Physics notes and concept map

Length, volume and time Prefixes: nano (10-9), micro (10-6), milli ( 10-3), kilo(103), Mega(106), Giga (10 9). Degree of accuracy of instruments: Metre rule and measuring tape (0.1 cm), Vernier calipers (0.01 cm), Micrometer (0.01 mm). Types of errors: Zero error, parallax error Mass vs Weight mass is a measure of the amount of substance in a body BUT weight is a measure of gravitational force on a mass W = m g electronic balance: mass (kilogram, kg) spring balance: force/weight (newton, N) Mass remains constant but weight can change because of acceleration due to gravity. Density is mass per unit volume D = m / V (SI Units: kg/m3 ) Density of water is 1 g/cm3 or 1 000 kg/m3) Less dense (lower density): float Denser (higher density): sink Speed and acceleration Speed is the rate of distance travelled S = D/t Acceleration is the rate of change of velocity a = (v-u)/t (Unit: m/s2) Average speed = total distance / total time or = (u + v) / 2 Speed-time graph: gradient = acceleration, area under graph = distance Distance-time graph: gradient = speed MECHANICS Forces F = m a Resultant force = 0 when (i) object is at rest or (ii) object is at constant speed Zero resultant force means that the forces are opposite and equal. Eg. friction = forward force, weight = air resistance. Vector drawing Note: choose an appropriate scale, state both resultant force and direction. Single arrow for forces but double arrow for resultant force. For drawing of parallelogram, all forces point away from the same starting point. Tracing of lines in dotted lines. Centre of gravity It is the point where the whole weight seems to act. For a body to be stable, base area of object must be large and its CG is low. The line of weight through the CG must lie within the base area of the object. Energy is the capacity to do work. (Unit: Joule) Energy stores: chemical potential, kinetic, gravitational potential, elastic potential, internal, nuclear. Methods of transfer: by heating, mechanically, electrically, by propagation of electromagnetic waves/ sound waves. Energy cannot be created or destroyed, but can be transferred from one store to another. The total amount of energy in an isolated system remains constant. Work, energy and power Work done is the product of the magnitude of a force and the distance moved in the direction of the force (Unit: Joule) W = F x // d Formulas of energy: - Kinetic = ½ mv2 - Gravitational Potential = mgh Power is the rate of doing work.(Unit: Watts) Power = Energy / time or Work / time P = E / t or P = W / t Scalar & Vector Scalar quantity is a quantity that has magnitude only. Eg. mass, speed, distnace. Vector is a quantity that has both magnitude and a direction. Eg. force, velocity, acceleration, weight, displacement. Turning effect of Forces Moment is the product of a force and its perpendicular distance from the pivot to the line of action of the force. (Unit: Nm or Ncm) Moment = F x ⏊ d Principle of Moment states that for a body to be in equilibrium, the sum of anticlockwise moments is equal to the sum of clockwise moments about the same pivot. Types of equilibrium: stable, unstable, neutral. Pressure Pressure is force per unit area. (Unit: Pa or N/m2 , cmHg, N/cm2) P = F / A and P = h⍴g (liquid pressure)

Thermal Processes Conduction Solids >Liquids>Gases Works by molecular vibration. Good conductors: Metals (electron diffusion and molecular vibration) Convection (for fluids i.e. liquids/gases) Due to density changes When heated Expands Becomes less dense Rises When cooled Contracts Denser Sinks Convection current is set up. E.g. air-conditioner, heating a liquid etc. Conduction and convection cannot take place in vacuum. Radiation can occur without medium i.e. in a vacuum or outer space. Shiny and silver surface can reduce radiation. Factors affecting absorption/emission: (i) nature of the surface (shiny/dull & light/dark) Black and dull surfaces are better absorbers (get hot more quickly) and also better emitters (lose energy more quickly) than white and shiny surfaces. (ii) Temperature A hotter body gives out more thermal energy. (iii) surface area A body with a larger surface area loses energy more quickly. E.g. oven, solar heating pipes, teapots, cooling fins of refrigerator etc. THERMAL PHYSICS Kinetic Model of Matter Solid: closely packed together by very strong attractive forces, vibrate about fixed positions. Liquid: quite closely packed together by strong attractive forces, sliding past over each other. Gas: far apart from one another with weak or negligible attractive forces, moving randomly at high speed. Brownian motion: bombardment of smoke particles by air molecules gives rise to the random motion of smoke particles. Pressure ⍺ temperature at constant volume At constant volume, when temperature increases, the overall k.e. increases and the air molecules move faster. This results in more frequent collisions with the walls of container, thus increases the pressure of air. Volume ⍺ temperature at constant pressure At constant pressure, increasing the temperature of the air will result in the increase in k.e. of air molecules and causes the molecules to exert a greater force on the piston and the walls. Hence, this leads to an expansion of the air and results in an increase in the volume.

THERMAL PHYSICS Thermal properties of Matter Melting point (Boiling point) is a temperature in which a substance changes from solid to liquid (liquid to gas). During melting (boiling), energy is absorbed to break the forces of attraction changing from solid to liquid (liquid to gas), increasing potential energy as particles move further apart. Average kinetic energy of particles remains constant, hence temperature remains constant. During Freezing (Condensation), energy is released to strengthen the forces of attraction changing from liquid to solid (gas to liquid), decreasing the potential energy as particles move closer together. Average speed of particles remains constant, hence temperature remains constant. Cooling curve Temperature gas liquid + gas liquid liquid +solid solid time Gases expand more than liquids and liquids expand more than solids. Boiling and evaporation involves in enerhy gain changing from liquid to gas. Differences between boiling and evaporation. Boiling Evaporation -Bubbles are formed.- No bubbles present. - Faster process.- Slower process. - Heat source is required.- Heat from surrounding air. - Occurs throughout the liquid.- Occurs only the surface of liquid. - Occurs at fixed temperature. - Occurs at any.temperature. Evaporation causes cooling as it takes energy away resulting in a drop of temperature. Heat capacity is the amount of internal energy needed to raise the temperature of an object by 1 ℃. SI unit: J℃-1 or JK-1 . Q = c△⍬ Specific heat capacity is the amount of internal energy needed to raise a unit mass of a substance by 1℃.. SI unit: Jkg-1℃-1 or Jkg-1K-1 Q = mc△⍬ Latent heat is the amount of internal energy required by a substance to change state without a change in temperature. Latent heat of fusion is the amount of internal energy required to change a substance from solid to liquid states without a change in temperature. Latent heat of vaporization is the amount of internal energy required to change a substance from liquid to gaseous states without a change in temperature. Specific latent heat is the amount of internal energy required to change the state of a unit mass of substance without a change in temperature. SI unit: Jkg-1 Q = ml Note: when a heater is used, Q