WBBSE Madhyamik Physical Science key terms

The smallest measurement that a given instrument can read reliably. On an ordinary metric ruler it is 1 mm; on a vernier calliper it is the difference between one main-scale division and one vernier division.

A physical quantity that has magnitude only and no direction. Mass, time, speed, work and temperature are scalars, so they combine by ordinary arithmetic without any directional rule being needed.

A physical quantity that needs both magnitude and direction to be fully described. Displacement, velocity, force and momentum are vectors, and they must be added by geometrical rules rather than simple arithmetic.

The straight-line distance from a body's starting point to its finishing point, taken with the direction of that line. It can be zero even when the distance actually travelled is large.

The rate at which velocity changes with time. Since velocity is a vector, acceleration may arise from a change of speed, a change of direction, or both; its SI unit is metre per second squared.

The natural tendency of a body to resist any change in its state of rest or of uniform motion in a straight line. It depends on the mass, which is why a loaded truck is harder to start or stop than a cycle.

The product of a body's mass and its velocity, measured in kilogram metre per second. It is a vector directed along the velocity and changes only when a net external force acts.

Work is done when a force moves a body through a distance in the direction of the force; it equals force multiplied by displacement and is measured in joules. No work is done when the displacement is perpendicular to the force.

The rate at which work is done or energy is transferred, obtained by dividing work by the time taken. Its SI unit is the watt, where one watt means one joule of work done each second.

The energy a body possesses because of its motion, equal to half its mass multiplied by the square of its speed. Doubling the speed makes the kinetic energy four times as large.

Energy stored in a body because of its position or condition, such as a stone raised above the ground or a stretched spring. Near the Earth's surface it equals mass times acceleration due to gravity times height.

The quantity of heat needed to raise the temperature of one kilogram of a substance by one degree Celsius. Water's large value, about 4200 J kg⁻¹ °C⁻¹, explains why it heats and cools slowly.

The heat absorbed or released by a substance while it changes state at constant temperature, so no thermometer reading changes. Melting ice and condensing steam both involve large hidden exchanges of heat.

The increase in a body's length, area or volume when its temperature rises, because its particles vibrate more vigorously and occupy more space. Gaps left in railway tracks and bridges make room for this expansion.

The maximum displacement of a vibrating particle from its mean position. In a sound wave the amplitude decides loudness, whereas the frequency decides the pitch of the note heard.

The number of complete vibrations or cycles completed in one second, measured in hertz. A higher frequency of a sound wave gives a note of higher pitch, that is, a shriller sound.

The sound heard a second time after reflection from a distant hard surface such as a cliff, wall or hillside. For a distinct echo the reflecting surface should be at least about 17 metres from the listener.

Sound whose frequency is above the upper limit of human hearing, roughly 20,000 hertz. Bats and ship sonar use ultrasonic pulses whose returning echoes reveal obstacles or the depth of water.

The ratio of the speed of light in vacuum to its speed in a given medium. It measures how strongly the medium bends a ray; glass has a higher value than water.

When light travelling in a denser medium strikes the boundary with a rarer medium at more than the critical angle, it is reflected wholly back into the denser medium. Optical fibres and mirages depend on it.

The distance between the optical centre of a lens, or the pole of a mirror, and its principal focus. It decides how strongly the device converges or diverges a beam of light.

The reciprocal of the focal length measured in metres, expressed in dioptres. A short focal length means a large power; converging lenses have positive power and diverging lenses negative power.

At constant temperature, the current through a metallic conductor is directly proportional to the potential difference across its ends, so V equals I times R. It fails for non-ohmic devices such as filament lamps.

A property of the material of a conductor, numerically the resistance of a conductor of unit length and unit area of cross-section. It changes with temperature and with the nature of the metal.

The rate at which electrical energy is converted into other forms in a circuit, equal to voltage multiplied by current. It is measured in watts and is printed on the rating plate of every appliance.

A practical unit of electrical energy equal to the energy consumed by a 1 kW appliance running for one hour, that is 3.6 million joules. Domestic electricity meters record consumption in this unit.

The production of an induced current or induced e.m.f. in a coil when the magnetic field linked with it changes, for example when a magnet is moved into or out of it. Generators and transformers work on this principle.

The amount of a substance that contains as many elementary particles as there are atoms in exactly 12 g of carbon-12. It is the chemist's counting unit for atoms, molecules and ions.

The number of particles contained in one mole of any substance, approximately 6.022 × 10²³ per mole. It connects the microscopic world of atoms with masses that can actually be weighed.

The volume occupied by one mole of any gas at standard temperature and pressure, about 22.4 litres at 273 K and 1 atmosphere. It lets you convert a measured gas volume into moles and then into mass.

At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure, so pressure multiplied by volume stays constant. Slowly compressing a trapped gas at room temperature obeys it.

Atoms of the same element that have the same number of protons but different numbers of neutrons, and hence different mass numbers. Chlorine-35 and chlorine-37 are isotopes with identical chemical properties but different masses.

The combining capacity of an element, usually the number of hydrogen atoms or chlorine atoms with which one atom of it combines, or twice the number of oxygen atoms it combines with.

A measure of how strongly an atom in a molecule pulls the shared electrons of a bond towards itself. A large difference in electronegativity between two atoms leads to ionic bonding instead of covalent bonding.

A chemical bond formed by the mutual sharing of one or more pairs of electrons between two atoms, typically both non-metals. The shared pair is attracted by both nuclei and holds them together.

The decomposition of an electrolyte by passing direct current through its solution or molten state, giving oxidation at the anode and reduction at the cathode. It is used to refine metals and to electroplate articles.

The existence of an element in two or more different forms in the same physical state, such as diamond and graphite for carbon. The forms differ in arrangement of atoms and therefore in properties.

A family of organic compounds sharing the same general formula and similar chemical properties, in which successive members differ by one carbon atom and two hydrogen atoms. Methane, ethane and propane form one.

An atom or group of atoms that gives a family of organic compounds its characteristic chemical behaviour, such as the hydroxyl group of alcohols or the carboxyl group of carboxylic acids.