Electric Charges and Fields

Class XII

 Electric Charges 

Gold Leaf Electroscope

Conductors And Insulators

Electric Charge

Electric charge is a scalar quantity. The SI unit of charge is coulomb. It is generally denoted by Q.

There are two types of charges. These charges are named by American Scientist Benjamin Franklin.

1. Positive Charge

2. Negative Charge

When glass rod is rubbed with silk. Something is acquired by both glass rod and silk. This is called Electric Charge.

The glass rod acquires Positive Charge.

When two glass rods are rubbed with silk and brought close to each other. They repel each other.

From this it is clear that

Like or Same charges repel to each other whereas opposite or unlike charges attract to each other.

When a plastic rod is rubbed with woolen cloth. The plastic rod acquires Negative Charge.

Hence, Two Positive or Negative Charges repel each other whereas Positive and Negative charges attract to each other.

The charges acquired after rubbing are lost when the charged the bodies are brought in contact.

If a body has charge then it is said to be charged. Gold Leaf Electroscope is an apparatus which is used to detect the charge on a body.

Gold Leaf Electroscope

The apparatus has a metal rod, two thin gold leaves and charged body. All of these are fitted in transparent glass window box. The figure is given below.

When charged body touches the metal knob at the top of the rod. The charge flows on the leaves and they diverge. The degree of divergence indicates the amount of charge.

Conductors and Insulators

Those substances which allow electricity to pass through them easily are said to Conductors. Conductors have electric charges (electrons). These electrons comparatively free to move inside the material.

There are following examples of conductors.

Metals, human and animal bodies and earth.

Those substances which offer high resistance to passage of electricity through them are said to Insulators.

There are following examples of insulators.

Non-metals, glass,porcelain, plastic, nylon, wood offer high resistance to passage of electricity through them.

Mathematics

Circle

Class X

Learning Points

1. Circle
2. Tangent to a Circle
3. Number of Tangents from a Point on a Circle

Circle

Circle is a collection of points. These points are located at a certain distance from a fixed point. The fixed point is known as centre. The certain distance is known as radius. A circle has centre, radius, diameter, circumference, Tangent, secant, chord. These are basic terminology of a circle.

Any lines which passing through one and only one point of circle is said to be tangent. Any lines which join two point of a circle is said to be secant. Any lines which passing through a circle without touching it is said to be non - intersectiong line.

Tangent to a Circle

Any lines which passing through one and only one point of circle is said to be tangent. Tangent is a Latin word derived from tangere means touching.

There is one and only one tangent at certain point of a circle. 

Tangent is a special types of secant in which two ends point coincides. Therefor all tangents are secant but all secant are not a tangent.

Theorem 10.1

The tangent at any point of a circle is perpendicular to the radius through the point of contact.

Given:-

There is a circle. O is the centre. P is a point on the circle. The tangent XY passing through P.

To Prove that :-

XY is perpendicular on OP.

Construction:-

Point Q is on XY outside of the circle. Join OQ.

Proof:

OQ is greater than OP.

If Q lies inside the circle.

XY will be secant.

OP is the shortest distance from centre O.

We know that the shortest distance from a point on line is perpendicular.

Therefore,

XY is perpendicular on OP. Proved.

1. By theorem above, we can also conclude that at any point on a circle there can be one and only one tangent.

2. The line containing the radius through the point of contact is also sometimes called the ‘normal’ to the circle at the point.

In next part I shall post "Number of Tangents from a Point on a Circle". Please do wait for this.