bulb; conductors vs insulators; heating effect in heater and fuse, MCB; Oersted 1820, magnetic effect; electromagnet in crane and bell; iron, nickel, cobalt magnetic; repulsion is the sure test; angle of incidence = angle of reflection; plane mirror image virtual, erect, laterally inverted; concave for dentists, convex for rear view; persistence 1/16 s; pitch-frequency, loudness-amplitude; electroplate the object at the negative terminal.