lecture notes PHYSICS

Transcription

1 Physics is the branch of science concerned with the properties and interactions of matter, energy, space and time. It explains ordinary matter as combinations of a dozen fundamental particles (quarks and leptons), interacting through four fundamental forces. It describes the many forms of energy (such as kinetic energy, electrical energy, and mass) and the way energy can change from one form to another. It describes a space-time and the way objects move through space and time. An understanding of universe begins with an understanding of physics. page 1

2 The study of the motion of objects The study of heat and the movement of energy within a system It is primarily concerned with the arrangement of electrons around the nucleus and the processes by which these arrangements change The branch of physics concerned with the nucleus of the atom Mechanics Thermodynamics Atomic Physics Nuclear Physics Physics Classical Physics Modern Physics Electricity & Magnetism Optics Condensed Matter Physics The study of the electric charges and magnetic properties of matter The study of the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it The field of physics that deals with the macroscopic physical properties (optical, electrical, magnetic and elasticity) of matter page 2

3 Aristotle taught that all things are made up four elements : earth, air, fire and water. Among those who held this view were Democritus and Epicurus. Both view-points were supported by complete systems of logic. Galileo Galilei studied the behaviour of falling bodies and formulated laws describing this behaviour. He also investigated the pendulum and put in to use in a clock. Kepler discovered that the orbits of the planets were elliptical in shape and that their motion could be generalised in the form of a mathematical formula that could be used predict their future motions. Newton developed the laws of motion and gravitation that explained the observations of many scientists who had preceded him. He also made basic discoveries concerning the nature and composition of light. During the 18th century, the scientific study of heat and electric energy. Franklin and Faraday did much of the pioneer work in electricity, while Rumford and Joule put the study of heat on a scholarly basis. Planck and Einstein in 113 to explain why the light from gas discharges was emitted at only a few, discrete frequencies; this light formed emission lines of different colours when the light was passed through a slit and dispersed by a prism. Heisenberg proved that it was impossible to determine both a particle s position and momentum with arbitrary precision; if one is known very accurately, then the uncertainty in the other becomes large. In1th century, the study of light was further advanced by the work of Young and Fresnel. then it became apparent that the various topics of physics (motion, light, electricity, heat and sound) could all be described in terms of energy. Before 20th century With the brilliant work of Maxwell in electricity and magnetism, physics was further simplified into only two main parts: motion (which includes heat and sound) and electromagnetism (which includes light). The discovery of X rays and radioactivity showed that matter was not as simple as had been supposed. The speculations of Einstein about the laws of motion and the relationship between matter and energy made it necessary to re-examine the whole structure of physics. Ernest Rutherford fired very small particles, emitted in radioactive decay, at a thin film of gold. From the scattering pattern of the particles, he determined that the atom consisted of a small, heavy, positively charged nucleus surrounded by very light electrons. Perhaps the greatest such unification that has taken place in this century is the integration of electromagnetism and quantum mechanics, in quantum electrodynamics (QED). This feat earned Richard Feynman, Julian Schwinger, and Sin-itiro Tomonaga the Nobel Prize for physics in 165. Beginning of 20th century to present day While the basic laws of physics were being studied, man was quick to put these discoveries to use. These applications of science to human needs is known as technology. For example, the development of the laws of planetary motion is a part of pure science. The use of these laws in the manipulation of a spaceship, on the other hand, is form of technology. page 3

4 Physics The science of modelling the universe around us, and then examining and manipulating those models so that we better understand how the universe works. Engineering Technology The discipline of applying physics models to the real world in order to accomplish a desired result. The product, or perhaps the result of physics and engineering. page 4

5 Snell s law Law of Refraction Telescope Uses the laws of refraction Space Telescope helps us to understand the universe A simple example for the relation between physics & technology. page 5

6 Study of the physical nature of the chemical molecules Study of the composition of astronomical objects and the changes that they undergo. concerned with the physical properties of molecules essential to living organisms. Physical Chemistry Astrophysics Biophysics Chemistry Physics Biology Interaction of energy and matter in chemical systems. All of the earliest chemists were actually physicists working on particular projects. The forces between chemical bonds, the rotation of electron around nucleus, electroplating, electrolysis, and electrochemistry all are explained with the help of physics. Geophysics The principles or laws of physics. For example, the circulatory system, respiratory system, excretory systems are all explained with the pressure, the muscular skeleton system is explained with the help of force concept. The physics and chemistry of the Earth s dynamic processes. page 6

7 Mathematics is the language of physics that has been expressed by Galileo as Mathematics is the language in which God wrote the Universe. Mathematics plays a role in physics that is great way to get a very concise statement that would take a lot of words. For example, The speed of an object is calculated by dividing distance to time. This expression is written as v = x / t. To a physicist both expressions say the same thing. The symbols of mathematics replace a lot of words. Galileo Galilei The mathematical expressions show the relationship among the variables or we can use the rules of mathematics to change the expressions such as x = v.t. page 7

8 Mass Length Time kilogram (kg) meter (m) second (s) Temperature Every idea in physics is explained in terms of fundamental ideas called Basic Units. Basic units are used to derive, Derived Units. kelvin (K) Electric Current ampere (A) Luminous Intensity candela (cd) Amount of matter mole (mol) page 8

9 Derived units obtain their meaning from the original definitions of the basic units. Table-1.2 Therefore, the definitions of basic units are the source of all knowledge in physics. Examples of SI derived units. Quantity Name Symbol area square meter m2 volume cubic meter m3 speed, velocity meter per second m/s acceleration meter per second squared m/s2 force newton kg.m/s2 energy joule pressure pascal kg.m2/s2 kg/m.s2 magnetic field strength tesla kg/a.s2 density kilogram per cubic meter kg/m3 power watt kg.m2/s3 page

10 Table-1.3 Decimal multiples of SI prefixes. Table-1.4 Decimal submultiples of SI prefixes. Factor Name Symbol Factor Name Symbol 1024 yotta Y 10-1 deci d 1021 zetta Z 10-2 centi c 1018 exa E 10-3 milli m 1015 peta P 10-6 micro µ 1012 tera T nano n 10 giga G pico p 106 mega M femto f 103 kilo k atto a 102 hecto h zepto z 101 deka da yocto y page 10

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12 Table-1.5 Physical quantities & measuring instruments. The most useful observations are those that can be expressed precisely in terms of numbers. Such observations are obtained by measurement. QUANTITY MEASURING INSTRUMENTS Mass Equal-Arm Balance In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the measurement results in a given number for the relationship between the item under study and the referenced unit of measurement. Digital Scale Bathroom Scale Time Sandglass Chronometer Watch Sundial Length Measuring Tapes Ruler Temperature Mercury Thermometer Digital Thermometer Bimetalic Thermometer Current Multimeter Analog Ammeter page 12

13 Measurement Measurement is the process of estimating the magnitude of an object, or comparing the magnitude of object with standards. Direct Measurement Indirect Measurement Compare similar quantities of the same dimension. Compare different quantities, or make some calculations for the result. Measuring the length of a pencil with a ruler, measuring mass of the stone with an equal-arm balance and measuring the time of flight of a parachutist with a chronometer are all examples of direct measurement. To measure all these physical quantities different kinds of measuring instruments are used. Measuring the distance covered by a runner and the time to cover that distance are needed to calculate the speed of the runner or measuring the mass of an object and its volume are necessary to calculate the density of that object. These could be examples of indirect measurement. page 13

14 As used by scientists, the word accuracy refers to how close a measurement is to a true or accepted value. For example, the boiling point of pure water at sea level is known to be 100 oc. Suppose a student s measurement of the boiling temperature of pure water at sea level is, oc. Because this o temperature is very close to 100 C, you can say that the student s measurement is accurate or has good accuracy. As used by scientists, the word precision refers to the reproducibility of a series of measurements. When several measurements of the same quantity are close to each other, they are said to have good precision. Suppose that three students each use a meter stick to measure the length of a room, and they get the following measurements: Student A s measurement Student B s measurement Student C s measurement : : : 10,4 meters 10,3 meters 10,3 meters Because all three measurements are close to each other (the first differs from the others by only 0,01 meter), the measurements can be described as having good precision or as being precise. page 14

15 A series of measurements can be precise without being accurate. For example, suppose that the meter stick used by the three students to measure the length of the room was defective. Perhaps the manufacturer made an error when marking the scale, or the end of the meter stick had become worn down after years of use. Then, all three measurements might be inaccurate even though they show good precision. When several measurements of the same thing are precise but inaccurate, the case often can be traced to the use of a defective measuring instrument. The precision is good, but the accuracy is poor. Both the accuracy and precision are poor. Both the accuracy and precision are good. page 15

16 Even when we try to measure things very accurately, it is never possible to be absolutely certain that the measurement is perfect. The errors that occur in measurement can be divided into two types, random and systematic. Random Errors Systematic Errors Random errors can be reduced by repeating the measurement many times and taking the average, but this process will not affect systematic errors. When you write up your practical work you need to discuss the errors that have occurred in the experiment. Systematic errors are due to the system or apparatus being used. Systematic errors can often be detected by repeating the measurement using different method or different apparatus and comparing the results. A zero offset, an instrument not reading exactly zero at the beginning of the experiment, is an example of a systematic error. Some examples of random errors: Changes in experimental conditions, such as temperature, pressure or humidity. A different person reading the instrument. Malfunction of a piece of apparatus. Some examples of systematic errors: An observer consistently making the same mistake. Apparatus calibrated incorrectly. page 16

17 Errors are also expressed as either absolute or relative errors. Absolute error is the actual difference between the measured value and the accepted value. Measured Absolute Accepted Value Error Value = (MV) (AE) (AV) In laboratory work involving measurements, absolute errors are usually called experimental errors. Notice that the absolute error carries with it the same unit that was used in the measured and accepted values. Also the sign of the error shows whether the measured value was above or below the accepted value. page 17

18 Relative error is expressed as a percentage, and it is therefore often called the percentage error. It is calculated as follows: Relative Error = Absolute Error (AE) Accepted Value (AV).100 page 18

19 Physics is a mathematical science. The mathematical quantities which are used to describe the motion of objects can be divided into two categories. The quantity is either a scalar quantity or a vector quantity. These two categories can be distinguished from one another by their distinct definitions: PHYSICAL QUANTITIES Scalar Quantity Vector Quantity Quantity that has a number with its appropriate unit (magnitude). Quantity that has a number with its appropriate unit (magnitude) and a direction. page 1

20 Examples of scalar quantities Examples of vector quantities Mass Force Volume Weight Density Velocity Length Acceleration Distance Displacement Speed Momentum Temperature Electric field Energy Torque page 20

21 Vector quantities are represented by vectors. A vector is a straight line with an arrow at one end. The direction of the arrow represents the direction of the vector and the length of the line represents the magnitude of the vector. Vectors are represented symbolically either with an arrow on top of the symbol or in bold type. Thus, either A or A respresents a vector, and its magnitude is denoted by A. application point A line of action magnitude A page 21