Physics 2, Chp 18 Pt. A - Electric Force

Insulator

(Dielectric) - poor conductor of electric charge, limited movement of electrons. Valence electrons in atoms are tightly bound. The electrons do not flow easily through an insulator.

Coulomb's Law of Electrostatics

*Electrostatic Force* - Suppose that two point charges, q1and q2 are a distance r apart in a vacuum. If q1 and q2 have the same sign, the two charges repel each other; if they have opposite signs they attract each other.

Total charge on an objec

*Q = Nqe* where N is # of excess or missing electrons

All interactions (that we know of) in the universe are produced by the four fundamental forces of nature

1. Gravitational force 2. Electromagnetic Force (Electric and magnetic force) 3. Weak force 4. Strong force

*Neutral charge*

1. equal number of positive and negative charge, zero net charge. 2. does NOT mean no charge

*Electron (e)*

1. is a subatomic particle that has a very small mass relative to the proton. The electron has a charge that is equal in magnitude to the proton but opposite in charge than the proton, the electron resides outside the nucleus of the atom. 2. has a very small mass, mass of the proton is about 2,000 time that 3. Upper limit on the diameter is about 10^-18 meters 4. All are exactly the same (mass and charge) 5. The electrostatic force is what keeps them bound to the atom 6. For an atom to be neutral there must be as many electrons (negative charge) as protons (positive charge) 7. Scientists believe that it is a fundamental particle and is not made up of any smaller particles.

*Proton (p)*

1. is a subatomic particle that is part of the nucleus of the atom, has a positive charge that is equal in magnitude to the electron and it has slightly less mass than the neutron. 2. *Charge on the electron (qp or p) qp = 1.6 x 10-19 C and is called positive* 3. defines what element the atom is and its chemical properties. 4. he atomic number (Z) gives the number of protons in the nucleus of the atom. 5. All are exactly the same (mass and charge) 6. The diameter is about 10^-15 m or 1 fm 7. made up of smaller particles known as quarks 8. The Strong Force is what keeps the protons and neutrons glued together inside the Atom

*Neutron (n)*

1. is a subatomic particle that is part of the nucleus of the atom, has no net charge and has slightly more mass than the proton. 2. *Has no net charge* 3. All are exactly the same (mass and charge) 4. made up of smaller particles known as quarks 5. The Strong Force is what keeps the protons and neutrons glued together inside the Atom

*Quarks*

A fundamental particle, protons and neutrons are made of two types (up and down ). There are other types and other particle made from them. is not made of any smaller particle and is pure energy.

Grounding

An object that is large enough that it can take a small excess charge without itself becoming significantly charged. Common examples are; the human body and the earth itself.

Temporary Induction of Charge

If there is a neutral wall and a negative charged balloon is brought near the wall, the charge is displacement in the neutral wall and the negative balloon induces a positive charge near the negative balloon and the a negative charge is moved away from the negative balloon. This is an induced charge on the wall but the wall still have no net charge, the charge in the wall is polarized, inducing a positive charge on the surface of the wall due to the negative charge of the balloon. The balloon will "stick" to the wall due to the electrostatic attraction on the negative (balloon) and positive (induced on wall) charges. When the charged balloon is removed the charges in the wall return to their pervious neutral positions.

Semiconductor

Insulating material that an impurity is added to change its electrical properties and can be changed from an insulator to a conductor by changing some eternal property.

*Electromagnetism*

The combined effect of the Electric and the Magnetic forces and fields.

Electrostatic Force

The force experienced by either charge due to the other is called a Coulomb or electric force 1. The forces on the two charges are equal in magnitude and opposite in direction even if the charges are not the same (Newton's 3rd law). 2. For two objects with a net charge, the force acts along a straight line drawn through both charges *is a vector quantity; magnitude and direction must be taken into account!*

*Coulomb's Law*

The magnitude F of the electrostatic force exerted by on point charge q1 on another point charge q2 is directly proportional to the magnitudes q1 and q2 of the charges and inversely proportional to the square of the distance r between them: *FE = k|q1||q2|/r^2 (in vacuum) Units are Newton's (N)* Electrostatic Constant k = 9.0 x 10^9 Nm^2/C^2 *1 .The "sign" of the charge is not used in the force equation* *2. The direction of the force determines the sign of the force and comes from the coordinate system.* *3. The force that q1 experiences is equal in magnitude to the force q2 experiences.* 4. The direction of the electrostatic force is determined by the relative charges that are producing the force (-,- or +,+ or -,+) and is a vector quantity. 5. This equations works for point sources and spherical bodies only. 6. Has the same form as Newton's Universal Gravitational Law

Force due to more than two point charge

The net force acting on a charged object due to the electrostatic force is the vector sum of each individual force.

Electric Dipole

Two equal but opposite charges that are separated by some distance but are still bound to each other. can be naturally accruing or they can be induced by bringing a charge near a neutral molecule (polarization).

Charging by Induction

Using the polarization effect a movement of charge can be induced in an object due to an external charge, this can produce a temporary or net charge on an objects surface.

Charging a conductor by induction

When a negatively charged object (like the Ebonite rod) is brought near a neutral metal conducting sphere but does not touch then the electron in the metal sphere are replied by the negative charge on the rod and move to the other side of the sphere. If the sphere is grounded, meaning the electrons are given a path to move away from the negatively charged rod then they will move to ground. If the path to ground is remove not letting the charge back on the sphere and then the rod is moved away the result is a metal sphere with a net positive charge. The same can be done with a positively charged rod and the net effect would be to produce a net negative charge on the sphere.

Electrification by Rubbing (Triboelectric Charging)

When two material are rubbed together they create an equal and opposite charge on each of the materials being rubbed. Charge can be positive or negative and is dependent on how strongly the material holds on to its electrons. When rubbed: *Positive Charged* - Fur, glass, hair, nylon and silk become positively charged (+), therefore they losses electrons. *Negative Charged* - Teflon, rubber, amber, wood and cotton become negatively charged (-) therefore they gain electrons.

Conductor

a material that electric charge can easily flow, abundant supply of free electrons (examples: metals, some liquids), valence electrons are not tightly bound and are free to move.

Conservation (in Physics)

a quantity is conserved when the total quantity does not change over time, it only moves around to different places or different forms. Examples are: Energy, momentum, mass and now charge

Ionization

adding or removing an electron from the atom leaving a net charge on the atom. Turning the atom into an Ion.

Charge Quantization

all charge is a multiple of the fundamental charge, all transfer of charge is a multiple of this fundamental charge

The Ion

an atom with a net charge due to losing or gaining an electron

*Electric Charge (q)*

an intrinsic property of the proton and electron. 1. There are only two kinds and they are called positive (+) & negative (-) 2. "Positive" and "negative" are only names and not algebraic signs 3. Like repel and unlike attract. 4. When positive is produced an equal amount of negative is also produced 5. can be transferred from one object to another 6. A net creates a force on other objects with a net

Direct Contact

charge is transferred by physical contact by a charged object. Because each individual charge on the charged rod repels each other, when the charge has a chance to move farther away, like moving to a metal sphere, then they wil

Electricity

derived from the Greek word for amber, elektron. It was the ancient Greeks who first made the observation that after amber had been rubbed with a cloth it attracted small pieces of lint. This is now known as the electrostatic force.

Superposition principle

determine the magnitude and direction of the forces on the charge one by the other charges then take the vector sum to find resultant force. The forces are superimposed on each other and the resultant force is the net force the particle experiences. Fresultant = F1 + F2 + F3 + ... vector sum

Conservation of charge

electric charges can be produced only in pairs of equally strong positive and negative charges. The net charge of the universe never changes 1. The total charge in a closed system does not changes over time 2. Charge is neither created nor destroyed it is transferred from one object to another.

Net Negative charge

electrons have been add to a neutral object

Net Positive charge

electrons have been removed from a neutral object

Weak Force

force in the nucleus of the atom responsible for radioactive decay

Electromagnetic Force (Electric and magnetic force)

generated by charge

Gravitational force

generated by the mass of an object

1-D net force due to multiple point charges

if more than two point charges act on an object with a net charge the vector sum of the electrostatic force from each point source acting is the net force acting on the charged object. For 1-D forces the direction is taken into account by the "sign" of the force, which is obtained from the direction of the induvial forces and the defined directions of the x-axis.

2-D net force due to multiple point charges

if the charged objects are not along the same line but are in 2 dimensional space then the magnitude and direction of the electrostatic forces must be taken into account by finding the resultant for the forces acting.

Quanta

in physics is the smallest amount a quantity can physically have and all quantities are made up of multiples of this

Charge mobility

is how the charge is able to move on an object. 1.If the object is a conductor (like metal) and this object has a net charge, then the charge is free to move and distributes itself over the entire surface of the conducing object. If you discharge a conductor, all the charge over the entire surface will discharge. 2. If the object is an insulator (plastic) and this object has a net charge, then the charge is not free to move, it stays where it was placed or created and does not move around the object. If you try to discharge this object only the surface that is touched discharges and the rest of the object will stay charged.

Polarization

is the relative shift of charge in an object due to an external change near but not touching the object. The charge in the object that is the same type as that of the external charge will be moved away from the external charge and opposite charge of the external charge will be moved toward the external charge resulting in a charge separation with alignment of the charge pairs.

Strong Force

keeps the protons and neutrons stuck together in the nucleus

*Valence electrons*

outer most electrons in an atom. The outermost electrons are responsible for how atoms and molecules will react with each other and there environment. determine the properties and chemical reactivity of the element and participate in chemical bonding

Fundamental Charge

qe = 1.6 x 10^-19 Coulombs is the smallest possible charge, charge is quantized. Integer number of indivisible charges. All charge is a multiple of qe. 1qe, 2qe, 3qe, 4qe, ... partial charges do not exist naturally in nature (maybe).

*Coulombs (C)*

the SI unit for charge

Charge and the Elements

the charge of an element is neutral unless electrons have been removed or added.

*The Atom*

the smallest particle of an element that can be identified as that element. made up of three fundamental particles; the electron, proton and neutron 1. There are an equal number of positive protons as there are negative electrons for an object that is neutral (has no net charge). 2. If there are an unequal number of protons and electrons then it is charged. 3. Most of the mass is in the nucleus

*Electrostatics*

the study of the electric charge at rest, the charge is not moving. Examples: static cling, Balloon on wall, hair standing up after combing, lightning, hocked by door knob because shoes rub across on rug

Methods For Producing a Net Charge

there are three methods: 1. Electrification by Rubbing (Triboelectric Charging) 2. Direct Contact 3. Charging by Induction

Negative Ion

when a neutral atom gains a negatively charged electron.

Positive Ion

when an electron is removed from a neutral atom, this gives the atom a net positive charge and the separated electron is a free negative charge.

*Net Charge*

when there is an excess of positive or negative charge on an object.