Science textbook solutions for Class 7 Science Chapter 8 Static Electricity

Important points to remember :

  1. Electric charge is stored even in our own bodies. All substances are made of very tiny particles. Electric charge is  an intrinsic property of  these particles.
  2. Two rods carrying  similar  charges push each  other  away. This is called  repulsion. 
  3. We learn from the second experiment that rods carrying opposite types of charges get pulled towards each other. This is called  attraction. 
  4. The scientist  Benjamin  Franklin named the electric charges positive  charge (+) and negative  charge (-). 
  5. Each atom is electrically neutral. It has equal amounts of positive and  negative charge. 
  6. If for some  reason, the  negative charge decreases, the atom becomes positively charged. The  electric  charge  generated by  friction is  called frictional electricity.  
  7. This charge  is produced  only at the  place  of friction. Hence,  it is called  static  electricity.  
  8. The number of positive and negative  charges are equal on an uncharged or neutral  object. 
  9. The  electric  charge generated  by induction stays only as long as the charged object  is near to it.

Question 1: Choose the correct option and fill in the blanks.
(always repulsion, always attraction, displacement of negative charge, displacement of positive charge, atom, molecule, steel, copper, plastic, inflated balloon, charged object, gold)

a) There is always repulsion between like charges.

b) Displacement of negative charge is responsible for generation of electric charge in an object.

c) A lightning conductor is made of a copper strip.

d) Copper/gold does not get electrically charged easily by rubbing.

e) There is always attraction when opposite electric charge come near each other.

f) A charged object can be detected with an electroscope.

Question 2: Explain why it is not safe to go out with an umbrella when there is heavy rain, lightning or thunder.
Answer : It’s not safe to go out with umbrella during lightning because –
– Lighting gets attracted to metal objects.
– If you go out with umbrella in lightning, the iron rod attracts & conducts lightning.
– This increases your chances of being electrocuted.
– Therefore, it’s nlt safe to out with umbrella in heavy rains / lightning / thunderstorms.

Question 3: Answer in your own words.
a) How will you protect yourself from lightning?
Answer :

  1. Stay away from concrete floors or walls. Lightning can travel through any metal wires or bars in concrete walls or flooring. Avoid water during a thunderstorm. Lightning can travel through plumbing.
  2. Avoid electronic equipment of all types. Lightning can travel through electrical systems and radio and television reception systems.
  3. Avoid corded phones. However, cordless or cellular phones are safe to use during a storm.
  4. Avoid concrete floors and walls.

b) How are charges generated?
Answer : Charges generated when there is loss and gain of electrons . Then we also say that when two materials are in contact there is movement of the electrons takes place in a two materials this give rise to the positive charge and negative charge is also equal to the positive charge and thus become neutral

c) In the lightning conductor, what provision is made for spreading the electricity into the ground?
Answer : if lightning strikes , it will hit the top of lightning conductor . The electrical energy passes through the metal strip and gets discharged safely into the ground through buried metal plate. Hence buildings are protected

d) Why do farmers stick an iron staff into the ground while working in the field in rainy conditions?
Answer : Farmers stick an iron staff into the ground while working in the field in rainy conditions to protect from Lightning. Usually during rainy season, there are chances of lightning and thunder storms. … Hence farmer is protected from damage himself and to his field

e) Why is lightning not seen everyday during the rainy season?
Answer : Lightning is not seen everyday during the rainy season because When the sun heats the air close to the surface of the Earth, it rises. As the moisture cools, it condenses and forms heavy clouds.

Question 4: What are the characteristics of a static electric charge?
Answer : Electrically charged objects have several important characteristics:

  1. Like charges repel one another; that is, positive repels positive and negative repels negative.
  2. Unlike charges attract each another; that is, positive attracts negative.
  3. Charge is conserved. A neutral object has no net charge. 
  4. If the plastic rod and fur are initially neutral, when the rod becomes charged by the fur, a negative charge is transferred from the fur to the rod. 
  5. The net negative charge on the rod is equal to the net positive charge on the fur.

Question 5: What is the damage caused by lightning? How will you create awareness to prevent it?
Answer : Major hazards caused by lightning
Fire damage: The biggest threat lightning poses to a structure is fire. Wood and other flammable construction materials can easily explode when exposed to the high temperature of a lightning strike. Lightning current travelling through wires and pipelines instantly burns them up causing complete damage to property.

Power surge damage: If lightning chooses electrical wiring as its primary or secondary path, the explosive surge can damage all the appliances it is connected to.

Shock wave damage: Lightning produces shock waves that can be destructive. These shock waves can fracture concrete and brick and stone chimneys severely.

1. Turn off all the electrical appliances such as television and refrigerator.
2. Avoid taking shelter under trees.
3. Avoid being near to any kind of metal objects which may include metal wires, fences, machinery and power appliances.
4. Take shelter inside a fully enclosed car with all the windows shut.

# Use your brain power
1.  What kind of  damage is caused by a lightning strike ? 
Answer : Damage Caused by Lightning Strikes. Lightning current travelling through wires and pipelines instantly burns them up causing complete damage to property. Power surge damage: If lightning chooses electrical wiring as its primary or secondary path, the explosive surge can damage all the appliances it is connected to.

2. What measures will you  take to prevent the damage caused by lightning ?
Answer : To avoid getting hit by lightning, stay away from open fields, hilltops, swimming pools, and open water during storms. Additionally, don’t stand near trees, tall isolated objects, or metal objects that could attract lightning strikes.

3. Why is the upper end of the lightning  conductor pointed ? 
Answer : Lightning rods are pointed so a charge will be concentrated at the end. If the rod had a round end the charges would spread equally around its surface. As long as the air is prevented from ionizing it won’t conduct the lightning bolt.

4. Why are coal and salt added to the pit in the ground ?
Answer : The purpose of coal and salt is to keep wet the soil permanently. Coal is made of carbon which is good conductor minimizing the earth resistant. Salt use as electrolyte to form conductivity between GI Plate Coal and Earth with humidity. Sand has used to form porosity to cycle water & humidity around the mixture.

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