Electric Charges, Forces & Fields

Level : Conceptual Physics Instructor : Kim

*~Warm up Question~* Two hundred years ago young boys called ‘powder monkeys’ ran below the decks of warships to pick up sacks of black gunpowder for the cannons above. It was the ship law that they do this task barefoot. Why?

1.1 Electric Charges - Static charge is a buildup of charge on an object

- There are two different types of charge, positive charge and negative charge - Charging is accomplished by getting some electrons to leave from one object and to go another, leaving the first object with a deficiency of electrons. - The object with a deficiency of electrons becomes positively charged - The object with an excess of electrons becomes negatively charged

- Charge is conserved. That is, electrons are neither created nor destroyed but are simply transferred from one material to another. In every event, whether large-scale or at the atomic level, the principle of conservation of charge applies

- An electron has the smallest amount of negative charge possible. => elementary charges - A proton has the smallest amount of positive charge possible. - These elementary charges (e) are equal in magnitude, but opposite in type. - In physics we represent charge with the letter Q or q. We measure charges in units called Coulombs(C). - One Coulomb actually represents a relatively large charge, so the elementary charge, measures in Coulomb, is extremely small (1C contains 6.25×1018elementary charges)

Elementary Charges The charge on 1 electron(e-) = -1.6×10-19C The charge on 1 proton (e+) = +1.6×10-19C

- Charges can be expressed as Q=ne, where n = ±1, ±2, ±3, …. => Charge is quantized. That is, it is impossible to have a charge of Q=0.5e

*~ An object that has unequal numbers of electrons and protons is electrically charged. If it has more electrons than protons, the object is negatively charged. If it has fewer electrons than protons, then it is positively charged

*~ Electric charge cannot be created or destroyed~*

1e- = -1.6×10-19C 1e+ = +1.6×10-19C Q=ne

Q1) The sulfide ion( S2-) has two extra electrons. Determine the charge on this ion in Coulumbs a) Q= +3.2×10-19C b) Q= -3.2×10-19C c) Q= +1.6×10-19C d) Q= -1.6×10-19C

Q2) Determine the charge in Coulombs of an ion of aluminum, Al3+, which lost three electrons. a) Q= +2.4×10-19C b) Q= +3.2×10-19C c) Q= +3.9×10-19 C d) Q= +4.810-19C

Q3) A man walks across a carpet and obtains a charge of -4.56×10-16C. How many excess electrons does this represent? That is, find the number of electrons a) 2850e- b) 2850e+ c) 1480e- d) 1480e+

*~ If the man took 2850 electrons from the carpet, giving him a net charge of -4.56×10-16C, the carpet would have a deficiency of 2850 electrons, giving it a net charge of +4.56×10-16C, because the total charge is conserved

Demo) A styrofoam tied to a string is suspending in the air. The only force acting on it is the gravitational force. If a charged object is placed close to the styrofoam, it will move. This shows that electrostatic force is much stronger than the gravitational force

1.2 Electric Forces - Charge objects exert forces at a distance on other charged objects. For example, a charge balloon can pick small pieces of paper. . - Like gravitational force, electrostatic force decrease with the square of the distance between the charged objects in question. => Coulomb’s law

- If the charges have unlike signs, each charged object is attracted to the other by a force +q +F –F –q

directed along the line between them r - If the charges have the same signs(both positive or both negative), each charged object +q –F –F +q

is repelled from the other r

- The magnitude F of the electrostatic force exerted by one point charge on another point charge is directly proportional to the magnitudes Q and q of the charges and inversely proportional to the square of the distance r between them

| | F = k : Coulomb’s law ² 𝑸𝑸𝑸𝑸 where k is a proportionality constant whose value is k = 9×109 N·m²/C² 𝒓𝒓

Coulomb’s Law vs Newton’s Law Formulas Coulomb’s Law Newton’s Law

F = k F = G ² ² 𝑸𝑸𝑸𝑸 𝑴𝑴𝑴𝑴 The gravitational force of attraction between two objects is Newton’s law proportional to the𝒓𝒓 product of the two masses and𝒓𝒓 inversely proportional to the square of the distance between them

The electrostatic force of attraction or repulsion between two Coulomb’s law charged objects is proportional to the product of the two charges and inversely proportional to the square of the distance between them.

· Both laws follow what is called the inverse square law for Similarities distance · Both formulas contain a proportionality constant

· The force with Newton’s law is attractive only, but electrostatic Differences force can be attractive or repulsive · The proportionality constant k = 9×109 N·m²/C² · The proportionality constant G = 6.67×10-11 N·m²/kg² Q4) Gravitational forces are responsible for the motion of planets. Electrical forces are responsible for the motion of charged particles. Which two forces are stronger? a) Gravitational force!! b) Electrical force!!