A syrnbol is an image, object etc that stands for some meaning.
Ex (+) denotes addition

Periodic vibrations occuring with in the atom are used in atomic clocks.

i) Plasmodium
ii) Paramecium
iii) Euglena

Chemical substance used to alter the speed of the reaction is called a catalyst and the process is called catalysis.

i) When a kicking force is exerted on a football at rest, it raises.
ii) When a pushing force is applied on a box at rest it moves.

6.023 x 10²³

The SI unit of Luminous Intensity is Candela.

Solid angle is the angle formed by three or more planes intersecting at a common point. SI unit is steradian.

i) Bacilli - Rod shaped bacteria.
Eg. Bacillus anthracis
ii) Spirilla - Spiral shaped bacteria.
Eg. Helicobacter pylori.
iii) Cocci - Spherical or ball shaped bacteria
Eg. Streptococus pneumoniae
iv) Vibrio - Comma shaped bacteria
Eg. Vibrio cholera

The process of changing less valuable metal into gold is called alchemy.

Frictional force arises when two or more bodies in contact move or tend to move, relative to each other. It acts always in the opposite direction of the moving body.
Examples:
(i) To light a matchstick.
(ii) To stitch a cloth in a sewing machine.

Mole is defined as The amount of substance which contains 6.023 x 10²³ entities.

ampere:
One ampere is defined as one coulomb of charge moving in a conductor in one second.

Probiotics are live food supplements used in yoghurt and other fermented milk products. Eg. Lactobacillus acidophilus and Bifidobacterium bifidum. These bacteria improve the microbial spectrum in the gut and thus contribute to the following effects.
1. Decrease the risk of colon cancer.
2. Decrease cholesterol absorption.
3. Prevent diarrhoeal diseases by increasing the immunity power.

Bacteria are single-celled prokaryotes (cells without nuclei). They are considered to be the first living organisms on earth. Bacteria are grouped under the Kingdom Monera. The study of bacteria is called Bacteriology. The size of bacteria range from 1 μm to 5 μm (micrometer). Bacteria are of two types based on respiration


i) Aerobic bacteria (requires oxygen)
ii) Anaerobic bacteria (Does not requires oxygen)
Cell Structure:
A bacterium has on outer covering known as the cell wall. Nuclear material is represented by a nucleoid without nudear membrane. An extra chromosomal DNA called plasmid is present in the cytoplasm. Protein synthesis is carried out by 70 S ribosomes. Other cell organelles (mitochondria, golgi body, endoplasmic reticulum etc.) are absent flagella aid in locomotion.

(i) Spoilage of food and vegetables:
Food spoilage may be defined as any change that causes food unfit for human consumption
The chemical reactions catalyzed by the enzymes result in the degradation of food quality such as development of bad tastes and odour, detorioration and loss of nutrients.
Eg: Rotten eggs develop a bad smell due to the formation of hydrogen sulphide gas.
Decaying of vegetables and fruits due to microbes.
(ii) Rancidity of fishes and meat:
Fishes and meat contains high levels of polyunsaturated fatty acids.
These fatty acids undergo oxidation when exposed to air or light.
It results in bad odour. This process is called rancidity.
(iii) Apples and fruits turn brown when cut:
It is due to chemical reaction with oxygen in air.
The cells of apples contain an enzyme called tyrosinase that when in contact with oxygen catalyses a biochemical reaction of plant's phenolic compounds to brown pigments known as melanins.

Experiment:
i) Arrange some notebooks one over the other to form a platform on a table.
ii) Keep a wide scale, as a slide, such that one of its ends rests on the pile of books:
iii) Take different kinds of materials like cotton cloth, plastic paper, newspaper, writing pad etc.
iv) Place some glass marbles in a bowl placed on the table.
v) First, keep a rectangular piece of paper near the end of the scale which is in contact with the table.
vi) Now release a glass marble from the top end of the scale such that it rolls down the scale.
vii) Allow the marble to roll over the piece of paper and finally, come to rest.
viii) Measure the distance travelled by the marble over the paper using the meter scale.
ix) Replace the rolling surface by placing the plastic sheet, wooden plank, cotton cloth etc.
x) In each trial measure the distance travelled by the glass marble.
xi) Tabulate the distance covered by the marble over each surface.

Inference:
The marble covers a lesser distance over the cotton cloth in comparison with the distance it covers over the glass plate.
Reason:
i) A rough surface like the cotton cloth offers more frictional force. So the marble moves slowly and cover a minimum distance.
ii) The smooth surface of glass, offers lesser friction. So, the glass marble travels a greater distance over it.
Result:
i) The effect of the force of friction increases as the roughness of the surface increases.
ii) Thus friction depends on the nature of a surface.

Types of clocks:
I. Types of clocks based on display
(i) Analog clocks:
1. It looks like a classic clock.
2. It has three hands such as hours hand, minutes hand and seconds hand to show the time
3. Analog clocks can be driven either mechanically or electronically.
(ii) Digital clocks:
1. A digital clock displays the time directly
2. It shows the time in numerals or other symbols.
3. It may have a 12 hours or 24 hours display,
II. Types of clocks based on working mechanism
(i) Quartz clock:
1. These clocks are activated by electronic oscillations which are controlled by a 'quartz crystal'.
2. The quartz clock is more accurate than the mechanical clock.
3. Accuracy - One second in every 109 seconds.
(ii) Atomic clock:
1. These clocks are making use of periodic vibrations occurring within the atom
2. Accuracy - One second in every 10 seconds.
3. Atomic clocks are used in Global positioning System, Global Navigation Satellite System and International time distribution services.