7th Grade Science Term II Tamil Nadu PDF

www.tntextbooks.in GOVERNMENT OF TAMIL NADU STANDARD SEVEN TERM - II...

www.tntextbooks.in GOVERNMENT OF TAMIL NADU STANDARD SEVEN TERM - II VOLUME - 3 SCIENCE SOCIAL SCIENCE A publication under Free Textbook Programme of Government of Tamil Nadu Department of School Education Untouchability is Inhuman and a Crime VII Std Science Term-1 EM Introduction Pages.indd 1 20-07-2019 1.46.04 PM  www.tntextbooks.in Government of Tamil Nadu First Edition - 2019 (Published under New Syllabus in Trimester Pattern) NOT FOR SALE Content Creation The wise possess all State Council of Educational Research and Training © SCERT 2019 Printing & Publishing Tamil NaduTextbook and Educational Services Corporation www.textbooksonline.tn.nic.in II VII Std Science Term-1 EM Introduction Pages.indd 2 20-07-2019 1.46.05 PM  www.tntextbooks.in The Science textbook for standard Seven has been prepared following the guidelines given in the National Curriculum Framework 2005. The book enables the reader to read the text, comprehend and perform the learning experiences with PREFACE the help of teacher. The Students explore the concepts through activities and by the teacher demonstration. Thus the book is learner centric with simple activities that can be performed by the students under the supervision of teachers.  The Second term VII Science book has six units. HOW  Two units planned for every month TO USE including computer science chapter has THE BOOK? been introduced.  Each unit comprises of simple activities and experiments that can be done by the teacher through demonstration if necessary student’s can perform them.  Colorful info-graphics and info-bits enhance the visual learning.  Glossary has been introduced to learn scientific terms.  The “Do you know?” box can be used to enrich the knowledge of general science around the world.  ICT Corner and QR code has been introduced in each unit for the first time to enhance digital science skills. Lets use the QR code in the text books ! How ?  Download the QR code scanner from the Google play store/ Apple App Store into your Smart phone.  Open the QR code scanner application  Once the scanner button in the application is clicked, camera opens and then bring it closer to the QR code in the text book.  Once the camera detects the QR code, a URL appears in the screen.  Click the URL and go to the content page. III VII Std Science Term-1 EM Introduction Pages.indd 3 20-07-2019 1.46.05 PM  www.tntextbooks.in Table of Contents Unit 1 Heat and Temperature.................................................... 1 Unit 2 Electricity.......................................................................12 Unit 3 Changes Around Us.......................................................36 Unit 4 Cell Biology...................................................................57 Unit 5 Basis of Classification....................................................70 Unit 6 Digital Painting.............................................................87 E - book Assessment DIGI links Let's use the QR code in the text books! Download DIKSHA app from the Google Play Store. Tap the QR code icon to scan QR codes in the textbook. Point the device and focus on the QR code. On successful scan, content linked to the QR code gets listed. Note: For ICT corner, Digi Links QR codes use any other QR scanner. IV VII Std Science Term-1 EM Introduction Pages.indd 4 20-07-2019 1.46.05 PM  www.tntextbooks.in Unit 1 Heat and Temperature Learning Objectives  To understand the working principle of thermometer  To measure temperature using thermometer  To know about Thermometric Liquids  To differentiate between Clinical and Laboratory Thermometer  To know the various units of temperature  To convert a temperature from a thermometer scale to others. 1 VII Std Science Term-2 EM Unit 1.indd 1 20-07-2019 1.46.38 PM  www.tntextbooks.in Introduction 1.3 Measuring Temperature You shiver The temperature of the when it is cold object is well approximated outside and with the kinetic energy of sweat when it the substances. The high is hot outside, temperature means that the but how can molecules within the object you measure those weather temperatures? are moving at a faster rate. Temperature is involved in many aspects of our But the question arises, how to measure daily lives, including our own bodies and health; it? Molecules in any substance are very small the weather; and how hot the stove must be in to analyze and calculate its movement (Kinetic order to cook food. energy) in order to measure its temperature. You must use an indirect method to measure the The measurement of warmness or coldness of kinetic energy of the molecules of a substance. a substance is known as its temperature. We studied that solids expands when heat is It is a measure of the average kinetic energy supplied to it. Like solid substances, liquids are of the particles in an object. Temperature is also affected by heat. To know this let us do the related to how fast the atoms within a substance activity 1. are moving. In a thermometer, when liquid gets heat, it expands and when it is cooled down, it The gas particles on the right have more kinetic contracts. It is used to measure temperature. energy than those on the left. So, the gas on the right is at a higher Like solid and liquid objects, the effect of temperature. heat is also observed on gaseous objects. 1.4 Thermometer: 1.2 Temperature Units: Thermometer is the most common instrument There are three units which are used to to measure temperature. measure the temperature: Degree Celsius, Fahrenheit and Kelvin. There are various kinds of thermometers. Some of them are like glass tubes which look Degree Celsius: Celsius is written as°C and read thin and are filled with some kind of liquid. as degree. For example 20°C; it is read as twenty degree Celsius. Celsius is called as Centigrade as Why Mercury or Alcohol is used in well. Thermometer? Mostly Alcohol and Mercury are used in Fahrenheit: Fahrenheit is written as °F for thermometers as they remain in liquid form example 25°F; it is read as twenty five degree even with a change of temperature in them. Fahrenheit. A small change in the temperature causes Kelvin: Kelvin is written as K. For example change in volume of a liquid. We measure this 100K; it is read as hundred Kelvin. temperature by measuring expansion of a liquid  The SI unit of temperature is kelvin (K). in thermometer. 2 VII Std Science Term-2 EM Unit 1.indd 2 20-07-2019 1.46.38 PM  www.tntextbooks.in ACTIVITY 1 What is required? A small glass bottle, a rubber cork, an empty refill, water, colour, a candle, a fork, a paper. What to do? Take a small glass bottle. Fill it with coloured water. Make hole at the centre of the rubber cork. Pass empty refill from the hole of the rubber cork. Make the bottle air tight and observe the water raised in the refill. Make a scale on paper, place it behind the refill and note down the position of the surface of water. Hold bottle with fork and supply heat to it with candle. Then observe. Empty Refill What is the change in the surface of water? ---------------------------------------------------- Rubber cork Fork Stop the supply of heat. When water is cooled, observe the surface Water of water in the refill, what change takes place? Why? A small glass bottle ---------------------------------------------------- When, a liquid is heated, it expands and when it is cooled down, Candle it contracts. Properties of Mercury:- It can be coloured brightly and hence is Its expansion is uniform. (For equal amounts easily visible. of heat it expands by equal lengths.) 1.4 Types of Thermometers It is opaque and shining. There are different It does not stick to the sides of the glass tube. types of thermometers for It is a good conductor of heat. measuring the temperatures of different things like air, It has a high boiling point (357°C) and a low our bodies, food and many freezing point (−39°C). Hence a wide range other things. Among these, of temperatures can be measured using a the commonly used thermometers are clinical mercury thermometer thermometers and laboratory thermometers. Properties of Alcohol The freezing point of alcohol is less than 1.4.1 Clinical Thermometer −100°C. So it can be used to measure very These thermometers are used to measure low temperatures. the temperature of a human body, at home, clinics and hospitals. All clinical thermometers Its expansion per degree Celsius rise in have a kink that prevents the mercury from temperature is very large. 3 VII Std Science Term-2 EM Unit 1.indd 3 20-07-2019 1.46.39 PM  www.tntextbooks.in ACTIVITY 2 What is required? A big bottle, a balloon, threads, candle, water, fork What to do? Take one big bottle, and fill some water in it. Attach one balloon on the mouth of bottle and fix it with thread. Hold bottle with a fork. Heat the bottle with a candle and take Baloon observation. What change occurs in the state of balloon after heating the bottle? -------------------------------------------------------- Tongs What change occurs in the state of balloon after heating the bottle? Why? ------------------------------------------------ Now, let the bottle get cooled down. Big bottle What change occurs in the state of balloon after bottle gets cool down? Why? ----------------------------------- When gases substance gets heat, it expands; when it cools it contracts. Candle Why does a tyre get burst in summer? ---------------------------------- flowing back into the bulb when the Before use, the mercury level should be thermometer is taken out of the patient’s below 35°C or 94°F. mouth, so that the temperature can be Do not hold the thermometer by its bulb. noted conveniently. There are temperature Keep the mercury level along your line of scales on either side of the mercury sight and then take the reading. thread, one in Celsius scale and the other in Fahrenheit scale. Since the Fahrenheit Handle the thermometer with care. If it hits scale is more sensitive than the Celsius against some hard object, it may break. scale, body temperature is measured in Do not place the thermometer in a hot flame F only. A clinical thermometer indicates or in the hot sun. temperatures from a minimum of 35°C or 94°F to a maximum of 42°C or 108°F. 1.4.2 Laboratory Thermometers Laboratory thermometers are used to Precautions to be Followed While Using a measure the temperature in school and other Clinical Thermometer laboratories for scientific research. They are  e thermometer should be washed before Th also used in the industry as they can measure and after use, preferably with an antiseptic temperatures higher than what clinical solution. thermometers can record. The stem and the Jerk the thermometer a few times to bring bulb of a lab thermometer are longer when the level of the mercury down. compared to that of a clinical thermometer and 4 VII Std Science Term-2 EM Unit 1.indd 4 20-07-2019 1.46.39 PM  www.tntextbooks.in there is no kink in the lab thermometer. ACTIVITY 3 A laboratory thermometer has only the Celsius scale ranging from −10°C to Measure your body temperature 110°C. Wash the thermometer preferably with an antiseptic solution. Hold it firmly by the end Precautions to be Followed While and give it a few jerks. These jerks will bring Using a Laboratory Thermometer the level of Mercury down. Ensure that Do not tilt the thermometer while it falls below 35°C (95°F). Now place the measuring the temperature. Place it thermometer under your tongue or arm pit. upright. After one minute, take the thermometer out and note the reading. It tells you your body Note the reading only when the bulb has been surrounded by the temperature. What did you record as your body temperature?__________ substance from all sides. In humans, the average ACTIVITY 4 internal temperature is Use of Laboratory thermometer 37°C (98.6°F), though it varies among individuals. Take some water in a beaker. However, no person always has exactly Take a laboratory thermometer and immerse its bulb end in water; holding the same temperature at every moment of it vertically. Ensure to dip whole portion the day. Temperatures cycle regularly up of bulb end. The bulb end should not and down through the day according to touch the bottom or side of the beaker. activities and external factors. Observe the movement of rise of mercury. When it becomes stable, take the reading of the thermometer. Repeat this with hot water and take the reading. Difference between clinical and laboratory thermometer Clinical Thermometer Laboratory Thermometer Clinical thermometer is scaled from 35°C to 42°C Laboratory thermometer is generally scaled from or from 94°F to 108°F. 10°C to 110°C. Mercury level does not fall on its own, as there is a kink near the bulb to prevent the fall of mercury Mercury level falls on its own as no kink is present. level. Temperature is read while keeping the thermometer Temperature can be read after removing the in the source of temperature, e.g. a liquid or any thermometer from armpit or mouth. other thing. To lower the mercury level jerks are given. No need to give jerk to lower the mercury level. It is used for taking the body temperature. It is used to take temperature in laboratory. 5 VII Std Science Term-2 EM Unit 1.indd 5 20-07-2019 1.46.39 PM  www.tntextbooks.in 1.4.3 Digital Thermometer any object other than human body. Also we are Here is a lot of concern advised to avoid keeping it in the sun or near a over the use of mercury in flame. Why? thermometers. Mercury A Clinical thermometer has small is a toxic substance and temperature range. The glass will crack/ burst is very difficult due to excessive pressure created by expansion to dispose of if a of mercury. thermometer breaks. These days, digital thermometers are available which do Maximum _ minimum thermometer not use mercury. Instead, it has a sensor The maximum and which can measure the heat coming out minimum temperatures of from the body directly and from that the previous day reported in weather can measure the temperature of the reports are measured by a thermometer body. called the maximum - minimum Digital thermometers are mainly thermometer. used to take the body temperature. ACTIVITY 5 1.5 Scales of thermometers Celsius scale Use of Digital thermometer Celsius is the common unit of measuring Tip Probe Display temperature, termed after Swedish astronomer, Anders Celsius in 1742, before that it was On/Off Button known as Centigrade as thermometers using 1. Wash the tip with warm (not hot), soapy this scale are calibrated from (Freezing point water. of water) 0°C to 100°C (boiling point of water). 2. Press the "ON" button. In Greek, ‘Centium’ means 100 and ‘Gradus’ 3. Insert the tip of the thermometer into the means steps, both words make it centigrade and mouth, bottom, or under the armpit. later Celsius. 4. Hold the thermometer in place until it Fahrenheit Scale beeps (about 30 seconds). Fahrenheit is a Common unit to measure 5. Read the display. human body temperature. It is termed after the 6. Turn off the thermometer, rinse under name of a German Physicist Daniel Gabriel water, and put it away in a safe place. Fahrenheit. Freezing point of water is taken as 32°F and boiling point 212°F. Thermometers with Caution Fahrenheit scale are calibrated from 32°F to 212°F. Alex wanted to measure the temperature Kelvin scale of hot milk using a clinical thermometer. His Kelvin scale is termed after Lord Kelvin. teacher stopped him from doing so. It is the SI unit of measuring temperature and We are advised not to use a clinical written as K also known as absolute scale as it thermometer for measuring the temperature of starts from absolute zero temperature. 6 VII Std Science Term-2 EM Unit 1.indd 6 20-07-2019 1.46.40 PM  www.tntextbooks.in Temperature in Celsius scale can be easily converted to Fahrenheit and Kelvin scale as discussed ahead Fahrenheit / °F Celsius / °C Kelvin / K 212°F 100°C 373.15 K Boiling Point of Water 180°F 100°C 100 K Freezing Point of Water 32°F 0°C 273.15 K 32°F 273K 0°C Relation between Fahrenheit scale and Celsius scales is as under. (F 32) C = , K = 273.15 +C 9 5 The equivalence between principal temperature scales are given in Table for some temperatures. Celsius scale Temperature Farenheit scale (°F) Kelvin scale (K) (°C) Boiling point of water 100 212 373.15 Freezing point of water 0 32 273.15 Mean temperature of 37 98.6 310.15 human body Room temperature 72 23 296.15 (Average) 7 VII Std Science Term-2 EM Unit 1.indd 7 20-07-2019 1.46.40 PM  www.tntextbooks.in HEAT AND TEMPERATURE KEY CONTRIBUTORS 1032 KELVIN Temperature of the Universe in the earliest moments after the Big Bang 373.15 KELVIN Boiling point of water Lord Kelvin Anders Celsius 100 0C, 212 0F 329.85 KELVIN Hottest natural temperature ever recorded on Earth. 56.7 0C, 134.06 0F 310.15 KELVIN Average human body temperature Gabriel Fahrenheit Rankine 37 0C, 98.6 0F 273.15 KELVIN Freezing point of water 0 0C, 32 0F 178.45 KELVIN Coldest natural temperature ever recorded on Earth -94.7 0C, -138.460F 1 KELVIN The Boomerang Nebu- la maintains the coldest known natural temperature in the universe -272.15 0C, -457.87 0F Low Temperature High Temperature 0 KELVIN Absolute zero Temperature Thermometer Liquid Expands when Heated -273.15 0C, -459.67 0F 8 VII Std Science Term-2 EM Unit 1.indd 8 20-07-2019 1.46.41 PM  www.tntextbooks.in 36 Most of the people in the C = 5x = 20°C 9 world use the Celsius scale K = C + 273.15 = 20 + 273.15 = 293.15 to measure temperature for day to day purpose. The Thus, the temperature in Celsius = 200C Kelvin scale has been designed in such a and in Kelvin = 293.15 K way, it is not only an absolute temperature 2. At what temperature will its value be same in scale, but also 1°C change is equal to a 1K Celsius and in Fahrenheit? change. This makes the conversion from Celsius to absolute temperature scale Given : If the temperature in Celsius is C, then (Kelvin scale) easy, just the addition or the temperature in Fahrenheit (F) will be same, subtraction of a constant 273.15 i.e. F = C. (F-32) = C 9 5 But in United States they prefer to (or) use the Fahrenheit scale. The problem is, (C-32) C = converting Fahrenheit to absolute scale 9 5 (Kelvin) is not easy. (C 32) x 5 = C x 9 To sort out this problem they use 5C 160 = 9 C The Rankine scale. It named after the 4 C = 160 Glasgow University engineer and physicist C = F = 40 Rankine, who proposed it in 1859. It is an The temperatures in Celsius and in absolute temperature scale, and has the Fahrenheit will be same at – 40 property of having a 1°R change is equal to 3. Convert the given temperature : a 1°F change. Fahrenheit users who need 1) 45°C =........ °F 2) 20°C =........ °F to work with absolute temperature can be 3) 68°F =........ °C 4) 185°F =........ °C converted to Rankine by R= F+ 459.67 5) 0°C =........ K 6) -20°C =........ K 7) 100 K =........ °C 8) 272.15 K =........ °C 1.6 Numerical Problems POINTS TO REMEMBER Solved examples 1. The measurement of warmness or coldness 1. How much will the temperature of 68°F be in of a substance is known as its temperature. Celsius and Kelvin? 2. There are three units which are used to Given : measure the temperature: Degree Celsius, Temperature in Fahrenheit = F = 68°F Fahrenheit and Kelvin. Temperature in Celsius =C=? 3. The SI unit of temperature is Kelvin (K). Temperature in Kelvin =K=? 4. In a thermometer, when liquid gets heat, (F-32) C = it expands and when it is cooled down, it 9 5 (68-32) C contracts. It is used to measure temperature. = 9 5 9 VII Std Science Term-2 EM Unit 1.indd 9 20-07-2019 1.46.42 PM  www.tntextbooks.in 5. Relation between Fahrenheit scale and c. + 127. + 400.15 Celsius scales is d. + 450 + 733.15 (F-32) C = II. Fill in the blanks 9 5 K = 273.15 +C 1. Doctor uses _____________ thermometer to measure the human body temperature. 2.  At room temperature Mercury is in EVALUATION _______________ state. 3. Heat energy transfer from _______________ to ___________ 4.  7°C temperature is _______________ than I. Choose the correct answer 0°C temperature. 1. International unit of measuring temperature is.......................... 5.  The common laboratory thermometer is a ______________ thermometer a. Kelvin b. Fahrenheit c. Celsius d. Joule III. Match the following 2. In thermometer when bulb comes in contact i) Clinical thermometer A form of energy with hot object, liquid inside it ii) Normal temperature a. expands b. contracts 100°C of human body c. remains same d. none of above iii) Heat 37°C 3. The body temperature of a healthy man is; iv) Boiling point of water 0°C a. 0°C b. 37°C c. 98°C d. 100°C v) Melting point of water Kink 4. Mercury is often used in laboratory thermometers because it IV. Give very short answer _______________________ 1. Temperature of Srinagar (J&K) is 4°C and in Kodaikanal is 3°C which of them has a. is a harmless liquid greater temperature ? What is the difference b. is silvery in colour and is attractive in between the temperatures of these two appearance places? c. Expands uniformly 2. Jyothi was prepared to measure the temperature d. is a low cost liquid of hot water with a clinical thermometer. Is 5. Which of the following temperature it right or wrong? Why? conversions is incorrect 3. A clinical thermometer is not used to measure K ( Kelvin) = °C ( Celsius) + 273.15 the temperature of air, why? °C K 4.  What is the use of kink in clinical a. -273.15 0 thermometer? b. -123. +150.15 5. Why do we jerk a clinical thermometer before we measure the body temperature? 10 VII Std Science Term-2 EM Unit 1.indd 10 20-07-2019 1.46.42 PM  www.tntextbooks.in V. Give short Answer VI. Give long answer 1. Why do we use Mercury in thermometers? 1. Draw the diagram of a clinical thermometer Can water be used instead of mercury? and label its parts. What are the problems in using it? 2. State the similarities and differences between 2. Swathi kept a laboratory thermometer in hot the laboratory thermometer and the clinical water for some time and took it out to read thermometer. the temperature. Ramani said it was a wrong way of measuring temperature. Do you agree VII. Higher Order Thinking questions with Ramani? Explain your answer. 1. What must be the temperature in Fahrenheit, so that it will be twice its value in Celsius? 3. The body temperature of Srinath is 99°F. Is he suffering from fever? If so, why? 2. Go to a veterinary doctor (a doctor who treats animals). Discuss and find out the normal temperature of domestic animals and birds. 11 VII Std Science Term-2 EM Unit 1.indd 11 20-07-2019 1.46.42 PM  www.tntextbooks.in Unit 2 ELECTRICITY Learning Objectives ™™ Understanding the flow of electric current and learning to draw the circuit diagram ™™ Understanding the difference between conventional current and electron flow. ™™ Understanding the different types of circuit based on flow of electricity and the connection of bulbs in a circuit ™™ Distinguishing a cell and a battery ™™ Understanding the effects of electric current and factors affecting the effect of electric current ™™ Applying their knowledge in identifying the components of electrical circuits. ™™ Understanding the discrimination between different type of circuits. ™™ Doing numerical problems and drawing the circuit diagram of their own. 12 VII Std Science Term-2 EM Unit 2 Folder.indd 12 20-07-2019 1.47.04 PM  www.tntextbooks.in Introduction of the atom is called the nucleus. The nucleus In 1882, when it was sun set in the west that consists of protons and neurtrons. Protons are miracle happened in New york city. When positively charged. Neutrons have no charge. Thomas Alva Edison gently pushed the switch Negatively charged electrons revolve around the on 14,000 bulbs in 9,000 houses suddenly got nucleus in circular orbits. Electricity is a form lighted up. It was the greatest invention to of energy that is associated with electric charges mankind. From then the world was under the that exists inside the atom light even in the night. Many countries began using electricity for domestic purposes. Seventeen years after the New York, in 1899 electricity first came to India. The Calcutta Electric Supply Corporation Limited commissioned the first thermal power plant in India on 17 April 1899 Around 1900s, a thermal power station was set up at Basin Bridge in Madras city and power was distributed to the government press, general hospital, electric tramways and certain residential areas in Madras. Today electricity is ACTIVITY 1 a common household commodity. Comb your dry hair. Immediately after combing the dry hair, bring the comb closer to the bits of paper. what will you observe? When you are getting up from the plastic chair , the nylon shirt seems to be stuck to the chair and make crackling sound. What is the reason for the creation of the sound? A balloon sticks to wall without any In your class 6, we learned about electricity adhesive after rubbing on your hand. and their sources. From operating factories, Do you know the reason for all? running medical equipments like ventilator, In all the above activities, when a body is rubbed against some other body become communications like mobile, radio and TV, charged. drawing water to the agricultural field and light up homes electricity is important. What Electric charge is measured in a unit called is electricity? We can see that. it is a form of coulomb. One unit of coulomb is charge of energy, like heat and magnetism. approximately 6.242×1018 protons or electrons. We have learnt that all materials are made Electrical charges are generally denoted by up of small particles called atoms. The centre the letter `q` 13 VII Std Science Term-2 EM Unit 2 Folder.indd 13 20-07-2019 1.47.05 PM  www.tntextbooks.in 2.1. Electric Current Worked example 2.1 The flow of electric charges constitute an If 30 coulomb of electric charge flows through electric current. For an electrical appliance a wire in two minutes, calculate the current in to work, electric current must flow through it. the wire? An electric current is measured by the amount Solution of electric charge moving per unit time at any Given : point in the circuit. The conventional symbol Charge (q) = 30 coulomb for current is ‘I’. Time (t) = 2 min x 60s = 120 s Cross section area Current I = q/t = 30C/120s = 0.25 A Electric 2.1.1.  Conventional Current and charge Electron Flow Unit of Electric Current The SI unit for measuring an electric current is the ampere, which is the flow of electric charge across a surface at the rate of one coulomb per second. I=q/t Where I ⇒ current (in Ampere - A) q ⇒ charge (in coulomb - c) t ⇒ time taken (in seconds - s) CURRENT I e e e CHARGE TIME q t Before the discovery of electrons , scientists believed that an electric current consisted of moving positive charges. 14 VII Std Science Term-2 EM Unit 2 Folder.indd 14 20-07-2019 1.47.05 PM  www.tntextbooks.in This movement of positive charges is called = 2 × 10-3 × 106 µA conventional current. = 2 × 103 µA After the electrons were discovered, it 0.002A = 2000 µA was known that electron flow actually takes place from the negative terminal to the positive 2.2. Potential difference (v) terminal of the battery. This movement is Electrical charges need energy to push them known as electron flow. along a circuit. Water always flows from Conventional current is in the higher to lower ground. direction opposite to electron flow. Similarly an electric charge always flows from a point at 2.1.2. Measurement of electric current higher potential to a point at Electric current is measured lower potential. using a device called ammeter. The terminals of an ammeter An electric current can flow only when there is are marked with + and - a potential difference (V) or P.D. sign. An ammeter must be connected in series in a circuit. The potential difference between any two points in the circuit is the amount of Instruments used to measure smaller energy needed to move one unit of electric currents, in the milli ampere or micro ampere charge from one point to the other. range, are designated as milli ammeters or micro ammeters. 2.2.1. Unit of potential difference 1 milliampere (mA) = 10-3 ampere. Did you ever notice the precautionary = 1/1000 ampere board while crossing the railway track 1 microampere (µA) = 10-6 ampere and the electrical transformer? What does the = 1/1000000 ampere word high voltage denotes? Worked Examples 2.2 If 0.002A current flows through a circuit, then convert the current in terms of micro ampere? Solution: Given that the current flows through the circuit is 0.002A We know that 1 A = 106 µA 0.002A = 0.002 × 106 µA 15 VII Std Science Term-2 EM Unit 2 Folder.indd 15 20-07-2019 1.47.06 PM  www.tntextbooks.in The term metioned in the board volt is The higher the resistance in a component, the the measurement for the electric potential higher the potential difference needed to move difference. electric charge through the component. We can The SI unit of potential difference is volt (V). express resistance as a ratio. potential difference between two points is Resistance of a component is the ratio of the measured by using a device called voltmeter. potential difference across it to the current flowing through it. R = V Water at the top of the waterfall has more I potential energy The S.I unit of resistance is ohm Greater the ratio of V to I, the greater is the resistance Electrical conductivity (σ) Electrical conductivity or specific conductance is the measure of a material's ability to conduct an electric current. It is commonly represented by the Greek letter σ (sigma) The S.I Unit of electrical conductivity is Siemens/meter(S/m) Water near the base of the waterfall has less potential energy Electrical resistivity (ρ) Electrical resistivity (also known as specific electrical resistance, or volume resistivity) The electric current flow from the higher is a fundamental property of a material that potential level to the lower potential level quantifies how strongly that material opposes the flow of electric current. The SI unit of is just like the water flow. electrical resistivity is the ohm-metre (Ω.m). 2.2.2.  Electrical conductivity and Material Resistivity (ρ) Conductivity (σ) Resistivity (Ω m) at 20°C (S/m)at 20°C Electric Silver 1.59x10-8 6.30x107 charge Copper 1.68x10-8 5.98x107 Annealed 1.72x10-8 5.80 x107 Copper Electrical component with resistance Aluminum 2.82x10-8 3.5x107 Resistance (R) 2.2.3. Analogy of Electric Current with An electrical component resists or hinders the Water Flow flow of electric charges, when it is connected in An electric current is a flow of electrons through a circuit. In a circuit component, the resistance a conductor (like a copper wire). We can't see electrons, however, we can imagine the flow of to the flow of charge is similar to how a narrow electric current in a wire like the flow of water channel resists the flow of water. in a pipe. 16 VII Std Science Term-2 EM Unit 2 Folder.indd 16 20-07-2019 1.47.06 PM  www.tntextbooks.in Let us see the analogy of flow of electric current ACTIVITY 2 with the water flow. Shall we produce electricity at our home? Water flowing through pipes is pretty good Materials required: mechanical system that is a lot like an electrical Zinc and copper electrodes, a light blub, circuit. This mechanical system consists of a connecting wires, and fruits such as pump pushing water through a closed pipe. lemons, orange, Imagine that the electrical current is similar apples, grapes, to the water flowing through the pipe. The and bananas. following parts of the two systems are related Procedure: The pipe is like the wire in the electric circuit 1. Set up a circuit and the pump is like the battery. as shown in The pressure generated by the pump drives figure water through the pipe. 2. Note the brightness of the blub when the The pressure is like the voltage generated by circuit is connected to a lemon. the battery which drives electrons through the 3. Repeat the experiment using the other electric circuit. fruits listed above. Do you notice the Suppose, there are some dust and rust that differences in the brightness of the plug up the pipe and slow the flow of water, bulb when it is connected to different fruits? Which fruit gives the greatest creating a pressure difference from one end to brightness? Why? (If you do not know the other end of the pipe. In similar way, the please get the appropriate reason from resistance in the electric circuit resists the flow your teacher) of electrons and creates a voltage drop from Inference: one end to the other. Energy loss is shown in In the above activity what makes enabled the form of heat across the resistor. the bulb to glow. Why there is a difference Current - Water Analogy in the brightness of the bulb? The reason is that the fruits which you have connected to the bulb produces the electric energy at different levels More water current Less water current The sources which produce the small amount of electricity for shorter periods More electric current Less water current of time is called as electric cell or electro chemical cells. Electric cell converts chemical energy into electrical energy 2.3. Sources of Electric current - In addition to electro chemical, we use electro Electro chemical cells or electric cells thermal source for generating electricity for An electric cell is something that pro- large scale use. vides electricity to different devices that It has two terminals. When electric cells are are not fed directly or easily by the supply used, a chemical reaction takes place inside the of ­electricity. cells which produces charge in the cell. 17 VII Std Science Term-2 EM Unit 2 Folder.indd 17 20-07-2019 1.47.07 PM  www.tntextbooks.in Primary Cell Secondary Cell Lithium Automobile Dry cell Button cells Alkaline cells cylindrical cells battery 2.3.1. Types of cell – primary cell and 2.3.2.Difference between primary cell secondary cell and secondary cell In our daily life we are using cells and batteries PRIMARY CELL SECONDARY CELL for the functioning of a remote, toys cars, clock, 1. The chemical The chemical cellphone etc. Event hough all the devices produces electrical energy, some of the cells reaction inside ­reaction inside the are reusable and some of them are of single use. the primary cell is secondary cell is Do you know the reason why? Based on their ­irreversible reversible type they are classified into two types namely – 2. It cannot be It can be recharged primary cell and secondary cell. recharged. 3. Examples of I t is used to operate Primary cell secondary cells are devices such as The dry cell commonly used in torches is an lead accumulator, mobile phones, example of a primary cell. It cannot be recharged Edison accumulator cameras, computers, after use. and Nickel – Iron and emergency accumulator. lights. Secondary cells 4. Examples- simple Examples of Secondary cells are used in automobiles and voltalic cell, Daniel secondary cells are generators. The chemical reaction in them cell, and lechlanche lead accumulator, can be reversed, hence they can be recharged. cell and dry cell Edison accumulator Lithium cylindrical cells, button cells and and Nickel – Iron alkaline cells are the other types that are in use. accumulator. ACTIVITY 3 2.3.3. Primary cell – simply Dry cell I am so exhausted. I am A dry cell is a type of chemical cell going to faint. What first commonly used in the common form batteries aid will you give me to wake up? for many electrical appliances. It is a convenient source of electricity available in portable and compact form. It was developed in 1887 by Yei Sakizo of Japan. 18 VII Std Science Term-2 EM Unit 2 Folder.indd 18 20-07-2019 1.47.09 PM  www.tntextbooks.in The dry cell is not really dry in nature but the quantity of water in it is very small, as the electrolyte is in the from of a paste. In other cells, the electrolyte is usually a solution 2.3.4. Batteries Dry cells are normaly used in small devices such as remote control for T.V., torch, camera and toys. Batteries are a collection of one or more cells A dry cell is a portable form of a leclanche cell. whose chemical reactions create a flow of It consists of zinc vessel which acts as a negative electrons in a circuit. All batteries are made up electrode or anode. The vessel contains a moist of three basic components: an anode (the ‘+’ paste of saw dust saturated with a solution of side), a cathode (the ‘–’ side), and some kind ammonium chloride and zinc chloride. of electrolyte. Electrolyte is a substance that The ammonium chloride acts as an electrolyte. chemically reacts with the anode and cathode. Electrolytes are substances that become 2.3.5. Invention of the Battery ions in solution and acquire the capacity to conduct electricity. One fateful day in 1780, Italian physicist, physician, The purpose of zinc chloride is to maintain biologist, and philosopher, Luigi the moistness of the paste being highly Galvani, was dissecting a frog hygroscopic. The carbon rod covered with a attached to a brass hook. As he brass cap is placed in the middle of the vessel. It touched the frog’s leg with an iron scapel, the acts as positive electrode or cathode. leg twitched. It is surrounded by a closely packed mixture Galvani theorized that the energy came from of charcoal and manganese dioxide (MnO2) in the leg itself, but his fellow scientist, Alessandro a muslin bag. Here MnO2 acts as depolarizer. Volta, believed otherwise. The zinc vessel is sealed at the top with pitch or shellac. A small hole is provided in it to allow Volta hypothesized that the gases formed by the chemical action to the frog’s leg impulses were escape. The chemical action inside the cell is the actually caused by different same as in leclanche cell. metals soaked in a liquid. He repeated the experiment using cloth soaked 19 VII Std Science Term-2 EM Unit 2 Folder.indd 19 20-07-2019 1.47.10 PM  www.tntextbooks.in in brine instead of a frog corpse, which resulted ACTIVITY 4 in a similar voltage. Volta published his findings Make your own switch in 1791 and later created the first battery, the Let us make a switch of voltaic pile, in 1800. our circuits. Take 10 cm The invention of the modern – long iron strip. Bend it battery is often attributed to twice as shown in figure. Alessandro Volta. It actually Now drive a nail into started with a surprising accident the bend of the wooden involving the dissection of a frog. block. Nail one end of the strip to the other end of the wooden block so that its free end rests just above the first nail without 2.4. ELECTRIC SWITCH touching it. Your switch is ready. Our country faces a shortage of electricity. So Would you like to test your switch? To do so, wastage of electricity means you are depriving first set up the circuit as shown in the figure. someone else of electricity. Your electricity bill goes up. So, we must use electricity very How would you use the switch to open or carefully and only when it is needed. We must close the circuit. use the electricity as long as we need it in our If the bulb in your circuit glows when the house hold activities. metal strip of your switch is pressed on the Can you remember what you did last year to nail and turns off when it is not, then your turn the current on or off? switch is working. The switch you made is a simple one. You may have seen many This time, we shall use different types of switches on switchboards a switch to turn the and appliances at your home and school. current on or off. You The switches are designed according to their may have used different usage, convenience and safety. But all of kinds of switches to turn them work on the same principle. Switch your household electric is a mechanical component that consists of appliances on or off. Switches help us to start or two or more terminals that are internally stop the appliances safely and easily. connected to a metal strip. Commonly used switches are listed below: 3.4.1. Electric Circuit Tapping Toggle Illuminated Circuit you made with switch is a simple circuit. key switch switch Plug Rocker Slide switch key switch 20 VII Std Science Term-2 EM Unit 2 Folder.indd 20 20-07-2019 1.47.11 PM  www.tntextbooks.in Circuit diagrams are used to draw Symbol Symbol Symbol Symbol open closed Battery Wire Bulb Switch Energy source Connects electrical Light source Controls the flow Has a positive components to one Consists of metal of electric current terminal and a another filament, glass bulb, negative terminal Usually insulated metal casing and with a rubber covering metal tip such as Electrial components are made up of Electric Circuits can be Closed Open No electric current Electric current flows flows in the electric Electric Electric in the electric circuit circuit conductors insulator Metal such as Wood, Glass copper, iron Bulb lights up Bulb does not Plastic, Rubber light up 21 VII Std Science Term-2 EM Unit 2 Folder.indd 21 20-07-2019 1.47.12 PM  www.tntextbooks.in 2.5. Electric circuit 2.5.1. Types of electrical circuits It is difficult to draw a realistic diagram of this In the above experiment, we make a circuit with circuit. The electrical appliances you use at home a bulb and a cell. We make only one kind of the have even more difficult circuits. Can you draw circuit with a cell and a bulb. But we can make realistic diagrams of such circuits which contain many types of circuits if we have more than one many bulbs, cells, switches and other components? bulb or cells by connecting these components in Do you think it is easy? It is not easy. different ways. Scientists have tried to make the job easier. 2.5.2. Series circuit They have adopted simple symbols for different Two kinds of circuits can be made with two components in a circuit. We can draw circuit bulbs and a cell. In this experiment we shall diagrams using these symbols. make one of them and study it. Symbols for bulbs, cells and switches are shown Look at the circuit with two bulbs, and a cell in figure. and a switch given here (Figure) In a cell, the longer line denotes the positive (+) terminal and the short line denotes the negative It is clear from the circuit diagram, that the (-) terminal. We shall use these symbols to two bulbs are connected one after the other. show components in the circuits we draw. Such The circuit diagram shows the sequence of the diagrams are called circuit diagrams. bulbs and cell, not their real position. The way All muscles of our bodies in which the bulbs have been connected in this move in response to electrical circuit is called series connection. impulses generated naturally Now make the circuit by joining the two bulbs in our bodies and cell. Do both the bulbs light up? Do both glow equally bright? If one glows less bright, will it shine more brightly if we change its place in the sequence? Change the sequence of bulbs and notice. Sometimes bulbs appear to be similar can differ from each other. So, similar looking bulb do not always glow equally bright when connected in series. The circuit can be broken at several places. For example, between the cell and the bulb, between the two bulbs etc. 22 VII Std Science Term-2 EM Unit 2 Folder.indd 22 20-07-2019 1.47.12 PM  www.tntextbooks.in 2.5.3 Parallel Circuit contains electrically charged particles. Many Figure - shows a circuit in which two bulbs of these particles are fixed to the atoms but in are connected in