Chapter 21 - Electrostatics Conceptual Physics

Electrostatics Electrostatics, or electricity at rest, involves electric charges, the forces between them, and their behavior in materials The fundamental rule at the base of all electrical phenomena is that like charges repel and opposite charges attract.

Electrical Forces and Charges The Atom Electrical forces arise from particles in atoms. The protons in the nucleus attract the electrons and hold them in orbit. Electrons are attracted to protons, but electrons repel other electrons.

Electrical Forces and Charges The fundamental electrical property to which the mutual attractions or repulsions between electrons or protons is attributed is called charge. Electrons are negatively charged. Protons positively charged. Neutrons have no charge, and are neither attracted nor repelled by charged particles.

Electrical Forces and Charges The helium nucleus is composed of two protons and two neutrons. The positively charged protons attract two negative electrons.

Electrical Forces and Charges Every atom has a positively charged nucleus surrounded by negatively charged electrons. All electrons are identical. The nucleus is composed of protons and neutrons. All protons are identical; similarly, all neutrons are identical. Atoms usually have as many electrons as protons, so the atom has zero net charge. A proton has nearly 2000 times the mass of an electron

Electrical Forces and Charges Attraction and Repulsion

Charges and Ions An object that has unequal numbers of electrons and protons is electrically charged. A positive ion has a net positive charge; it has lost one or more electrons. A negative ion has a net negative charge; it has gained one or more extra electrons.

Electrons The innermost electrons in an atom are bound very tightly to the oppositely charged atomic nucleus. The outermost electrons of many atoms are bound very loosely and can be easily dislodged. How much energy is required to tear an electron away from an atom varies for different substances.

Electrons When electrons are transferred from the fur to the rod, the rod becomes negatively charged.

Conservation of charge Electrons are neither created nor destroyed but are simply transferred from one material to another. This principle is known as conservation of charge.

Objects that tend to give up electrons and become positive: Glass Nylon Fur Hair Wool

Objects that tend to attract electrons and become negative: Rubber Polyester Styrofoam Saran Wrap PVC

Coulombs Law Coulomb’s law states that the force between charges varies directly as the product of the charges and inversely as the square of the distance between them. d is the distance between the charged particles. q1 represents the quantity of charge of one particle. q2 is the quantity of charge of the other particle. k is the proportionality constant.

Coulombs Law k = 9,000,000,000 N·m2/C2 or 9.0 × 109 N·m2/C2 The SI unit of charge is the Coulomb, abbreviated C. A charge of 1 C is the charge of 6.24 × 1018 electrons. A coulomb represents the amount of charge that passes through a common 100-W light bulb in about one second Electron charge (-e): -1.60 x 10-19 C Proton charge(+e): +1.60 x 10-19 C

Practice Problems There are two positive charges, both are 4uC and they are 2.5m apart. What is the repelling force between them? There are two charges, one is -8uC and the other is 6.4uC. They are located 9.6 cm apart. What is the force between them? Is it an attractive or repelling force? Two electrons are located 3m apart. What is the force between them?

Comparison What is the chief significance of the fact that G in Newton’s law of gravitation is a small number and k in Coulomb’s law is a large number when both are expressed in SI units? The small value of G indicates that gravity is a weak force the large value of k indicates that the electrical force is enormous in comparison.

Problems If an electron at a certain distance from a charged particle is attracted with a certain force, how will the force compare at twice this distance? Answer: In accord with the inverse-square law, at twice the distance the force will be one fourth as much. Is the charged particle in this case positive or negative? Answer: must be positive because it is attracted to the electron.

Conductors Materials through which electric charge can flow are called conductors. Metals are good conductors for the motion of electric charges because their electrons are “loose.”

Insulators Electrons in other materials—rubber and glass, for example—are tightly bound and remain with particular atoms. These materials, known as insulators, are poor conductors of electricity.

Semiconductors Semiconductors are materials that can be made to behave sometimes as insulators and sometimes as conductors. Thin layers of semiconducting materials sandwiched together make up transistors.

Transfer of charge Two ways electric charge can be directly transferred are by friction and by contact. Electrons are being transferred by friction when one material rubs against another. Combs, balloons, rugs, vehicles, etc…

Transfer of charge Electrons can also be transferred from one material to another by simply touching. This method of charging is called charging by contact. If the object is a good conductor, the charge will spread to all parts of its surface because the like charges repel each other.

Induction If a charged object is brought near a conducting surface, even without physical contact, electrons will move in the conducting surface.

Charging by Induction Charging by induction can be illustrated using two insulated metal spheres. Uncharged insulated metal spheres touching each other, in effect, form a single noncharged conductor.

Charging by Induction When a negatively charged rod is held near one sphere, electrons in the metal are repelled by the rod. Excess negative charge has moved, leaving the first sphere with an excess positive charge. The charge on the spheres has been redistributed, or induced.

Charging by Induction When the spheres are separated and the rod removed, the spheres are charged equally and oppositely. They have been charged by induction, which is the charging of an object without direct contact.

Electric Fields An electric field is a force field that surrounds an electric charge or group of charges. A gravitational force holds a satellite in orbit about a planet an electrical force holds an electron in orbit about a proton.

Electric Fields An electric field has both magnitude and direction. That means it is a VECTOR! The magnitude can be measured by its effect on charges located in the field. Electric fields travel from positive to negative Out of the positive Into the negative

Electric Fields Electric field lines can be used to represent an electric field. The lines always hit a charge at a 90° angle

Electric Fields Inside a conductor the electric field is zero. The charges naturally spread out evenly due to repulsion and attraction forces, therefore, their charges cancel each other out. This is called electric shielding. We can shield electric forces, but we can’t shield out gravity…why? Gravity only attracts!

Electric Potential Energy A charged object can have potential energy by virtue of its location in an electric field. To push a positive test charge closer to a positively charged sphere, we will expend energy to overcome electrical repulsion. Work is done in pushing the charge against the electric field. This work is equal to the energy gained by the charge. The energy a charge has due to its location in an electric field is called electrical potential energy.

Electric Potential energy If the charge is released, it will accelerate away from the sphere and electrical potential energy transforms into kinetic energy. Increasing electrical potential energy:

Electric Potential Electric potential is not the same as electrical potential energy. Electric potential is electrical potential energy per charge.

Electric Potential An object of greater charge has more electrical potential energy in the field of the charged dome than an object of less charge, but the electric potential of any charge at the same location is the same.

Electric Potential = Voltage The SI unit of measurement for electric potential is the volt (V), named after the Italian physicist Allesandro Volta. Potential energy is measured in joules and charge is measured in coulombs,

Electric Potential A potential of 1000 V means that 1000 joules of energy per coulomb is needed to bring a small charge from very far away and add it to the charge on the conductor. To add one proton to the conductor would take only 1.6 × 10–16 J.

Capacitors Electrical energy can be stored in a device called a capacitor. Examples: Computer memories use very tiny capacitors to store the 1’s and 0’s of the binary code. Capacitors in cameras store larger amounts of energy slowly and release it rapidly during the flash. Enormous amounts of energy are stored in banks of capacitors that power giant lasers in national laboratories.

Capacitors The simplest capacitor is a pair of conducting plates separated by a small distance, but not touching each other. Charge is transferred from one plate to the other. The capacitor plates then have equal and opposite charges.

Capacitors The charging process is complete when the potential difference between the plates equals the potential difference between the battery voltage. The greater the battery voltage and the larger and closer the plates, the greater the charge that is stored.

Capacitors A charged capacitor is discharged when a conducting path is provided between the plates.

Capacitors The energy stored in a capacitor comes from the work done to charge it. The energy is in the form of the electric field between its plates. Electric fields are storehouses of energy.

Van de Graaff Generator The voltage of a Van de Graaff generator can be increased by increasing the radius of the sphere or by placing the entire system in a container filled with high-pressure gas.

Van de Graaff Generator