Electricity

Charge and current

• Charge (unit coulomb, C) is positive or negative. Like charges repel; unlike attract. The charge of one electron is 1.6 x 10^-19 C.
• Rubbing transfers electrons: glass rubbed with silk becomes positive; plastic or ebonite rubbed with fur becomes negative.
• The gold-leaf electroscope detects charge. Lightning is a huge electric discharge; lightning conductors protect buildings.
• Electric current is the rate of flow of charge: I = Q / t. Unit: ampere (A) (1 A = 1 C/s). Measured by an ammeter, connected in series.
• Conventional current flows from positive to negative; electron flow is the opposite.
• Potential difference (voltage): work done per unit charge, V = W / Q. Unit: volt (V). Measured by a voltmeter, connected in parallel.
• A cell is a source of potential difference; a battery is a group of cells.

Conductors and insulators

Ohm's law and resistance

Ohm's law: at constant temperature, current through a conductor is proportional to the potential difference: V = I R. R is resistance (unit ohm).

• Resistance R = (resistivity x length) / area: it increases with length and decreases with thicker area. It rises with temperature for metals.
• Resistivity depends on the material only. Alloys (nichrome, constantan, manganin) have higher resistivity than pure metals, so they are used in heating elements.
• Rheostat: a variable resistor. Fuse: a thin wire of low melting point that melts when current is too high.

Series and parallel combination

Worked example. Two resistors of 4 ohm and 6 ohm in series: R = 10 ohm. In parallel: R = (4 x 6) / (4 + 6) = 2.4 ohm. On a 12 V supply, the series current is 12/10 = 1.2 A.

Heating effect and power

• Joule's law: H = I^2 R t. Used in heaters, geysers, irons, toasters, electric bulbs, fuses.
• Electric power P = V I = I^2 R = V^2 / R. Unit watt. 1 kWh = 1 unit.
• Filament of a bulb is of tungsten (very high melting point, 3380 degrees C); the bulb is filled with argon or nitrogen to protect it.
• Fuse wire is made of a tin-lead alloy.

Domestic circuits

• India's supply: 220 to 240 V, 50 Hz AC.
• Three wires: live (red) carries current, neutral (black) returns it, earth (green) is the safety wire.
• Earthing sends leakage current to the ground, preventing shocks.
• MCB (miniature circuit breaker) and fuse protect from short circuit and overloading. Short circuit happens when live and neutral touch; overloading when too many appliances draw current.
• Appliances are connected in parallel. Switch is on the live wire.

Cells and batteries

• Volta made the first voltaic cell; Galvani studied "animal electricity".
• Electroplating (silver, gold, chromium coating) and electrolysis use the chemical effect of current.