HEAT

Heat- is a form of energy that gives us the sensation of hot or cold.

Temperature- The degree of hotness or coldness of a body.

1. With the help of a diagram, explain the structure of a clinical thermometer.

Ans A clinical thermometer is used for measuring human body temperature. It consists of a long narrow uniform glass tube. It has a bulb at one end. This bulb contains mercury. Outside the bulb, a small shining thread of mercury can be seen. A kink prevents mercury level from falling on its own. the thermometer scale is marked from 98°F to 108°F or 35°C to 42°C on the glass.

 

2. State the precautions to be taken while using a clinical thermometer.

Ans

•  Thermometer should be washed before and after use, preferably with an antiseptic solution.
• Ensure that before use the mercury is below 35°C or 104°F.
• Read the thermometer keeping the level of mercury along the line of sight.
• Handle the thermometer with care. If it hits against some hard object, it can break.
• Don't hold the thermometer by the bulb while reading it.

3. What is the normal temperature of human body?

Ans The normal human body temperature is 37°C. The normal human body temperature is the average body temperature of large number of healthy people.

4. Why is the range of a clinical thermometer is from 35°C to 42°C?

Ans The clinical thermometer is designed to measure the temperature of human body only. the temperature of human body normally doesn't go below 35°C or above 42°C.

5. Explain the structure of a laboratory thermometer with the help of a diagram.

Ans Laboratory thermometer is used to measure temperature of different substances in the laboratory. the scale in this thermometer ranges from 0° to 100°C.

The lower fixed point is the temperature at which pure ice melts (melting point of ice/ 0°C). And the upper fixed point is the temperature at which pure water boils. (boiling point of water/ 100°C)

Mercury/ thermometric liquid

Mercury is taken in a thermometer because:-

• It doesn't stick to the glass.
• It is a good conductor of heat.
• Its silvery shine helps us to see the temperature accurately.
• Its expansion is uniform.
• It doesn't vaporise easily.

Transfer of Heat

Heat transfer from a hot body to a cold body. It can take place in three different ways-

• Conduction
• Convection
• Radiation

Conduction

is the process of transfer of heat from one particle to the next particle in a solid without the movement of particles from their position.

Conductors and Insulators

Materials which allow heat to pass through them easily are called Conductors. Eg Iron, Aluminium, Mercury (liquid metal)
Materials which don't allow heat to pass through them easily are called insulators or poor conductors. eg plastic, cotton, wool, etc.

Applications for Conductors and Insulators

• Cooking utensils are made of different metals which are good conductors of heat. This helps in saving fuel.
  

• The handles of cooking utensils are made of wood or plastic as both are insulators of heat.
  
• Two thin blankets are warmer than one thick blanket because a layer of air is trapped in between the thin blankets and doesn't allow body heat to escape.
• We wear woolen cloths in winter because wool is a poor conductor of heat. The wool fibres trap air inbetween them which prevents lose of body heat.
• Insulators like bricks and mud are used in the construction of our houses.

Convection

The process of transfer of heat through a fluid (in liquids and gases) due to the actual movement of particles of the medium is called convection.

2. With the help of a diagram explain the formation of sea breeze and land breeze.

Ans

• During the day the land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. This cycle is called Sea breeze.
• At night the water cools down more slowly than the land. So, the cool air from the land moves towards the sea. This is called Land breeze. 

Applications of Convection

• Ventilation are provided in the room near the ceiling so that hot air escapes and cold air from outside enters the room through windows and cold air from outside enters the room through windows and doors.
  
• Room heaters are installed at the ground level. The warm air rises up, cold air comes down making the romm warm.
  
• Chimneys are provided for hot air to escape out.
  

Radiation

The process of transfer of heat in which a medium in not necessary and heat is directly transferred from a hot body to the cold body.

Application of Radiation

• Light coloured clothes are preferred in summer because they absorb less heat energy than darker clothes or reflect most of the heat that falls on them.
  
•  Solar cookers and heaters are painted black to absorb maximum heat.
  
• During summer buildings are painted in light colours because they reflect maximum heat that falls on them.
  
• Room heaters have shiny reflectors to reflect heat into the room.
  
• The radiators of auto mobile engines are painted black because black surfaces are good radiators of heat.
   

 Conduction   

• Heat is transmitted by the molecules in the medium.
• Medium is necessary.
• Slower process.
• Heat travels in any path.

Convection

• Heat is transmitted by the movement of molecules from one part to another in the medium.
• Medium is necessary.
• Slow process.
• Heat travels in any path.

Radiation

• Heat is transmitted with or without a medium.
• Heat can pass through vaccum
• Quick process
• Heat travels in straight path.

NOTE

• SI unit of heat energy = Joule
• SI unit of temperature = Kelvin (K)
• Least count (L.C) = Value of one division = Difference between two bigger divisions/ No of divisions between them
• The commonly used temperature scales are Celsius and Fahrenheit and Kelvin scale.

Conversion Formula                                                                                        

To Fahrenheit  = C*( 9/5 )+32

To Celsius = ( 5/9 )*( F-32 )

Eg 

Convert 86°F to °C

Formula = ( 5/9 )*( F-32 ) =

( 5/9 )*( 86-32 ) =

( 5/9 )*( 54/1 ) =

= 30°C

Express 45°C in °F

C*( 9/5 )+32 =

45*( 9/5 )+32 =

81+32 =

113°F

LIGHT

Light is a form of energy which gives us the sensation of vision.

Rectilinear propagation of light- The property of light travelling in a straight line is called rectilinear propagation of light.

Ray of light- Direction of light to which light travels.

Beam of light- It is the stream of light and is shown by a number of rays all of which may be parallel or diverging or converging.

Reflection of light- The process of sending back the rays of light which fall on the surface of an object.

Mirror- Any polished or shiny surface can act as a mirror.

Lateral inversion- In an image formed by a mirror, the left side of the object is seen as the right side of the image and right side of the objects is seen as the left side of the image.

Write the differences between real image and virtual image.

REAL IMAGE

➤It can be obtained on a screen.

➤It is inverted.

➤It is formed when rays of light actually meet at a point on the screen after reflection.

VIRTUAL IMAGE

➤It can't be obtained on a screen.

➤It is erect.

➤It is formed when rays of light do not meet at a point but appear to meed after reflection.

Characteristics of the image formed by a plane mirror.

➤The image formed by a plane mirror is virtual and erect.

➤It is of the same size as the object.

➤The image is at the same distance behind the mirror as the object is in front of it.

➤It is laterally inverted.

Spherical Mirror- A smooth and polished curved surface which reflects light. It is a part of a sphere.

Convex Mirror(draw diagram)- IF the reflecting surface of a spherical mirror is curved outward, it is called a convex mirror.

Concave Mirror(draw diagram)- IF the reflecting surface of a spherical mirror is curved inwards, it is called a concave mirror.

Characteristics of an image formed by a concave mirror.

➤The nature and size of the image formed by a concave mirror depends on the distance of the object from the mirror.

➤For very distant objects, a concave mirror produces a real and inverted image, which is smaller than the object.

➤As the distance of the object is reduced gradually, by bringing it more and more towards the concave mirror the image remains real and inverted but its size is increasing. When the object is placed very close to the concave mirror, the image formed is virtual anderect and enlarge.

Uses of concave mirror➤The reflectors of torches, headlight of cars and scooters are concave in shape.
➤Dentist use concave mirrors to see enlarged image of the teeth.
➤They are used as shaving mirrors and makeup mirrors.
➤Doctors use concave mirrors for examining eyes, nose, ears and throat.
➤Concave mirrors can concentrate the suns rays at a point. So, they are used in solar power projects.

Characteristics of an image formed by a convex mirror.➤The image formed by a convex mirror is virtual, erect and diminished.

Uses of Convex mirror
➤Convex mirror is used as rear view in cars or vehicles. They give a wider field of view. As compared to that of a plane mirror. Hence the driver of the car can see a larger area behind the car when he looks into the rear view mirror in front of him. It also gives an erect image smaller than the object.
➤They are used in supermarkets parking lot etc to get a larger field of view. 

"OBJECTS IN THE MIRROR ARE CLOSER THAN THEY APPEAR." Why is this warning message put on rear view mirrors of cars?
Convex mirrors give a wider field of view and also makes objects appear smaller. Since smaller appearing objects seem farther away than they actually are, a driver might make a mistake during lane change assuming that adjacent vehicle is at a safe distance behind when in fact it is quite closer.

Lenses- A piece of transparent material (glass or plastic) which has one or two spherical surfaces.

Converging and diverging lenses. (draw diagram)
A convex lens converges (bend inward) the light generally falling on it and it is called converging lens. A concave lens diverges (bends outward) the light and is called diverging lens.

CONCAVE LENS
➤A lens which is thicker at the edges and thinner in the middle.
➤When held close to the object, it forms a virtual, erect and diminished image.
➤When held away from the object it forms virtual, erect and diminished image for all positions of the object.

CONVEX LENS
➤A lens which is thicker in the middle and thinner at the edges.
➤When held close to the object it forms a virtual, erect and magnified image.
➤When held away from the object it forms real and inverted image. The image may be diminished or magnified or of same size depending upon the position of the object form eh lens.

White light-Any light which is composed of 7 colours.

Dispersion of light- The splitting of white light into 7 different colours.

Spectrum- The band of 7 colours. (VIBGYOR)

Causes of Dispersion
Light of all colours travel at the same speed, but in any transparent medium like water or glass, the light of different colours travel with different speed (wavelength) Red light travels the fastest while the violet light travels the slowest. As a result red bends the least and violet the most.

Splitting of light using prism(draw diagram)
Take a glass prism. Allow a narrow beam of sumlight through a small hole in the window of a dark room to fall on one face of the prism. Let the light coming out of the other face of the prism fall on a white sheet of paper or on a white wall. We obsever that white light splits into 7 colours. This shows that the sunlight consist of 7 colours. The sunlight is said to be white light.

Newton's disc
Take a circular cardboard disc. Divide this disc into 7 segments. Paint the seven rainbow colours on these segments. Make a small hole at the centre of the disc. Fix the disk loosely on the tip of a refill of a pen. If you rotat the disc fast enough the colours merges and appear whitish.