Electrical Charge and Coulomb’s Law of Electrostatic Force Honors Physics April 16, 2018

Focus Questions: write them down and leave spaces for the answers Define static electricity What is the fundamental rule of all electrical phenomena? What is in imbalance which causes an object to be electrically charged? What is the principle of conservation of charge? What is Coulomb’s Law? What does one coulomb of charge equal? What are three differences between Newton’s Law of Gravitation and Coulomb’s Law?

Static Electricity As you already know (or, at least, as you have already seen in the videos), static electricity (or electricity at rest) involves electric charges, the forces between them, and their behavior in materials. 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.

Static Electricity, continued 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 are POSITIVELY charged Neutrons have no charge The fundamental rule of all electrical phenomena is that like charges repel and opposite charges attract.

Static Electricity, continued When an object has equal numbers of electrons and protons it has no net electric charge. But, when there is an imbalance in the number of electrons and protons the object becomes electrically charged. An imbalance occurs by adding or removing electrons.

Static Electricity, continued The innermost electrons in an atom are bound very tightly to the oppositely charge atomic nucleus. The outermost electrons of many atoms are bound very loosely and can be easily “dislodged.” The amount of energy required to tear an electron away from an atom varies for different substances.

Static Electricity, continued Electrons are neither created nor destroyed but are simply transferred from one material to another. This is called the principle of conservation of charge.

Static Electricity, continued Outer electrons of the atoms in a metal are not anchored to the nuclei of particular atoms, but are free to “roam” in the material. Materials through which electric charge can flow are called conductors. Metals are good conductors for the motion of electric charges. Electrons in other materials (rubber and glass) are tightly bound and remain with particular atoms. Materials through which electric charge cannot flow are called insulators. Semiconductors are materials that can be made to behave sometimes as insulators and sometimes as conductors. Atoms in semiconductors hold their electrons until given a small energy boost. Silicon is a good example of a semiconductor

Coulomb’s Law When two charged objects, near one another experience acceleration either toward or away from each other because each object exerts a force on the other object this force is called electric force. + F - + F

Coulomb’s Law, continued Recall from Newton’s Law of Universal Gravitation that the gravitational force between two objects of mass (m1), and mass (m2) is proportional to the product of the masses and inversely proportional to the square of the distance (d) between them:

Coulomb’s Law, continued The electrical force between any two objects obeys a similar inverse-square relationship with distance. The relationship among electrical force, charges, and distance – Coulomb’s Law – was discovered by the French physicist Charles Coulomb in the eighteenth century.

Coulomb’s Law, continued For charge objects, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them. Where: 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 = 8.99x109 Nm2/C2

Coulomb’s Law, continued The SI unit of charge is the coulomb, abbreviated C. A charge of 1 C is the charge of 6.25x1018 electrons. A coulomb represents the amount of charge that passes through a common 100-W light bulb in about one second.

Coulomb’s Law, continued Newton’s law of gravitation for masses is similar to Coulomb’s law for electric charges. Whereas the gravitational force of attraction between a pair of one-kilogram masses is extremely small, the electrical force between a pair of one-coulomb charges is extremely large. The greatest difference between gravitation and electrical forces is that gravity only attracts but electrical forces may attract or repel.

Comparison with Gravitational Force What are 3 differences between the electrical force and the gravitational force?

Comparison with Gravitational Force What are 3 differences between the electrical force and the gravitational force?

Example 1 Two 40 gram masses each with a charge of 3μC are placed 50cm apart. Compare the gravitational force between the two masses to the electric force between the two masses. (Ignore the force of the earth on the two masses) 3μC 40g 50cm

The electric force is much greater than the gravitational force

Example 2 Two charges are separated by a distance r and have a force F on each other. q1 q2 r F If r is doubled then F is : ¼ of F If q1 is doubled then F is : 2F If q1 and q2 are doubled and r is halved then F is : 16F

Example 3 A charge of 2mC is 0.5 m from a charge of 3mC. Find the electric force.

Example 3 A charge of 2mC is 0.5 m from a charge of 3mC. Find the electric force.

Example 4 Two equal charges are located 1m from each other. The force acting between them is 2N. How many Coulombs is each charge?

How many Coulombs is each charge? Example 4 Two equal charges are located 1m from each other. The force acting between them is 2N. How many Coulombs is each charge? Answer: 15x10-6 C = 15μC

Example 5 The electron (9.109x10-31 kg; -1.60x10-19 C) and proton (1.673x10-27 kg; +1.60x10-19 C) of a hydrogen atom are separated, on average, by a distance of 5.3x10-11 meters. Find the magnitudes of the electric force and the gravitational force that each particle exerts on the other.

Example 5 The electron (9.109x10-31 kg; -1.60x10-19 C) and proton (1.673x10-27 kg; +1.60x10-19 C) of a hydrogen atom are separated, on average, by a distance of 5.3x10-11 meters. Find the magnitudes of the electric force and the gravitational force that each particle exerts on the other. -8.2x10-8 N and 3.6x10-47 N

Example 6 Three charges are positioned as shown. Find the force acting on the +2C charge.

Example 6 Three charges are positioned as shown. Find the force acting on the 2 C charge.

Example 6 Three charges are positioned as shown. Find the force acting on the 2 C charge.

Example 6 Three charges are positioned as shown. Find the force acting on the 2 C charge.

Example 6 Three charges are positioned as shown. Find the force acting on the 2 C charge.

Assignment page 566: 1-3, problem like example #6 Answers are given in significant figures #1 – 230 Newtons (attractive) #2a – 2.2x10-5 Newtons (attractive) #2b – 9.0x10-7 Newtons (repulsive) Hint: the charge of EACH after equilibrium is the average of the two charges before equilibrium. #3 – 0.393 meters Solve like example #6  1x1011 N, 18.4o 0.75 m 0.75 m