Class 8 Unit-7 (Energy)

 

Unit-7 (Energy)

Introduction

Energy is the capacity of doing work. There are different forms of energy.

1.      Mechanical energy

2.      Heat energy

3.      Light energy

4.      Sound energy

5.      Electrical energy

6.      Magnetic energy

7.      Chemical energy

8.      Nuclear energy

 

Heat

-Heat is a form of energy that transfers from one body to another. It gives us the sensation of warmth.

-Heat is measured by a device called calorimeter.

-The SI unit of heat is Joule (J). Heat is also measured in calorie unit.

 

Temperature

-The measurement of hotness or coldness of a substance is known as its temperature.

-Temperature is measured by a device called thermometer.

-The SI unit of temperature is Kelvin (K).

-We often use two units of temperature, degree Celsius (⁰C) and degree Fahrenheit (⁰F)

 

Transfer of heat

Mechanism of Heat Transfer

Heat is the energy that flows from a hot object to a cold object. There are three main ways heat can be transferred:

1. Conduction
• Definition: Transfer of heat through a solid material without the movement of the material itself.
• How it happens: Heat is passed from one particle to another by vibration.
• Example: A metal spoon becomes hot when its one end is in hot water.
2. Convection
• Definition: Transfer of heat through a liquid or gas by the movement of the particles.
• How it happens: Hot particles rise and cold particles sink, creating a circular flow called a convection current.
• Example: Boiling water in a pot—hot water rises and cold water sinks.
3. Radiation
• Definition: Transfer of heat in the form of electromagnetic waves without needing any medium.
• How it happens: Heat can travel through empty space.
• Example: Sunlight warming the Earth.

 

Applications of radiation of heat in daily life

Ø The Sun heats the Earth through radiation.

Ø We feel warmth from a fire without touching it because of heat radiation.

Ø Microwave ovens cook food using heat radiation.

Ø Electric heaters warm a room by radiating heat.

Ø Solar panels absorb heat from sunlight through radiation.

 

Wave

A wave is a repeated disturbance that moves through a medium (like air, water, or a solid) and transfers energy from one place to another without moving the matter itself.

Types of Waves:

1. Mechanical Waves

o Need a medium (like air, water, or solid) to travel.

o Example: Sound waves, water waves, and waves on a string.

2. Electromagnetic Waves

o Do not need a medium; they can travel through empty space.

o Example: Light waves, radio waves, and X-rays.

3. Transverse Waves

o The particles of the medium move perpendicular to the direction of wave motion.

o Example: Light waves, waves on a string.

4. Longitudinal Waves

o The particles of the medium move in the same direction as the wave.

o Example: Sound waves, compressions in a spring.

 

Thermos

A thermos is a container which is used to keep hot drinks hot or cold drinks cold.

 

Greenhouse Effect:
The Greenhouse Effect is the process by which the Earth’s atmosphere traps heat from the Sun to keep the planet warm.

Impacts of Greenhouse Effect

1. Global Warming: The Earth’s temperature rises, causing hotter summers and warmer winters.
2. Melting of Ice Caps: Polar ice caps and glaciers melt, leading to rising sea levels.
3. Floods: Rising sea levels can cause flooding in low-lying areas.
4. Climate Change: Weather patterns become unpredictable, causing storms, droughts, and heavy rainfall.
5. Loss of Biodiversity: Plants and animals may lose their habitats and face extinction.
6. Agricultural Impact: Crops may fail due to extreme heat or irregular rainfall.
7. Health Problems: Heat waves and increased air pollution can harm human health.

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Methods to Reduce the Greenhouse Effect

1. Plant More Trees: Trees absorb carbon dioxide (CO₂), which reduces greenhouse gases.
2. Use Renewable Energy: Use solar, wind, and hydro power instead of coal, oil, and gas.
3. Save Energy: Switch off lights, fans, and electronic devices when not in use.
4. Use Public Transport: Walking, cycling, or using buses and trains reduces vehicle emissions.
5. Reduce, Reuse, Recycle: This decreases waste and lowers greenhouse gas emissions from landfills.
6. Use Energy-Efficient Appliances: Use LED bulbs, energy-saving fans, and refrigerators.
7. Reduce Fossil Fuel Use: Avoid excessive burning of coal, petrol, and diesel.
8. Promote Awareness: Educate people about climate change and the greenhouse effect.

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Applications of Greenhouse Effect

1. Greenhouses in Gardening: Greenhouses trap heat from sunlight, helping plants grow even in cold weather.
2. Solar Energy Devices: Solar panels absorb heat from the Sun, similar to how greenhouse gases trap heat.
3. Winter Warming: Glass windows in homes trap sunlight, keeping rooms warm in winter.
4. Preserving Food: Some food storage methods use heat trapping to maintain temperature, like in solar cookers.
5. Weather and Climate Studies: Understanding the greenhouse effect helps scientists predict climate changes and plan for agriculture.

 

Light

Light is a form of energy that allows us to see objects around us. It gives the sensation of vision to the eyes.

 

Luminous objects produce light themselves (like the Sun or a bulb).

Non-luminous objects do not produce light but can reflect light (like the Moon or a mirror).

 

Transparent Objects: Objects that allow all light to pass through them.

• Effect: You can see clearly through them.
• Examples: Glass window, clear water, clear plastic sheet.

 

Translucent Objects: Objects that allow some light to pass through, but not clearly.

• Effect: You cannot see clearly through them.
• Examples: Frosted glass, thin paper, lampshade.

 

Opaque Objects: Objects that do not allow any light to pass through.

• Effect: You cannot see through them at all.
• Examples: Wood, metal, wall, book.

 

Reflection of light

Reflection of light is the phenomenon when light rays fall on an object, their direction changes and they turn back.

Basic Terminologies of Reflection of Light

1.   Incident Ray – Ray coming from the source.

2.   Reflected Ray – Ray returns from the surface.

3.   Normal – Perpendicular line on the reflecting surface.

4.   Angle of Incidence (i) – Angle between the incident ray and normal.

5.   Angle of reflection – Angle between the reflected ray and normal.

6.   Reflecting surface – The surface from which the light is reflected.

7.   Glancing angle – Angle between the reflecting surface and incident ray.

8.   Angle of deviation – The angle between reflected ray and direction of incident ray.

 

Laws of reflection

1. The incident ray, reflected ray, and the normal all lie in the same plane.

2. The angle of incidence (i) is equal to the angle of reflection (r).

3. A normal incident ray returns back along the same path after reflection.

 

 

Real image and virtual image

Real Image

1. Formed when light rays actually meet.
2. Can be seen on a screen.
3. Usually upside down (inverted).
4. Can be smaller, same size, or bigger than the object.
5. Example: Image formed by a concave mirror or convex lens.

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Virtual Image

1. Formed when light rays only appear to meet.
2. Cannot be seen on a screen.
3. Always upright (same as the object).
4. Usually same size or bigger than the object.
5. Example: Image seen in a plane mirror.

 

 

Plane Mirror

A flat mirror that forms a virtual image of objects.

Characteristics of Image by Plane Mirror

1. The image is upright (same orientation as the object).
2. The image is virtual (cannot be projected on a screen).
3. The image is laterally inverted (left and right are reversed).
4. The image is of the same size as the object.
5. The image appears to be behind the mirror at the same distance as the object is in front.
6. The image is as far as it looks from the mirror.

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Spherical Mirrors

A spherical mirror is a mirror whose surface is part of a sphere.

Types of Spherical Mirrors:

1. Concave Mirror

§ Curved inward like the inside of a bowl.

§ Converges light rays to a point (focus).

§ Can form real or virtual images depending on the object’s position.

§ Uses: Makeup mirrors, shaving mirrors, torch reflectors, solar cookers.

2. Convex Mirror

§ Curved outward like the back of a spoon.

§ Diverges light rays (spreads them out).

§ Always forms virtual, upright, and smaller images.

§ Uses: Vehicle rearview mirrors, security mirrors.

 

 

1. Plane Mirror

• It is used as a looking glass.
• Used in periscopes to see over obstacles.
• Used in rearview mirrors of vehicles sometimes.
• Used in decoration and interior design to make rooms look bigger.

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2. Concave Mirror

• Used in makeup and shaving mirrors to get a magnified view.
• Used in torch and car headlights to focus light into a beam.
• Used in solar cookers to focus sunlight for cooking.
• Used in dentist’s mirrors to see small areas clearly.

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3. Convex Mirror

• Used as rearview mirrors in vehicles for a wider field of view.
• Used in road safety mirrors at sharp turns to see oncoming traffic.
• Used in security mirrors in shops to watch large areas.

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Sound

Sound is a form of energy that gives the sensation of hearing.

 

Sound wave

Sound wave is produced due to the vibration of medium. The wave carries energy from one point to another.

 

Characteristics of sound waves

1.      Amplitude of wave: Amplitude is the distance of maximum oscillation position from the mean position.

2.      Wavelength: The total distance travelled by a complete wave is called wavelength.

3.      Frequency: The number of waves produced per sound is called frequency.

4.      Speed of sound: Speed of sound is the distance travelled by a wave in one second.

 

Intensity of sound

The intensity of sound is defined as the energy transfer per second per unit area. It is denoted by I.

Intensity (I)

                   =             

              (I)=

 

Factors affecting the intensity of sound

Greater loudness of sound is called greater intensity and lower loudness of sound is called low intensity of sound. There are several factors that affect the intensity of sound:

1.      Amplitude of sound

2.      Distance of listener from the source

3.      Density of the medium

4.      Area of the vibrating body

 

Does intensity of sound depend on its frequency?

The intensity of sound depends on its amplitude (loudness) and not on its frequency (pitch).

 

Infrasonic, audible, ultrasonic

1. Infrasonic Sound

• Definition: Sound with a frequency below 20 Hz.
• We Cannot Hear It: Humans cannot hear these sounds.
• Example: Earthquakes, volcanic eruptions, and some animal sounds (like elephants).

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2. Audible Sound

• Definition: Sound with a frequency between 20 Hz and 20 kHz.
• We Can Hear It: Humans can easily hear these sounds.
• Example: Speech, music, and everyday sounds.

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3. Ultrasonic Sound

• Definition: Sound with a frequency above 20 kHz.
• We Cannot Hear It: Humans cannot hear these sounds.
• Example: Bats use it for navigation, ultrasound in medical imaging, and cleaning devices.

 

 

Applications of ultrasonic waves

1.      Ultrasonic waves are used in medical imaging to see unborn babies and internal organs.

2.      They are used in ultrasonic cleaners to clean jewelry, watches, and instruments.

3.      Bats and dolphins use ultrasonic waves to find objects and prey.

4.      Ultrasonic waves help in detecting cracks or flaws in metals and structures.

5.      They are used in sonar systems to measure water depth and locate underwater objects.

6. Some devices use ultrasonic waves to repel insects and rodents.

 

 

Noise, noise pollution and its control

Noise pollution is defined as any level of nuisance caused by sound that is generally harmful to the environment and creates a disturbance in human life.

 

Causes of noise pollution

1. Vehicles make noise with horns and engines.

2. Factories and machines produce loud sounds.

3. Construction work like drilling and hammering makes noise.

4. Loud music and speakers increase noise in cities.

5. Trains and airplanes create strong noise.

6. Household machines like mixers and generators add to noise.

 

 

Health problems due to noise pollution

1. Hearing Loss: Loud noise can damage the ears and reduce hearing.

2. Stress and Anxiety: Continuous noise can make people feel tense and worried.

3. Sleep Disturbance: Noise can disturb sleep, leading to tiredness.

4. High Blood Pressure: Long exposure to noise can increase blood pressure.

5. Headaches: Loud noise can cause frequent headaches.

6. Heart Problems: Prolonged noise pollution may affect the heart and circulation.

 

Remedial measures for noise pollution

1. Plant Trees: Trees can reduce noise by acting as sound barriers.

2. Use Earplugs or Headphones: Protect your ears in noisy places.

3. Limit Loudspeakers: Reduce the use of loud music and public announcements.

4. Control Traffic: Follow traffic rules and avoid unnecessary honking.

5. Use Quieter Machines: Industries should use machines that make less noise.

6. Urban Planning: Build residential areas away from airports, highways, and factories.

7. Public Awareness: Educate people about the harmful effects of noise pollution.