# Get the Knowledge that sets you free...Science and Math for K8 to K12 students

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## Electric Charge and Electric Field

Wireless power transmission: A technology most awaited Yes! transfer of power is possible with out any contact, as shown in the figure. Wireless power is a most wanted technology. It has already been invented by Nikola Tesla in 1888. A low–cost PET bottle serves as the power source of the coil. Around 200 windings of insulated copper wire are manually applied to the bottle. A transmission in the range of 10 meters was reached and the power used was 100mW. Lets learn such interesting applications of electric field in this topic.

## After completing the topic, the student will be able to:

• Explore about electric charge and its properties by investigating about various interactions between the objects in real world.
• Understand the concept of charge polarization and probe its application in daily life.
• Inspect and investigate about the Coulomb's law of electric charge.
• Reinforce the importance of superposition of forces to determine the resultant electrostatic force in a system of charges.
• Learn about the electric field lines around a charge.
• Discover electric dipole and determine dipole moment, field intensity and torque on the dipole when placed in a uniform electric field.
• Observe and illustrate how an electron behaves in an electric field.
• Understand and determine the electric flux through any Gaussian surface using Gauss's law.
Static electricity After a plastic comb is used for straightening dry hair (or rubbed on wool) it becomes electrically charged. The charge creates an electric field which then induces charge separation in nearby paper bits thus leading to a net electrostatic attractive force between the bits and the comb.

# Try the following

• Rub your feet on a carpet and touch a doorknob. What happens?
• On a cold dry morning, have you ever felt that while removing a pullover, the hair on your hands stand up?
• Take a plastic comb and comb your hair quickly. (Your hair has to be dry). Take a few pieces of paper and hold the comb close to the papers. The papers will tend to get attached to the comb.
• Instead of a comb, try the same experiment using a glass rod. Rub the rod on a silk cloth, which will then get lifted towards the rod.
Electric Charge

The main basis of electricity and magnetism is the electric charge. The charges tend to move so as to achieve an electrically neutral situation. Nature likes to be electrically neutral. A charge creates a region of influence that we call electric field. Charge, the electric field around it, and motion of charge, make up the entire branch of electricity and magnetism. To understand the basic concepts of electric charge, electric field and electric force, we will discuss very simple situations where the charges are stationary or when they move, they do not disturb the electrical field.

Experiences of electric charges in our everyday lives:

One of the best examples of observing effects of electric charges is lightning. It is one of nature’s spectacular offerings. Moisture bearing clouds are initially electrically neutral. But as the masses of clouds increase in size, they start rubbing against each other. Friction makes electrons jump, so that between two clouds there will be a charge imbalance - one cloud is positively charged and the second cloud is negatively charged.

When a positively charged cloud collides with a negatively charged cloud, charge neutralization occurs and sparks are produced. These sparks are seen as lightning. Sometimes the charge imbalance can occur between a cloud and the earth; lightning is seen when the charges are exchanged between the cloud and the earth.

What is making the paper stick to the comb or the glass rod? An easy interpretation of these observations is that some objects when rubbed acquire a property that we intuitively know to be charge. The paper piece also gets a charge imbalance due to presence of charged comb or rod.

Physics behind lightning As the negative charge is created at the bottom of the cloud, positive charge is created on the ground, which leads to a flow of charge, in the form of lightning.
Electric charge and matter

Electrical behaviour of materials is divided into two classes: Loosely bound electrons from a material can be persuaded to leave their atoms and migrate to other atoms, causing charge. The persuasion is the external energy in the form of, say, rubbing action. This can give the electrons enough energy to migrate from one surface to another.

Let us consider a real life example of lightning. Lightning is formed by an imbalance of electrical charges in a cumulonimbus cloud. These large clouds have strong vertical and downdraft winds. As the water particles and ice crystals are violently bounced around within the cloud the positive and negative charges begin to congregate at the bottom and top of the cloud. Positive charges build up on the ground and negative charges build up at the base of the cloud. The negative charges begin to move downward in a zigzag stroke called a stepped leader (they cannot be seen at this point).

Positive charges build up on the ground, congregate around the highest point (a tree, lightning rod, or even a person). Since positive and negative charges attract, the positively charged upward streamer begins to reach up toward the negatively charged, downward stepped leader. When the two strokes meet a bright bolt of lightning, called the return stroke.