Electric Forces and Fields Chapter 18

ELECTRIC CHARGE Section 1

Chemistry Review Atoms are small particles that make up matter Atoms are neutral when they have equal numbers of ______________and _____________ Atoms can lose electrons, because electrons are located in the _________________ Protons are in the _______________and do not leave the atom When atoms gain electrons, they become _____________ charged ions When atoms lose electrons, they become ______________ charged ions

Electric forces and charges The electric force is one of four fundamental forces. The electric force is a field force. The electric force is the result of the interaction between electric charges.

Discovery Lab: Electrostatics Observe what happens at each step below: Step 1: Rub the rubber rod with the fur and bring it near the electroscope. Touch the rubber rod to the electroscope. Scrape the rod against the sphere. Touch the sphere with your hand. Step 2: Rub the glass rod with the silk. Bring the glass rod near the electroscope. Scrape the rod against the sphere. Touch the sphere with your hand. Step 3: Repeat Steps 1 and 2, but this time don’t touch the electroscope with your hand in between steps. © 2013 Mark Lesmeister/Pearland ISD

Properties of Electric Charge There are two kinds of electric charges. Protons have a positive charge; electrons negative. Opposite charges attract; Like charges repel.

Properties of Electric Charge Electric charge is conserved. When a neutral atom gives up an electron, it becomes a positive ion. The electron is the smallest unit of charge that can be isolated AKA elementary charge - +

Insulators and Conductors A conductor is a material that allows charges to flow easily throughout the material. Most metals are conductors. An insulators is a material that does not allow charges to flow easily throughout the material. Glass, rubber, silk and plastic are examples of insulators.

Transfer of Electric Charge: Contact – (friction and Conduction) Conductors and insulators can be charged by contact. Example- Rubbing rubber rod with fur, glass rod with silk. = charging by friction

18.4 Charging by Contact and by Induction Charging by contact.

Transfer of Electric Charge: Contact – (friction and Conduction)

Transfer of Electric Charge: Induction Conductors can be charged by induction. A charged object is brought near the conductor. The conductor is then grounded, allowing the excess charge to flow away. The grounding is then broken. The conductor remains charged, even when the charged object is removed. © 2013 Mark Lesmeister/Pearland ISD

18.4 Charging by Contact and by Induction Charging by induction.

Transfer of Electric Charge: Polarization A surface charge can be induced on an insulator by polarization. Ex. Walking across a carpet on a dry day. + - © 2013 Mark Lesmeister/Pearland ISD

LAW OF CONSERVATION OF ELECTRIC CHARGE During any process, the net electric charge of an isolated system remains constant (is conserved).

What is happening in this diagram There are two kinds of electric charge The two balloons must have the same charge and as a result, repel each other like charges repel A rubbed balloon and your hair do not have the same charge and as a result, they are attracted to each other Unlike charges attract

III - Semiconductors These materials have properties of both insulators and conductors. Silicon and Germanium are examples of semiconductors used in electrical devises.

VI - Superconductors These materials appear to show no resistance The electrons passing through a super conductor generate no heat Ex. include metals that are perfect conductors when at or below certain temperatures.

ELECTRIC FORCE Section 2

Electric Force When two charged objects are brought close together, they may experience forces of attraction or repulsion. The closer the two objects are to each other, the stronger the force between them. The greater the mass (number) of the charges the stronger the force between them

Magnitude and direction of the electric force depends on: The amount of charge carried by each object. Charge is measured in Coulombs (C). An electron (negative elementary charge) has a charge (q) of x C. A proton (positive elementary charge) has a charge (q) of 1.60 x C.

Coulomb’s Law The force between 2 charged objects can be calculated using coulomb’s Law K c = 8.99 X 10 9 N.m 2 /C 2

Fundamental forces of nature

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Inverse square of distance – complete the chart Original dNew dNew/orig. Change in distance reduceinverseSquare Change in Fg /102/11/2¼ x /201/22/14x

Example 1 What is the magnitude of the electric force between a proton and an electron in a hydrogen atom? The two particles are separated by a distance of 5.3 x m on average. G: q 1 = x C, q 2 = 1.60 x C, r = 5.3 x m, K c = 8.99 x 10 9 N.m 2 /C 2 U: F electric = ? E: S: S: 8.19 x N  An attractive force!!

Example 2: A Model of the Hydrogen Atom

YESTERDAY’S CLASS ASSIGNMENT DUE NOW will not accept late work

The Electric Field Section 6

Electric Field The electric force is a field force. An electric field exists in a region if a stationary charged object (test charge – usually positive) experiences an electric force because of its charge.

Electric Field Vector The electric field at any point is a vector. The direction of the electric field is the direction in which a small positive charge would move if placed there. The magnitude of the electric field (E) is the force per unit charge at that point: The electric field is measured in N/C.

18.6 The Electric Field It is the surrounding charges that create the electric field at a given point.

18.6 The Electric Field Electric fields from different sources add as vectors.

Electric Field Lines Electric field lines or lines of force provide a map of the electric field in the space surrounding electric charges. Electric field lines represent the strength and direction of an electric field at any point around a charged object. Electric field lines do not really exist, however, they offer a useful means of analyzing electric fields.

Electric Field Lines Electric field lines run in the direction of the field at each point. Lines begin on positive charges (or infinity) and end at negative charges (or infinity). Lines do not cross. The magnitude of the field is shown by the number of lines in a given area.

Electric Field Lines – sketch this!!

18.7 Electric Field Lines The number of lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge. Electric Field Lines – sketch this!! Write this in the box!!

18.7 Electric Field Lines Electric Field Lines – sketch this!! Now sketch two negative charges!!

Electric Field Strength

Example Problem #3 A typical thundercloud has an electric field of about 3.0 x10 5 N/C. If the electric field is directed toward the ground. What is the electric force on an 18 nC charge in this field? G: E = 3.0 x 10 5 N/C, q = 1.8 x C U: F electric = ? E: S: S: 5.4 x N directed towards the ground

Gravitational Field Strength of a Point Mass (or Spherically Symmetric Mass The gravity field around a point mass, or a spherically symmetric mass, depends only on the mass and the distance away from the center.

CLASS ASSIGNMENT DUE TOMORROW Will not accept late work