These DAV Class 7 Science Notes and DAV Class 7 Science Chapter 11 Notes – Electric Charges at Rest act as excellent revision resources, particularly in preparation for board exams.
Electric Charges at Rest Class 7 DAV Notes
→ Charged and Uncharged Objects: If an object acquires the property of attracting light objects, it is said to be electrically charged or ‘charged’. But most of the substances are uncharged.
→ When an object is rubbed against another object, both of them may become charged. This happens because of transfer of charges from one object to another. Following table shows some objects which can get charged on rubbing.
→ Conductors: Metallic materials allow electric charges to flow through them and hence are called conductors. As electric charge can flow through them, so they do not get charged up on rubbing.
→ Insulators: Non-metallic materials do not allow electric charges to flow through them and hence are called insulators. As electric charge cannot flow through them, so they get charged up on rubbing.
→ Properties of Charged Objects:
- A charged object exerts a force not only on uncharged objects but also on charged objects.
- When the charge on two objects is similar, they repel each other.
- When the charges on two objects are different, they attract each other.
- Like charges repel each other, while unlike charges attract each other.
- Charges are of two kinds, viz. positive and negative.
- Charges are always produced in pairs. This means that when two objects charge each other, one of them becomes positively charged and another becomes negatively charged.
→ Methods of Charging Up:
- Charging by rubbing or friction.
- Charging by contact or conduction.
- Charging by induction.
→ Earthing: Putting a charged object in contact with the earth causes it to lose its charge. This process is called earthing. Earth wire is used in household wiring to protect appliances from damage and to prevent accidental electric shocks.
→ Electrical Induction: When a charged object is brought near an uncharged object, the charges get transferred to the uncharged object. The opposite charges move to the nearer end and the like charges move to the farther end of the uncharged object. This process of separation of charges is called electrical induction.
→ Charges in Nature:
→ Lightning and Thunder: The bigger and heavier droplets in clouds tend to acquire negative charges. The smaller and lighter droplets tend to acquire negative charges. The negative and positive charges in the clouds exert a strong force of attraction on each other. Air is a bad conductor of electricity. But when the charges accumulate in the clouds to a great extent, the charge is passed down to the earth. This is called electric discharge. We witness this in the form of a streak of light and this is called lightning. Lightning is also accompanied by thunder.
→ Advantages of Lightning: Lightning converts nitrogen into its oxide which then comes down along with rains. This helps in nitrogen fixation in soil. Lightning also helps in conversion of oxygen into ozone and thus helps in formation and maintenance of ozone layer in the atmosphere.
→ Lightning Conductor: This is a device which is installed on buildings to prevent the building from harmful effects of lightning. It is made up of a long flat strip of metal. A conducting wire is attached to it which goes deep into the ground. When lightning strikes, the electric charge is transferred via the metallic wire to the earth. Thus, building is saved from harmful effects of lightning. Lightning conductor can be seen on tall buildings, telephone towers, temples and churches.
→ Safety Precautions During Lightning
For a person caught outside in the open:
- At the first signs of lightning, immediately go to a safe place; such as a house or a building.
- Inside of car and bus is also safe; if the windows and doors are closed.
- Avoid open areas and sheds.
→ Stay away from trees.
For a person inside a house or building:
- Avoid contact with water.
- Do not use electric equipments. Electric lights can remain on.
- Do not use a phone with cord. Cordless phones are safer.
→ Charged body: When an object acquires the property of attracting light objects, it is called a charged body.
→ Electrical Induction: Separation of electric charges is called electrical induction.
→ Earthing: The electrical discharge to the earth is called earthing.
→ Lightning: The process of electric discharge from clouds to the earth is called lightning.
→ Thunder: The heavy sound; accompanied by lightning.
→ Conductors: Materials which allow electric charges to pass through them with ease.
→ Insulator: Material which does not allow electric charges to pass through it.
| Material | Metallic/non-metallic | Electrical property |
| Plastic comb | Non-metallic | Gets charged |
| Metal comb | Metallic | Does not get charged |
| Drinking straw (plastic) | Non-metallic | Gets charged |
| Ebonite rod | Non-metallic | Gets charged |
| Glass rod | Non-metallic | Gets charged |
| Metal pan | Metallic | Does not get charged |