Class 8, Physics
We shall study about Lightning and Earthquake in detail
Lightning
Lightning occurs because of the massive electric charge flowing from clouds, from one part of cloud to another or from a cloud to ground.
Electrostatic Force:
Rubbing a comb against dry hair / rubbing a balloon against woollen material produce a force that can attract as well as repel the objects. This is called Electrostatic Force
Charged Object:
An object having electric charge on it is called Charged object.
Charging the Object:
The process of giving charge to an object is called Charging the Object.
Charging by Friction:
Charging of an object by rubbing it with another object is called Charging by Friction.
What are the two kinds of Charges??
- Positive charge and Negative charge
- when rubbed with Electric charge on glass rod when rubbed with silk cloth it is considered as Positive.
- Electric charge on ebonite rod when rubbed with fur/wool is considered as Negative.
- Benjamin Franklin discovered the two kinds of charges
- Like charges repel and unlike charges attract each other.
Explain how repulsion is sure test of Electrification?
To test whether an object is charged or not bring Positively charged and negatively charged object near the object to be tested one by one
a) If it is attracted by both, it is not charged.
b) If it is repelled by Negatively charged object, it is also positively charged.
c) If it is repelled by Negatively charged object, it is also Negatively charged.
What are the Electric charges in atoms?
- An atom consists of three main particles. Protons, Electron and Neutrons.
- Neutron- A proton carries Positive charge, electron carries a Negative charge and neutron has no charge
When an atom/object is said to be at neutral?
Under normal conditions, the number of protons is equal to number of electrons, so the atom/object is said to be at Neutral.
When an object is said to be negatively charged?
When an atom gains electrons, the number of electrons is more than that of +vely charged proton and hence object is said to be negatively charged.
When an object is said to be Positively charged?
when an atom loses electrons, there is less number of electrons than protons, hence the object is said to be positively charged.
What are three methods of charging?
- 1.Friction
- 2.Conduction.
- 3. Induction
Explain charging by Friction?
- When an object us rubbed against another object, there is transfer of charge from one object to another.
- Ex: Plastic comb rubbed on dry hair acquires charge and attract small pieces of paper.
- This transfer of charge is caused due to friction.
Explain Charging by Conduction?
when an uncharged object is touched with Positively or negatively charged object, it acquires the charge of the rubbed object. This process is called Charging by Conduction.
Explain in detail Charging by Induction??
The process of charging an object by keeping it near a charged object & not touching it is called Charging by Induction.
- In charging by induction, no charge flows from charged object to uncharged object.
- when object is Positively charged it repels the electrons in the uncharged object.
- Due to this, nearer end of uncharged object becomes electron deficient that is positively charged.
- The farther end becomes negatively charged and has excess of electrons.
- Vice versa also is possible.
What is a Gold leaf Electroscope??
- An electroscope is an instrument used for detecting electrical charge and its nature on an object.
- It is very sensitive and delicate instrument invented by William Gilbert.
- Gold leaf electroscope consist of brass rod
- It passes through rubber stopper in glass jar.
- Top end of brass rod has brass disc.
- Lower end carries have two thin gold leaves that are hanged parallel to each other.
- Glass jar protect gold leaf from outside air currents
- In the lower part of glass jar in interior it is surrounded by metal foil.
- When electroscope is used metal foil is earthed.
- This is done to increase the sensitivity of the electroscope, instead of gold leaves, silver, copper, aluminium, brass is used.
Define Earthing??
The process of transferring charge from a charged object to Earth is called Earthing.
How Gold leaf electroscope is used to detect the charge?
- It is used to detect charge on an object.
- When an object to be tested is placed on the brass cap of the gold leaf, if it shows divergence then the object has either a positive or negative charge.
How to identify the nature of charge?
- Touch the brass disc of the charged electroscope with the object to be tested.
- If there is further divergence of leaves, then the object is Positively charged.
- If there is a decrease in divergence, the object is said to be negatively charged / uncharged.
What is flow of charge in electric current?
- When a charged object is connected to an uncharged object through conductor, electric current flows until the two objects are equally charged.
- To make electric current move continuously we need a electric cell / generator.
Note: Franklin experiment showed that his lightning is Static Electricity. He invented lightning rod and lightning conductor.
Define Electric Discharge?
The passage of electric current in air due to movement of electric charges is called Electric Discharge
How charges are formed in clouds?
- Clouds contain tiny droplets of ice/ water that moves against each other.
- This causes huge amount of charge to build onto the clouds.
How charge moves from one cloud to another?
- When the charge accumulation is greater, wind brings the clouds together.
- The insulation of air breaks and charge jumps from one cloud to another through air.
How lightning is caused?
- The sudden rush of electrical charge heats
- up air to about 30,000°C and produce bright glow of lightning.
- Rapid flow of electric charge through air between two oppositely charged clouds are seen as intense spark of electricity in sky called as Lightning.
How do we hear Thunder?
- Due to the heat produced at time of lightning air gets heated up and expand suddenly.
- This rapid expansion of air send disturbance in the form of vibrations through air producing loud sound
- This loud sound at time of lightning is called Thunder.
How do lightning hits on tall buildings?
- Cloud has the negative charge concentrated at base and Positive charge in the upper region.
- When a charged cloud passes over tall building /tree it induces negative charge on them.
- Negative charge at base of cloud pulls the positive charge on tall building at tremendous speed.
- Negative and Positive charges connect, a continuous path is formed from flow cloud to tall building / tree.
- Large amount of Negative charge accumulated in cloud rushes down its path giving a lightning strike.
- Lightning strike is more frequent in hilly areas because clouds are comparatively closer to them.
List the Harmful effects of lightning?
- Lightning strike generates huge amount of electric charge. Temperature is around 30,000°C
- It can cause forest Fire.
- It can cause fire and shatter buildings resulting in lot of damage
- Sometimes it can even kill humans and animals instantly.
What is a lightning conductor?
Lightning conductor is a device fixed on the top of buildings to protect them from lightning damage.
How lightning conductor protects the building?
- It consists of long lightning rod made up of iron or copper with number of sharp spikes at top end.
- Rod runs from top to bottom along the outer wall of building.
- Lower end of rod is connected to metallic plate (copper) buried deep into earth to provide good earth connection.
- When the lightning Strikes, the lightning conductor provides easy path for the charge to pass through to the Earth and protect the building.
What are the safety measures to be taken during lightning?
- Move to appropriate shelter inside
- the building or car with closed windows.
- Do not lie flat on ground.
- Do not take shelter under tree.
- When there are group of people, stay about 15 feet from each other.
- Stay away from all water bodies
- Do not go in the open vehicles.
- Do not stand on the top of building / mountain.
- Inside a forest, stand under shorter trees.
State some of the indoor safety measures?
- Electrical lines are excellent conductor of electricity. Unplug all the electrical appliance during lightning.
- Do not take bath / shower during storm.
- Do not use telephone cords, electrical wires, metal pipes.
List few useful effects of lightning?
- Lightning fixes atmospheric nitrogen to the soil.
- Heat and light released during lightning help in combination of atmospheric nitrogen & oxygen to form oxides of Nitrogen.
- They get dissolved in water to form Nitric acid.
- Nitric acid comes with rain water and react with salt present in soil to form nitrates
- These are used by plants as nutrients and helps in the growth of plant.
2. Ozone layer in atmosphere absorbs harmful UV rays of Sunlight and protect living organism from its harmful effect. Electric discharge during lightning forms ozone from atmospheric oxygen
3. Lightning played an important role in the evolution of life on the Earth.
Earthquake
1. Define Earthquake?
Earthquake is a sudden shaking of the earth’s crust that last for very short time.
2. How Earthquake is caused?
Earthquake is caused by disturbance deep inside Earth’s crust
The energy stored in the Earth’s deeper layers, over long period of time is suddenly released in the form of strain in rock resulting in Earthquake.
3. What are the three layers inside the Earth?
(i) Crust (i) Mantle (iii) Core
4. List the causes of Earthquake?
The Main cause of Earthquake is
- Movement of Tectonic plates
- Volcanic eruptions
- Dislocation of crust
- Underground nuclear explosion
5. Define Tectonic plates?
Surface of Earth is not a continuous layer. They are broken into many pieces some large and some small pieces called Tectonic plates.
6. What are major Tectonic plates?
World is divided into seven large tectonic plates and several smaller plates.
- North American
- South American plate
- African plate
- Antarctica plate
- Eurasian Plate
- Indian plate
- pacific plate
7. How Earthquake is caused by movement of Tectonic plates?
- There is always a slow movement of these plates
- That it each plate moves few inches in a year from each other.
- Movement of plates sometimes causes edges of tectonic plate to grind each other, deformed, displaced, crushed or fractured
- They made slide or slide under each other.
- During this they send a set tremor/ vibration through crust causing Earthquake
8. Define Volcano?
Volcano is hole in Earth’s crust through which molten rocks and hot gases under high pressure comes out.
9. How volcanic eruption cause Earthquake?
Day when volcano erupt, the energy released causes vibrations in the cust. These vibrations can cause earthquakes around that area.
10. What are Faults?
Over millions of years, Tectonic plates movement has caused / created weak points in earth’s crust called Faults.
11. What are Fault/ Seismic zone?
Fault occur along the boundaries of tectonic plates and in this zone only earthquakes occur. These weak zones are called Fault/Seismic zones.
12. List the regions in India that has Fault zone?
- Kashmir, western & central, Himalayas
- Whole of North East
- Rann of Kutch
- Rajasthan
- Indo Gangetic plain
13. Underground nuclear explosion
When Nuclear explosion takes place in the interior of earth it causes severe powerful vibrations. These vibrations result in earthquake in that site.
14. Define Seismic focus ?
Place of origin of an earthquake inside Earth is called Seismic focus.
15.What is Focal depth?
The Depth of seismic focus below the Earth’s surface is called Focal depth.
There are 3 types of focal depth Shallow, Intermediate, Deep
- Earthquake having focal depth 60-70 km – Shallow
- Earthquake having focal depth 70-300 km – Intermediate
- Earthquake having focal depth > 300 km – Deep
16. Define Epicentre?
- Point on the surface of earth directly above the focus of an earthquake is Epicentre.
- The impact of earthquake is strong at this point.
17. Seismic waves: Waves generated inside Earth’s surface due to earthquake are called seismic waves / Shock waves.
18. Define Seismograph?
An instrument that detects and records the intensity of seismic waves due to earthquake is called Seismograph.
19. How do Seismograph work?
- Seismograph is a vibrating pendulum when Earth shakes, seismograph fixed to earth and suspended bob of the pendulum also shakes.
- Pen is attached to Pendulum.
- Under this there is fixed roller with graph the paper.
- Graph paper on rotating drum shakes with shaking of Earth.
- Pen attached to suspended bob draw vibrations produced due to earthquake on the paper.
20. What is Seismogram?
The graphical record of the intensity of seismic waves is called Seismogram.
21.Define Richter Scale?
- Richter scale is used for measuring the magnitude of Earthquake.
- It was devised by Charles F. Richter
- It gives measure of energy released at earthquake focus
- Range of Richter range from 0 to 10
- Change in value on the scale by one corresponds to 10 times change in energy associated with earth more
- An earthquake of 5.0 magnitude is ten times more powerful than earthquake of 4.0 magnitude
- An earthquake of 5.0 magnitude is 100 times more powerful than earthquake of 3.0
- So the Richter scale is said to be logarithmic.
22. What are the damaging effects of Earth quake ?
- Human made buildings railway track, road, bridge, factories are severly damaged.
- 2 People get trapped inside collapsed buildings & may die.
- Fire break out may caused by sparking from short circuits.
- Ground water pipes rupture and cause water supply issues
- Earthquakes in hilly and Mountain areas cause land slides.
- Sewage system burst open leading to epidmenic
- Earthquake can change course of rivers and cause flood.
- Communication system electric poles, mobile phone, towers collapse.
- Earthquakes under ocean can cause powerful sea waves called Tsunami.
- Tsumani have a width of 100-200 km and travel hundreds of km across deep ocean reaching speed of 700-800 km/h.
- Tsunami causes great loss of life and property in coastal regions.
23.How to make your buildings Earthquake safe?
- In high seismic area, constriction materials used can be light, as mud/timber instead of heavy stones etc, so during earthquake damage may not be heavy.
- Fix the cupboard and shelves to walls.
- Hang photo frame, heater, wall clocks properly
- All tall buildings should have firefighting equipment in working condition.
- Install outward opening doors.
24. What are the outside safety measures to be taken, if you are outdoors?
- Move to an open area immediately
- If you are travelling do not come out drive slowly and move away from tall buildings, bridges, and electric cables etc.
- Move away from trees, tall building, flyers since these may collapse and fall down.
25.What are the indoor safety measures to be taken if you are indoor??
- Stay under table until the shaking stop.
- If you are in bed, don’t get up, protect your head with pillow.
- If you can come out, go to open space area.
- 4.Turn off power supply and gas supply.
- Stay away from Window, Fridge, Gas stoves, Fridge etc.