Static Electricity

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What is Electricity? Historical Ideas The ancient Greeks knew about static electricity and the existence of 2 types of charge. The ancient Greeks knew about static electricity and the existence of 2 types of charge. In the 1700’s there was an emerging idea that some items were “electrics” (they attracted objects when rubbed) and others were “non- electrics” (did not attract objects when rubbed) In the 1700’s there was an emerging idea that some items were “electrics” (they attracted objects when rubbed) and others were “non- electrics” (did not attract objects when rubbed) No one could explain why! No one could explain why!

“Charge” is the name we apply to the property, and objects can be charged by friction “Charge” is the name we apply to the property, and objects can be charged by friction Charge gives rise to forces of attraction and repulsion Charge gives rise to forces of attraction and repulsion Observing attraction and repulsion, we can say that two charge states exist Observing attraction and repulsion, we can say that two charge states exist

Opposites Attract Charges that are like (that is, created in similar circumstances) repel Charges that are like (that is, created in similar circumstances) repel Charges which are unlike (that is, created in different circumstances) attract A neutral object is attracted to both positive and negative charges A neutral object is attracted to both positive and negative charges

Charles du Fay (1698 – 1739) A French physicist A French physicist Began his experiments in the 1700’s Began his experiments in the 1700’s Discovered that a cork ball touched by an “electrified” glass rod would attract another cork ball touched by an “electrified” resin rod. Discovered that a cork ball touched by an “electrified” glass rod would attract another cork ball touched by an “electrified” resin rod.

Two Fluid Theory His results led du Fay to conclude that there were two kinds of electric “fluids” His results led du Fay to conclude that there were two kinds of electric “fluids” 1. Vitreous Electricity 1. Vitreous Electricity 2. Resinous Electricity 2. Resinous Electricity

The Two Fluid Theory Says 1. All objects contain both fluids 1. All objects contain both fluids 2. An “electrically neutral” object has equal amounts of vitreous and resinous fluids. 2. An “electrically neutral” object has equal amounts of vitreous and resinous fluids. 3. An object is vitreous if it has more vitreous fluid and resinous if it has more resinous fluid 3. An object is vitreous if it has more vitreous fluid and resinous if it has more resinous fluid 4. When two objects come into contact with each other the fluids transfer from one object to another to balance out and become electrically neutral. 4. When two objects come into contact with each other the fluids transfer from one object to another to balance out and become electrically neutral.

The Problem What happens when two vitreous objects touch? What happens when two vitreous objects touch? What happens when two resinous objects touch? What happens when two resinous objects touch? Why do objects attract and repel each other? Shouldn’t everything attract everything else if everything contains both fluids? Why do objects attract and repel each other? Shouldn’t everything attract everything else if everything contains both fluids?

Benjamin Franklin Began studying electricity in Began studying electricity in Rejected du Fay’s two fluid theory. Rejected du Fay’s two fluid theory. Franklin was the first to use the terms positive and negative to describe electricity. Franklin was the first to use the terms positive and negative to describe electricity.

One Fluid Theory Franklin proposed that there is only one electrical fluid and an object is positively or negatively “charged” depending on how much fluid it has. Franklin proposed that there is only one electrical fluid and an object is positively or negatively “charged” depending on how much fluid it has. A “neutral” object has just enough fluid A “neutral” object has just enough fluid A “positive” object has too much fluid A “positive” object has too much fluid A “negative” object has too little fluid A “negative” object has too little fluid

How the one fluid theory solves the problems of the two fluid theory Positive and negative objects attract each other because the extra fluid from the positive object flows into the negative object Positive and negative objects attract each other because the extra fluid from the positive object flows into the negative object Like charges repel because: Like charges repel because: 1. A positive charged object won’t take the extra electrical fluid from another positive object 1. A positive charged object won’t take the extra electrical fluid from another positive object 2. A negative charged object doesn’t have extra fluid to give to another negative charged object 2. A negative charged object doesn’t have extra fluid to give to another negative charged object

A Modern Look at Electricity

The Modern Atomic Theory ( A review of Atoms and Elements) 1. Atoms are made up of positive particles called protons, negative particles called electrons and neutral particles called neutrons. 1. Atoms are made up of positive particles called protons, negative particles called electrons and neutral particles called neutrons. 2. Protons and neutrons are found in the nucleus of the atom while electrons orbit around the nucleus 2. Protons and neutrons are found in the nucleus of the atom while electrons orbit around the nucleus 3. Atoms are electrically neutral so: 3. Atoms are electrically neutral so: The # of protons = The # of electrons The # of protons = The # of electrons

The Particle Model of Electric Charge Developed from 20 th century experimentation. This is the model currently accepted as correct. Developed from 20 th century experimentation. This is the model currently accepted as correct. This theory states that charge is a result of the presence of protons and electrons. This theory states that charge is a result of the presence of protons and electrons.

The 3 Electrical States According to the particle model of electric charge, an object can exist in 3 electrical states. According to the particle model of electric charge, an object can exist in 3 electrical states. 1. An object can be Neutral 1. An object can be Neutral # of protons = # of electrons # of protons = # of electrons 2. An object can be Positive 2. An object can be Positive more protons than electrons more protons than electrons 3. An object can be Negative 3. An object can be Negative More electrons than protons More electrons than protons

Neutrons are found in the nucleus where they are fixed and cannot move Neutrons are found in the nucleus where they are fixed and cannot move Since protons (+) are found in the nucleus they are fixed and cannot move. Since protons (+) are found in the nucleus they are fixed and cannot move. Electrons (-) are moving around the nucleus and as a result are free to move. Electrons (-) are moving around the nucleus and as a result are free to move.

Neutral Attraction Bring a positively charged rod near bits of paper, and the paper will be attracted to the rod Bring a positively charged rod near bits of paper, and the paper will be attracted to the rod However, the paper is not negatively charged! However, the paper is not negatively charged! How do we know? If the paper was negatively charged, then the bits of paper would be repelling each other How do we know? If the paper was negatively charged, then the bits of paper would be repelling each other Bring a negatively charged rod near the paper, and the paper will be attracted to the rod Bring a negatively charged rod near the paper, and the paper will be attracted to the rod

Conductors Substances that allow electrical charge to pass through them easily. Substances that allow electrical charge to pass through them easily. Conductors have electrons that are not tightly held by their nuclei which allows electrons to move easily. Electrons are able to “flow” Conductors have electrons that are not tightly held by their nuclei which allows electrons to move easily. Electrons are able to “flow” Examples Examples Metals Metals Ions in water Ions in water The human body The human body

Insulators Substances that do not allow electric charge to pass through them easily. Substances that do not allow electric charge to pass through them easily. Insulators have electrons that are more strongly attached to their nuclei. Insulators have electrons that are more strongly attached to their nuclei. Examples Examples Glass Glass Plastics Plastics Water Water Air Air

Charging by Friction An object can be charged by rubbing it (friction) because this action can cause the electrons to be pulled off of or deposited onto the object An object can be charged by rubbing it (friction) because this action can cause the electrons to be pulled off of or deposited onto the object The electrons are unable to flow and therefore the transferred electrons will stay where they are placed. The electrons are unable to flow and therefore the transferred electrons will stay where they are placed.

Electron Affinity The transfer of electrons is determined by the Electron Affinity of the object. The transfer of electrons is determined by the Electron Affinity of the object. When 2 objects are rubbed together, the object with the highest Electron Affinity will take electrons from the other object becoming negative. When 2 objects are rubbed together, the object with the highest Electron Affinity will take electrons from the other object becoming negative. The object that loses electrons becomes positive. The object that loses electrons becomes positive.

Answer these Questions in your Notes: Electrostatic Phenomena 1. Why is a glass rod positively charged when it is rubbed with silk? 1. Why is a glass rod positively charged when it is rubbed with silk? 2.Why is a glass rod negatively charged when rubbed with wool? 2.Why is a glass rod negatively charged when rubbed with wool?

Charge is not created or destroyed, but rather moved around Charge is not created or destroyed, but rather moved around Diagram (p. 3.9) Diagram (p. 3.9)

Charging Objects by Conduction A neutral object can be “charged” by coming in contact with a charged object. A neutral object can be “charged” by coming in contact with a charged object. Charge is transferred between the neutral object and the charged object. Charge is transferred between the neutral object and the charged object.

Charging by Induction Charging a neutral object WITHOUT contact Charging a neutral object WITHOUT contact There are two types of induced charge; There are two types of induced charge; Temporary Induced Charge Temporary Induced Charge Permanent Induced Charge Permanent Induced Charge

Temporary Induced Charge A (+) charged object is brought near a neutral conductor. A (+) charged object is brought near a neutral conductor. The (+) charged object attracts the (-) charges to the end of the conductor nearest it. The (+) charged object attracts the (-) charges to the end of the conductor nearest it. This creates a Charge Separation This creates a Charge Separation the conductor now has a positive and a negative end the conductor now has a positive and a negative end

BUT: BUT: IF THE (+) CHARGED OBJECT IS REMOVED, THE CONDUCTOR BECOMES NEUTRAL AGAIN

Permanent Induced Charge A (+) charged object is brought near a neutral conductor and a charge separation is created. A (+) charged object is brought near a neutral conductor and a charge separation is created.

Permanent Induced Charge: Grounding The conductor is touched with a finger, producing a conducting path between the object and the Earth The conductor is touched with a finger, producing a conducting path between the object and the Earth The Earth (Ground) The Earth (Ground) has a huge reservoir of charges. It can easily absorb or release these charges to an object when necessary.

The finger is removed leaving the excess electrons in the conductor attracted to the (+) charged object. The finger is removed leaving the excess electrons in the conductor attracted to the (+) charged object.

The (+) charged object is removed allowing the excess (-) charges to distribute evenly. This leaves the object with a permanent (-) charge. The (+) charged object is removed allowing the excess (-) charges to distribute evenly. This leaves the object with a permanent (-) charge. The charge that is The charge that is induced is opposite to the charge of the original object.

Why Does this Work? The charge on the object is unable to move because it is an insulator. The charge on the object is unable to move because it is an insulator. The charge on the sphere can move because it is a conductor The charge on the sphere can move because it is a conductor The (-) charges are forced to move to one end of the sphere because they are attracted to the (+) charge. The (-) charges are forced to move to one end of the sphere because they are attracted to the (+) charge.

If you touch the sphere with your finger, you become a “ground” and provide a path for the electrons from the Earth to move towards the positive object. If you touch the sphere with your finger, you become a “ground” and provide a path for the electrons from the Earth to move towards the positive object. When you remove your finger, the pathway is broken and the electrons are left on the sphere. When you remove your finger, the pathway is broken and the electrons are left on the sphere.

Who Did What? Thomson Thomson Discovered the atom Discovered the atom Milikan Milikan Oil drop experiment Oil drop experiment Rutherford Rutherford Gold foil experiment Gold foil experiment Provided consistency between the atomic model of matter and the particle model Provided consistency between the atomic model of matter and the particle model

Particle Model of Electricity Two types of charge particles, positive (protons) and negative (electrons), exist Two types of charge particles, positive (protons) and negative (electrons), exist Charge cannot be created or destroyed – charge is conserved Charge cannot be created or destroyed – charge is conserved Positive charges are fixed and negative charges are free to move Positive charges are fixed and negative charges are free to move A neutral object has equal numbers of positive and negative charge A neutral object has equal numbers of positive and negative charge

Particle Model A negative object has an excess of negative charge A negative object has an excess of negative charge A positive object has a deficit of negative charge A positive object has a deficit of negative charge Charge is shared by contact. Materials which allow charge to move easily are called conductors and materials which do not allow charge to move easily are called insulators Charge is shared by contact. Materials which allow charge to move easily are called conductors and materials which do not allow charge to move easily are called insulators Like charges repel; unlike charges attract Like charges repel; unlike charges attract