ELECTROSTATICS ELECTROSTATICS – the study of electric charges, forces and fields Two types of charges exists, arbitrarily named POSITIVE and NEGATIVE By Benjamin Franklin So what is positive and what is negative?
We know that charged particle exist in atoms Electrons negative Electrons are responsible for negative charges and Protons for positive charge Benjamin Franklin did not know about the existence of these particles but, he did investigate the behavior of static discharge and lightning.
coulomb (C). The SI unit of charge is the coulomb (C). 1 C = 6.25 x 10 18 electrons or protons The charge carried by the electron is represented by the symbol -e, and the charge carried by the proton is +e. A third particle, which carries no electrical charge, is the neutron. The charge of a single electron = -1.6 x 10 -19 C The charge of a single proton = +1.6 x 10 -19 C
Ben knew that if certain electrically neutral objects are rubbed, they can become charged. For example; when rubber is rubbed with a wool cloth, both become charged. The rubber scrapes electrons from fur atoms. So the rubber is negatively charged and the cloth is positively charged. or a comb through hair
Ben also knew that a charge separation occurs when a glass rod is rubbed with a silk cloth In the case of the glass and silk, the glass rod loses negative charge and becomes positively charged while the silk cloth gains negative charge and therefore becomes negatively charged.
Ben experimented with the interactions between the charge objects. He suspended one and brought other charged objects near… Ben observed that like charged unlikeattract object repel & unlike charges attract repel attract
A NEGATIVE charge is an EXCESS of electrons POSITIVE SHORTAGE A POSITIVE charge is a SHORTAGE of electrons PRINCIPLE OF CONSERVATION OF ELECTRIC CHARGE moved aroundnot created or destroyed. In the process of rubbing two solid objects together, electrical charges are moved around, not created or destroyed. The negative charges So… The negative charges are transferred between the two objects. Leaving one with an excess of positive charge and the other with an excess of negative charge. The quantity of excess charge on each object is exactly the same.
electroscope An electroscope is a device that detects static charge. The metal leaves of the electroscope move apart if a charged object is brought near the knob. Benjamin Franklin used a similar device when he investigated charges. Positively charged Negatively charged Demo
insulator An insulator is a material in which the electrons are tightly held by the nucleus and are not free to move through the material. Examples of good insulators are: glass, rubber, plastic and dry wood. A material can be an electric … conductor free to move A conductor is a material through which electrons are free to move through the material. Examples of good conductors are: metals, such as silver, copper, gold and mercury.
Objects become charged by… Friction Induction Conduction Electrons are rubbed off one insulator on to another insulator Charging an object WITHOUT touching a charged object Charging by CONTACT with a charged object
Electrons are rubbed off one insulator on to another insulator http://phet.colorado.edu/en/simulation/balloons
Charging by Conduction Charging by Conduction A charged object touches the uncharged object and some electrons leave the charged rod and move spread over sphere. same charge Charging by conduction results in an object with the same charge
POLARIZATON. Neutral objects can be temporarily attracted to charged objects by a process called POLARIZATON. To understand the last way to charge an object, we need to look at what happens to a neutral object when a charged object is brought near.
Electrons are free to move in metals. Nuclei remain in place; electrons move to bottom polarization Polarization occurs because the electrons are attracted or repelled by the charged object. This result is in a polarization or temporary separation of the charge, and attraction results.
Charging by Induction The rod does not touch the sphere. It pushes electrons out of the back side of the sphere and down the wire to ground. The ground wire is disconnected to prevent the return of the electrons from ground, then the rod is removed. The resulting charge on the object is opposite polarization grounding permanent charge
Grounding is allowing charges to move freely along a connection between a conductor and the ground. The Earth (the ground) is a practically infinite reservoir of electric charge. Here a positively charge rod attracts electrons from the ground into the electroscope Here a negatively charge rod repels electrons into the ground from the electroscope
To review… Induction results in an OPPOSITE CHARGE
COULOMB’S LAW Coulomb’s Law states that two point charges exert a force (F) on one another that is directly proportional to the product of the magnitudes of the charges (q) and inversely proportional to the square of the distance (r) between their centers. The equation is: F = electrostatic force (N) q = charge (C) k = 9x10 9 N m 2 /C 2 r = separation between charges (m)
Example 1 Two negatively charged balloons are 0.70m apart. If the charge of each is 2.0 x 10 -6 C, What is the electric force between the two balloons? q 1 = q 2 = 2.0 x 10 -6 C d = r = 0.70 m F = 9.0 x 10 9 N m 2 /C 2 (2.0 x 10 -6 C) 2 (0.70m) 2 F = 0.073 N An attracting or repelling force?
Example 2: Two equally charged balloons repel each other with a force of 4.0 x 10 -3 N. If they are 0.015 m apart, what is the charge of the each balloon? F = 4.0 x 10-3 N d = 0.015 m q 2 = (4x10 -3 N)(0.015m) 2 (9x10 9 Nm 2 /C 2 ) q 2 = Fd 2 k q 1 = q 2 = 1.0 x 10 -8 C
Example 3 : Two charges q 1 = -8 μC and q 2 = +12 μC are placed 120mm apart in the air. What is the resultant force on a third charge q 3 = -4 μC placed midway between the other charges? q 1 = -8 μC q 2 = +12 μC q 3 = -4 μC r = 0. 120m -q1-q1 +q2+q2 -q3-q3 F1F1 F2F2 FRFR = 80 N to the right = 120 N to the right F R = 80N + 120N = 200 N, to the right