Charge & Coulomb’s Law 18.1-18.5
What is Electric Charge? An intrinsic property of protons and electrons, which make up all matter, is electric charge. A proton has a positive charge, and an electron has a negative charge.
Properties of electric charge There are two forms of charge which Ben Franklin called positive (+) and negative (-). The SI unit for measuring the magnitude of electric charge is the Coulomb (C). The electric charge is said to be quantized. The smallest amount of free charge (i.e. “elementary charge”) is e = 1.6×10-19 C. Any electric charge, q, occurs as integer multiples of the elementary charge e Protons have a charge of e and electrons have a charge of –e.
Sample Problem A certain static discharge delivers -0.5 Coulombs of electrical charge. How many electrons are in this discharge?
Properties of electric charge It is possible to transfer electric charge from one object to another. Usually electrons are transferred, and the body that gains electrons acquires an excess of negative charge. The body that loses electrons has an excess of positive charge. During any process, the net electric charge of an entire isolated system remains constant (is conserved). This is referred to as the law of conservation of electric charge.
Properties of electric charge Two electrically charged objects exert a force on one another, called an electrostatic force : like charges repel and unlike charges attract each other.
Conductors and Insulators Not only can electric charge exist on an object, but it can also move through an object. Substances that readily conduct electric charge are called electrical conductors. Materials that conduct electric charge poorly are known as electrical insulators. Semiconductors are materials that are intermediate between conductors and insulators Why are some material better conductors of electricity than others?
Charging by Friction, Contact and by Induction By Friction: rub two objects together, electrons are literally stripped off one and transferred to the other By Contact: By Induction :
CHARGING BY INDUCT ION
Checkpoint The figure shows five pairs of plates: A, B, and D are charged plastic plates and C is an electrically neutral copper plate. The electrostatic forces between the pairs of plates are shown for three of the pairs. For the remaining two pairs, do the plates repel or attract each other?
Coulomb's Law The electrostatic force is directed along the line joining the charges, and it is attractive if the charges have unlike signs and repulsive if the charges have like signs. The magnitude F of the electrostatic force exerted by one 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.
Coulomb’s Law Coulomb’s law tells us how the magnitude of the force between two particles varies with their charge and with the distance between them. Coulomb’s law applies directly only to spherically symmetric charges.
Sample Problem A point charge of positive 12.0 μC experiences an attractive force of 51 mN when it is placed 15 cm from another point charge. What is the other charge?
Superposition Electrical force, like all forces, is a vector quantity. If a charge is subjected to forces from more than one other charge, vector addition must be performed. Vector addition to find the resultant vector is sometimes called superposition.
Sample Problem Three charges are in a line along the x-axis. A -3 μC charge is at x = 1m, a 2 μC charge is at x = 3m, and a 4 μC charge is at x = 7m. What is the force on the 4 μC charge?
Sample Problem Three charges form a right triangle. There is a -3 μC charge, a 2 μC charge 2 cm below the -3 μC charge, and a 4 μC charge 6 cm to the right of the 2 μC charge. What is the direction of the force on the 4 μC charge?