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Electrostatics: Principles, Laws, and Applications, Study notes of Nursing

Johns Hopkins University (JHU)Nursing

A concise introduction to electrostatics, covering fundamental concepts such as types of charges, methods of charging bodies, and coulomb's law. It also explains the construction and use of electroscopes, including charging methods and applications. The text further explores charge distribution on conductors, point action, and practical applications like electrostatic precipitators and lightning arrestors. It includes questions on electrostatics forces.

Typology: Study notes

2025/2026

Available from 11/01/2025

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ELECTROSTATICS
Introduction
Electrostatics is the study of static (stationary) charges.
SI nit of charge is coulomb (C)
Some substances get charged when rubbed against other substances i.e. nylon, plastic, paper etc.
The charge acquired stays within the body i.e. it does not move and therefore known as
electrostatic charge or static electricity.
Types of charges
There are two types of charges i.e. negative and positive charges. The negative charge consists of
electrons which are mobile.
The following materials becomes negatively charged by friction when rubbed:
❖ Polythene
❖ Most plastics
❖ Ebonite
The following on the other hand becomes positively charged when rubbed:
❖ Acetate
❖ Perspex
❖ Glass
❖ Fur
Methods of charging bodies
❖ Induction-the charging body is brought close but not touching the body being charged
❖ Contact- the charging body is brought into contact with the body being charged
❖ Separation-bodies are charged while in contact before there are separated.
The basic law of electrostatics states that unlike charges attract and like charges repel each other
The repulsion or attraction force between the charges is given by coulombs law which states that
The magnitude of the electrostatic force of attraction or repulsion between two point charges is
directly proportional to the product of the magnitudes of charges and inversely proportional to
the square of the distance between them.
𝐹 = 𝐾 𝑄1𝑄2
𝒅𝟐
K=9.0×109
pf3
pf4
pf5

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Download Electrostatics: Principles, Laws, and Applications and more Study notes Nursing in PDF only on Docsity!

ELECTROSTATICS

Introduction Electrostatics is the study of static (stationary) charges. SI nit of charge is coulomb (C) Some substances get charged when rubbed against other substances i.e. nylon, plastic, paper etc. The charge acquired stays within the body i.e. it does not move and therefore known as electrostatic charge or static electricity. Types of charges There are two types of charges i.e. negative and positive charges. The negative charge consists of electrons which are mobile. The following materials becomes negatively charged by friction when rubbed: ❖ Polythene ❖ Most plastics ❖ Ebonite The following on the other hand becomes positively charged when rubbed: ❖ Acetate ❖ Perspex ❖ Glass ❖ Fur Methods of charging bodies ❖ Induction-the charging body is brought close but not touching the body being charged ❖ Contact- the charging body is brought into contact with the body being charged ❖ Separation-bodies are charged while in contact before there are separated. The basic law of electrostatics states that unlike charges attract and like charges repel each other The repulsion or attraction force between the charges is given by coulombs law which states that The magnitude of the electrostatic force of attraction or repulsion between two point charges is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them. 𝐹 = 𝐾 𝑄 1 𝑄 2 𝒅𝟐 K=9.0×10^9

ℰ = 8. 85 × 10 −^14

Questions on electrostatics forces

  1. Calculate the electrostatic force between two objects +13μC and - 22μC which are 0.055m apart. And is it an attraction or repulsion.
  2. Calculate the electrostatic force between two objects - 14 μC and - 22 μC which are 0.86m apart. And is it an attraction or repulsion.
  3. The electrostatic force between two objects is 6.4N when the charges are - 4.5μC and – 8.8μC. how far apart are the objects.
  4. An attraction force of 1.1N is exerted between two charged objects when they are 0.43m apart. The charge on one object is - 5.7μC. what is the charge on the other object. ELECTROSCOPE An electroscope is a scientific device that is used to detect the presence of an electric charge on a body. Types of electroscopes ❖ Pith-ball electroscope ❖ Gold-leaf electroscope Pith-ball electroscope: A pith-ball electroscope is a simple device used to detect the presence and type of static electric charge. It consists of one or two lightweight, non-conductive pith balls suspended by thin threads from an insulating stand. When a charged object is brought near the pith balls, they will either be attracted to or repelled by the object, indicating the presence of charge. Construction diagram

Examples

  1. Describe the Charging an electroscope positively by induction method. Bring a negatively charged rod near the cap of the electroscope. Electrons flow downwards from the cap to the brass plate and the leaf. With the rod in that position, the electroscope earthed by touching the cap. Electrons flow from leaf and the brass plate to the earth. The leaf falls. The finger is then first removed then the rod. The positive charge then distributes itself throughout the electroscope, causing the positively charged gold leaves to repel each other.
  2. Describe the Charging an electroscope negatively by induction method. A positively charged rod is brought near the cap of the electroscope. Electrons flow upwards from the brass plate and the leaf to the cap. With the rod in that position, the electroscope is earthed by touching the cap. Electrons flow from the earth to the leaf and the brass plate to neutralize the positive charges there. The leaf then falls. The figure is then first removed then the rod. The negative charge then distributes itself throughout the electroscope, causing the negatively charged gold leaves to repel each other.
  3. Describe the charging of electroscope positively by contact. The positively charged object has a deficiency of electrons. When contact is made, electrons from the neutral electroscope are attracted to the positively charged object. This transfer of electrons leaves the electroscope with a net positive charge (fewer electrons than protons). The positive charge distributes itself throughout the conducting parts of the electroscope (knob, rod, and both gold leaves). Since both gold leaves now have a positive charge, they repel each other, causing them to move apart and diverge. The degree of divergence is proportional to the amount of positive charge transferred to the electroscope. Discharging of an electroscope If you touch the metal cap with your finger the leaf collapses showing that the charges have been discharged through your body. An uncharged body will always cause the leaf of a charged electroscope to collapse regardless of the charge on the electroscope. This shows that charge moves from the charged electroscope to the uncharged body.

Uses of the electroscope a) To detect presence of charge on a body.

  • The material is brought close to the cap of the electroscope, if the leaf rises (diverges) the material is charged and if the leaf does not rise (diverge) the material is not charged b) To identify insulators and conductors
  • The materials are brought near the cap of charged electroscope, if the leaf falls, the material is a conductor and if the leaf does not fall then the material is an insulator c) To test the quantity of charge
  • The quantity (amount of charge) is estimated by the leaf divergence. The greater the divergence, the more the charges the slighter the divergence the lesser the charges. d) To test sign of charge on a body
  • The electroscope is first given a known charge e.g. positive of negative, then the charged material brought close to the charged electroscope. If leaf divergence increases, it means similar charge and if the leaf divergence decreases, it means opposite charge. Charging curve of an electroscope The charging curve of an electroscope is exponential growth since the amount of charge increases exponential with time. Discharging curve of an electroscope The discharging curve of an electroscope is exponential decay since the amount of charge decreases exponential with time.

Demonstration of point action A highly charged sharp point is brought close to a Bunsen flame. Observation The flame is blown away. Explanation Burning flame contains positive and negative ions. When the sharp point is brought close to the flame, negative ions are attracted to the sharp point, while positive ions are repelled away from the rod. As the positive ions are repelled, they create an “electric wind” which blows away the flame. NOTE: If the conductor is brought very close to the flame, the flame splits Application of point action Point action is applied in the working of the lightning arrestor. Working of a lightning arrestor. When a negatively charged cloud passes over the arrestor it induces positive charge on the spikes and negative charge on the plate. The negative charge on the plate is immediately discharged to the surrounding ground. Negative ions are attracted to the spikes and are discharged by giving up their electrons. At the same time, positive ions are repelled upwards from the spikes and they neutralize the negative charge on the clouds.