Bio-medical Physics for Nurses K Thayalan
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Physics is a science dealing with nature and natural phenomena. It is concerned with the study of two concepts—matter and energy, and how they interact with each other. Matter is the one which occupies space and is made up of molecules or atoms. Examples are buildings, trees, flowers, water and air. All bodies are composed of matter.
Energy is the ability to do work and it has several forms. In general physicist studies the behavior of matter and energy under different physical conditions.
Matter is classified into three states viz, solid, liquid and gas. Bricks, wood and stone are solids and milk, water, oil are liquids. Hydrogen, oxygen and nitrogen are gases at room temperature. There is another state called plasma in which matter is present in the ionic state at very high temperatures.
A solid has a definite shape and volume. Its shape and size do not change by itself. A liquid has a definite volume but no shape of its own. It has the shape of the containing vessel. A gas has no shape or volume of its own. The gas occupies the space available to it. Liquids and gases are called fluids.
In common matter has the following properties;
  1. Matter occupies space: All bodies occupies space. The space occupied by a body is called its volume. No body can occupy the same space at the same time.
  2. Matter has weight: Everybody possesses mass and it is attracted towards the center of the earth. Therefore the body has weight.
  3. Inertia: Inertia means inactive. A body cannot change its state of rest or motion on its own, e.g. a book on a table.
  4. Resistance: Every matter offers resistance, e.g. cycling against wind.2
  5. Matter is divisible: All substances, solids, liquids or gases can be divided and subdivided into small particles, e.g. elements can be divided into molecules, atoms etc.
  6. Compressibility: All substances can be compressed to some extent by the application of force. Compressibility depends on the inter molecular spacing. Solids are incompressible, liquids are compressible and gases are highly compressible.
  7. Porosity: All substances are porous. Porosity is due to the spacing between the particles of matter. Metals are less porous than wood and earthen vessels are more porous.
  8. Elasticity: This is property by which material bodies regain their original shape and size after the external forces are removed. When a force is applied on a body there is a change in its length, shape and volume. Now the body is said to be strained. If the applied force is removed, the body regain its original shape and size. These bodies are said to be elastic bodies. The bodies which do not regain their original shape and size is called plastic bodies. No body is either completely elastic or plastic.
  9. Cohesion: The force of attraction between the molecules of the same substance is called cohesive force and this property is called cohesion. Because of this, molecules are held together. It also gives shape to solids. Cohesion is higher in solids than liquids.
  10. Adhesion: The force of attraction between molecules of two different substances is called adhesive force, e.g. water drop, clings to a glass plate.
To study the matter and energy and understand their properties, measurements of physical quantities are needed. Measurements plays an important role in experiments and daily walk of life. Quantities like, mass, length, volume and pressure are called physical quantities and are measurable. Accurate measurement of physical quantity requires a standard of its own kind. For example—distances are measured in meters, masses in kilograms and time in seconds. Therefore unit is a quantity adopted as a standard of measurement in terms of which similar quantities can be measured.3
Fundamental Units
The units which are independent of one another and have their own standard (base) are called fundamental units. The units of all other physical quantities can be obtained from the fundamental units: The three important fundamental units are (i) Length (ii) Mass and (iii) Time.
Systems of Units
There are many systems of units and over the years several systems of units have been used. They are FPS, (Foot, pound, second), CGS (Centimeter, gram, second) and MKS (Meter, kilogram, second). The FPS system is not used nowadays, only MKS and CGS systems are used.
SI System
In 1990, the new system of units the “systems international d' units” (SI Units) was introduced. This system is superior to all other systems and more convenient in practice. These systems do not vary with time as they are based on the properties of an atom. It is a coherent system having 6 basic units and 2 supplementary units. The basic units are meter, kilogram, second, ampere, kelvin and candela. The supplementary units are radian and steradian. The various SI units, symbol along with physical quantities are given in Table 1.1.
Table 1.1   Physical quantities and their SI units and symbol
Physical quantity
SI Unit
1. Length
2. Mass
3. Time
4. Current
5. Temperature
6. Luminous intensity
7. Plane angle
8. Solid angle
Units of Length, Mass and Time
The unit of length is meter. One meter is the length equal to 16507630.73 wavelength of the orange-red line of krypton-86 discharge lamp kept at 15°C and 76 cm of mercury. In practice several other units are used to measure length. The symbol and their relation with meter is given below.
Decimeter (dm)
= 10−1 m
Centimeter (cm)
= 10−2 m
Millimeter (mm)
= 10−3 m
Micron (μm)
= 10−6 m
Nanometer (nm)
= 10−9 m
= 10−10 m
Fermi (fm)
= 10−15 m
Kilometer (km)
= 103 m
Light year
= 9.467×1015 m
One light year is the distance traveled by light in one year
The unit of mass is kilogram. One kilogram is the mass of the platinum-iridium cylinder of diameter equal to it height kept at the international bureau of weights and measure near Paris, France. The other practical units of mass are as follows:
Gram (g)
= 10−3 kg
Milligram (mg)
= 10−6 kg
Quintal (q)
= 102 kg
Ton (T)
= 1000 kg
The unit of time is second. One second is defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two specified energy levels of cesium −133 atom. The other basic units of time are as follows:
Micro second (μs)
= 10−6 s
Millisecond (ms)
= 10−3 s
Minute (min)
= 60 s
Hour (hr)
= 3600 s
One solar day has 24 hours. One second is equal to 1/86400 of a mean solar day.
Derived Units
The units, which are not having their own standard (base) and obtained from the fundamental units are called derived units. Examples are; area, velocity, density and acceleration.
= meter × meter
= m2
= meter/second
= ms−1
= kilogram/meter3
= kgm−3
= meter/second 2
= ms−2
The powers of the fundamental units in terms of which a physical quantity can be represented are known as dimensions. The fundamental units of length, mass and time may be denoted by letter L, M and T. Any other physical quantity can be expressed in terms of these letters. Examples are;
= mass × acceleration
= MLT−2
= mass/volume
= M/L3 = ML−3
= distance/time
= L/T = LT−1
= Force/Area
= ML−1 T−2
Thus, the dimensions of force, density, speed and pressure are MLT−2 ML−3, LT−1 and ML−1 T−2 respectively. Dimensions are useful to derive an equation, to test the correctness of an equation and to convert one system of units into another.
Rules and Conventions in the use of Symbols
The following rules and conventions should be observed in the use of symbols in SI units.
  1. When the unit is named after a scientist it should not be written in a capital initial letter, e.g. newton, ampere. The symbol of the unit is expressed in capital letters, e.g. N for newton
  2. The symbol for all other units should be written with small letters, e.g. m for meter.
  3. Only singular form of the unit is to be used. No full stops or punctuation marks should be used at the end of the symbol.6
  4. Space is to be left between the numerical and symbol, e.g. 20 s and not as 20s
  5. In the temperature unit kelvin no degree sign is used, e.g. 273 K and not as 273°K
Accuracy and Errors in Measurement
All physical quantities needs to be measured very accurately. But during the measurement, certain error occurs. This is due to either faulty judgement or instrument defects. There are two types of errors namely (i) Random errors and (ii) Instrumental errors.
Random Errors
Small changes in readings while taking measurements is called random errors. It may be due to (i) small changes in the conditions of measurement and (ii) the incorrect judgement of the observer in making a measurement.
The magnitude of random error depends on the quality of the instrument and the observer. These errors can be minimized by taking arithmetic mean of a large number of measurements of the same quantity. The arithmetic mean will be very close to the correct result.
Instrumental Errors
If a instrument used for measurement is defective, it may lead to instrumental errors. Since the reason for these errors are known, they are also called as systemic errors. To eliminate these errors the measurements are corrected accordingly.
  1. What are fundamental units?
  1. What are derived units?
  1. How many basic units are there in SI system?
  1. What is one light year?
  1. Give the dimensional formula for pressure.
  1. Define the SI unit of length.
  1. Define the SI unit of mass.
  1. Define the SI unit of time.7
  1. What are the types of errors?
  1. What are the special features of the SI system?
  1. Dimension of force is
    1. MLT−2
    2. ML−1T−2
    3. ML−2T2
    4. ML−1T−1
  1. The dimensional formula for pressure is
    1. MLT−1
    2. ML2T−3
    3. ML−1T−2
    4. ML−1T−1
  1. Which is not a fundamental unit
    1. Mass
    2. Length
    3. Time
    4. Velocity
  1. One micron is equal to
    1. 10−6 m
    2. 10−4 m
    3. 1000 g
    4. 10−3 m
  1. One second is defined on the basis of transition between two specified energy levels of
    1. Cobalt—60 atom
    2. Cesium—137 atom
    3. Krypton—86
    4. Sodium atom
  1. One ton is equal to
    1. 10 kg
    2. 100 kg
    3. 1000 kg
    4. 10,000 kg
  1. That which possesses mass and occupies space is called
    1. Matter
    2. Energy
    3. Heat
    4. Semi conductor.
  1. The force of atrraction between unlike molecules is referred as
    1. Surface tension
    2. Diffusion
    3. Cohesion
    4. Adhesion
  1. Which of the following quantities has no unit
    1. Stress
    2. Strain
    3. Surface tension
    4. Velocity.
  1. The units of velocity in SI system is
    1. ms−1
    2. ms−2
    3. m2s
    4. cms−1