STATIC ELECTRICITY

STATIC ELECTRICITY

STATIC ELECTRICITY

Definition

This is the type of electricity due to charges in electrons which do not move.

Or

This is the type of electricity due to stationary charges.

Electrostatics

This is the study of stationary electric charges.

TYPES OF CHARGES

There are two types of charges;

1. Positive charge
2. Negative charge

Positive charge

-This is the type of charge with deficiency number of electrons.

Negative charge

-This is the type of charge with excessive number of electrons.

FUNDAMENTAL LAW OF ELECTROSTATICS

-It is called 'first coulombs’ law that states “like charges repel while unlike charges attract each other”.

CHARGING OF THE BODY (ELECTRIFICATION)

-This is the process of either adding or removing charged electrons from a body.

 METHODS OF CHARGING A BODY

There are three methods of charging the body and these are;

1. By friction or rubbing
2. By contact
3. By induction

1. BY FRICTION/ RUBBING

This is the method of charging a body by rubbing one body on the other.

When an ebonite rod is rubbed on fur some of the electrons are transferred from the fur to the ebonite. Therefore the ebonite becomes negatively charged while fur transferred from the fur to the ebonite. Therefore the ebonite becomes negatively charged while fur becomes positively charged.

While when the glass rod is rubbed in silk electron will be transferred from glass rod to silk, the glass rod will be positively charged while silk will be negatively charged.

2. BY CONTACT

This is the method of charging the body by touching each other. Let’s say x is positively charged and y is negatively charged.

Then the positive charge will be transferred to body y, and then after sometime they will repel each other since they will have  equal charge.

3. BY INDUCTION
This is the method of charging the body without touching. Let body x be negatively charged while body y lies both positive and negative.

An electrophorus works by induction and hence can be used to generate positive charge from a single negative charge. The charge produced on the insulating bulb is negative the top are then placed on it since the surface are only in contact at relatively few points a positive charged induced on the lower surface and a corresponding negative charge is produced on its top surface . The top of the upper disc is then touched with a finger there by carrying in away the negative charge to the earth. This is known as earthing.

Examples;

• Wood
• Plastic
• Rubber

•Detecting the presence of a charge in the body or on a body.


Charge distribution in a conductor

a)Spherical Conductor

In a spherical conductor charge is distributed equaly throughout the conductor.

                Hollow Conductor

Capacitor

This is a device which is used for storing the amount of charge.

Capacitor is used in:

• Television
• Radios
• Computer

 Capacitance (c)

This is the ability of a capacitor to store electric charges.

Always the amount of charge stored is directly proportional to the potential difference.

Q α v………….. (1)

 α is directly proportional

Q = cv

Where by c- capacitance

The SI unit of capacitance is Faraday

Other units are;

• Mil- mF = 1x10^-3 F
• Micro –µF = 1x 10^-6 F
• Nano –F = 1 x10^-9 F
• Pico –  F = 1x 10^-12 F

Example

1. A capacitor of capacitance 200mF is allowed to charge. The P.d between the plates is 10Volts. How much charge will accumulate on the plate during the period of changing data?

DATA

C=200mf=200 x 10^-6 F

v=10v

Q=?

Q= CV

   SOLUTION

Q =200 x100^-6x10

Q =2 x10² x10^-6 x10

Q =2 x 10^-3 C

The SI unit of the charge is coulombs

2. A 3mf capacitor has a potential difference of 12V. Determine the total charge.

DATA

C=mf=3 x10^-3F

V=12V

Q =?

Q =CV

SOLUTION

Q =3 x10^-3 x12

Q =36 x10^-3 C

The total charge is 36 x10^-3 C

TYPES OF CAPACITOR

Capacitors are categorized in:

1. Paper or plastic capacitor

This is a type of capacitor which uses paper or plastic as dielectric material.

    2. Mica capacitor

This is the type of a capacitor which uses the electrolyte as a dielectric materials i.e. water

All capacitor can be categorized due to theories;

• Cylindrical capacitor

Total charge (Q_T)

Q_T =  Q₁+Q₂+Q₃

But Q = CV

Q₁V = C₁V+C₂V+C₃V

C_T = c₁+c₂+c₃

For the parallel combination of the capacitor their equivalent or total capacitance is given by;

C_T = C₁+C₂+C₃

Example

1.Three capacitors A, B and C are arranged in series. Their capacitance were given 10µc, 20µc, and 30µc. Calculate the value of a single capacitor that would replace them.

DATA

Capacitor series

C_T=5.45µc


2. A 1000µf capacitor has been charged to a Potential difference of 25V. What is the charge on the plate of a capacitor?

 DATA

Capacitance= 1000µf = 1000 x 10^-6

P.d = 25V

Charge (Q)

Q = CV

SOLUTION

Q = 1000 x 10^-6 x 25

Q = 1 x 10^-3 x 25

Q = 25 x 10^-3

Q = 0.025c

The charge on the capacitor is 0.025 coulombs.


3.A capacitor of capacitance 250µf is allowed to change until the potential difference between the charges is 10V how much a charge accumulates on the plates during the charging process.

Data

Capacitance (C) = 250µf = 250 x10^-6

P.d (V) =10V

Q=CV

SOLUTION

Q=250 x10^-6 x10V

Q=250 x 10^-5C

Q=0.0025C

4.What value of the capacitor could be used to replace a set of 5µf, 10µf and 15µf capacitors connected in series

Solution

FACTORS AFFECTING CAPACITANCE OF A CAPACITOR

1. Cross section area (A) of the plates.

As the cross section area increases, the capacitance also increases.

 C α A …..(i)

2. Distance of the separation of the plates.

3. Di electric material

The capacitance varies with the variation of the dielectric material.

Combining equation i and ii

Examples

1.A capacitor of 2mm² cross section area and distance of separation of the plate of 2mm is connected   to a Potential difference of 20V.

1. Find the capacitance of a capacitor
2. Amount of the charge stored

Solution

A=2mm²

d=2mm

V=20volts

C=?

Lighting Conductor

Lightning is huge discharge of static electric charges between clouds or cloud and the ground.

A lightning conductor is a metal rod attached to a building and connected to a think copper strip that leads into the ground. Its tio has sharp spikes.

Lightning conductors can help to protect building and other structures from lightning strikes.

The lightning conductor is placed above the highest point on the building because lightning tends to hit the highest object within its region or path.

When lightning strikes the conductor ,electric flow along the wire and dissipated to the ground there by protecting the building.