TOPIC 1: STATIC ELECTRICITY – PHYSICS NOTES FORM 2

Welcome to our website scholars.co.tz. In this article, are you looking for TOPIC 1: STATIC ELECTRICITY – PHYSICS NOTES FORM 2, Download Physics Notes Form One Topic 1

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.

When x and y are brought together.

Charges from x will be separated as shown due to the law of electrostatics. Upon earthing of y negative charges will be migrating the earth and body y will remain positive charged.

Let body x be positive while body y with both positive and negatively charged.

When they are brought near to each other

If body y is touched

THE GOLD LEAF ELECTROSCOPE

Is an instrument or device for identifying the presence of electrons i.e. charge on an object.

 An electrophorus

This is an instrument that is used to determine the presence of charge.

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.

 Uses of the electrophorus

An electrophorus is used in the laboratory to produce electric esparto.

Testing the sign of the charge 

 Identifying the insulating properties of a material, the electroscope is positively charged can be used to test per insulation property of the material.

If a material is placed near a cap of the machine the metal leaf wings converge and diverge slowly on that of the material is an insulator.

A conductor – is a material which conducts heat and electricity easily.

Insulator – is a material which does not conduct heat and electricity.

Examples;

• Wood
• Plastic
• Rubber

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

TOPIC 1: STATIC ELECTRICITY – PHYSICS NOTES FORM 2

Charge distribution in a conductor

a) Spherical Conductor

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

b) Pearl Shaped Conductor

In a pearl shaped conductor more changes accumulate on the sharp corners of a conductor than the other parts.
Pearl Shaped Conductor

c) Hollow  Conductors

Charge distribution in a hollow conductors, such as a hollow cylinder in the figure below Is only on the outside of the cylinder. The inside of the cylinder has no charge

Hollow Conductor

Capacitor

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

Capacitor is used in:

• Television
• Radios
• Computer

•A capacitor stores charges by keeping them in its plates.

Dielectric material

This is a medium placed between the plates of a capacitor. It is supposed to be an insulator.

 Examples of dielectric material are;

• Paper
• Mica
• Electrolyte
• Vacuum

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 = 1×10^-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

• Spherical capacitor

 COMBINATION OF CAPACITORS

Capacitors can be combined in series or parallel.

SERIES COMBINATION
When capacitors are arranged in series each capacitor will have the same charge but with different potential difference.

Therefore capacitors arranged in series the total or equivalent capacitance given by;

Parallel combination

When capacitors are arranged in parallel they will have the same potential difference but their charge will differ;

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

C₁=10µc

C₂=20µc

C₃=30µc

SOLUTION

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

Take the reciprocal of the value

C_T= 2.727µf.

5. Three capacitors of values 2µf, 3µf and 6µf are connected in series and in parallel. What is the equivalent capacitance in each case?

Solution

When connected in series

=1µf

Case 2 parallel

C_T=C₁+C₂+C₃

C_T=2+3+6

C_T=11µf

6.Find the equivalent capacitance of the diagram shown and the total quantity of charge stored given that the total potential difference of the circuit is 10V

Solution

C₁= 2f

C₂= 3f

C₃ = 4f

Since C₂, C3 are in parallel

C₄ = C₂+C₃

= 3+4

= 7f

Since C₁ and C₄ are in series then

Take the reciprocal

Total charge

Q= C₁V₁

Q=15.56C

7. A capacitor of two parallel plates capacitated by air has a capacitance of 15pF. A potential difference of 18 volts is applied across the plates. Determine the charge on the capacitor

Solution

Capacitor = 15pf = 15 x10^-12

V = 18V

Q = CV

= 15 x 10^-12 X 18V

Q = 270 x10^-12C

(b) If the space between is filled with mica the capacitor ratio increase to 240pF

How much more charge can be put on the capacitor if there is 1.8V supply?

Solution

Capacitance =240pf = 240 x 10^-12f

Volts = 1.8v

Q=CV

Q=240 x10^-12 x 18

Charge will be 4.32 x10^-9C

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.

 

 As the distance increases the capacitance decreases.

3. Di electric material

The capacitance varies with the variation of the dielectric material.

Combining equation i and ii

G =permittivity of free space.

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.

2. Find the capacitance of a capacitor

3. Amount of the charge stored

Solution

A=2mm²

d=2mm

V=20volts

C=?

Q=?

G=1.8 x10^-12

C=1.8 x 10^-15

Q = CV

Q= 1.8 x 10^-15 x20

Charge = 3.6 x 10^-14C

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.

              Lightning Conductor