Electricity

Electric Charge & Force Electric Charge an electrical property of matter that creates a force between objects we experience this force as a shock Like charges repel, and opposite charges attract the two types of charges are called positive & negative when there is an equal amount of positive & negative charges on an object, it has no net charge

Electric Charge & Force

Electric Charge & Force The photo to the left is a Triboelectric Series. It shows that, when two objects are rubbed together, the object on top will become positive (lose electrons) while the object on the bottom will become negative (gain electrons).

Electric Charge & Force An object’s electric charge depends on the imbalance of its protons & electrons objects are made up of an enormous number of neutrons, protons, and electrons whenever there is an imbalance in the number of protons & electrons in an atom, molecule, etc. it has a net electric charge difference in the numbers of protons & electrons determines an object’s electric charge the SI unit of electric charge is the coulomb, C

Electric Charge & Force Conductors allow charges to flow; insulators do not Conductors a material that transfers charge easily ex. metal Insulator a material that does not transfer charge easily ex. cardboard, glass, silk, & plastic Objects can be charged by the transfer of electrons protons & neutrons are relatively fixed in the nucleus… electrons can be easily transferred ex. sliding across a fabric car seat = charging by friction/motion

Electric Charge & Force objects can also be charged without friction ex. by touching something with a charged object objects charged in this manner are said to be charged by contact Charges move within uncharged objects charges in a neutral conductor can be redistributed w/out contacting a charged object opposite sides will have an induced charge (induction)

Electric Charge & Force How can a negatively charged comb pick up pieces of neutral tissue paper? polarization of atoms in an insulator produces an induced charge on the surface of the insulator Electric force the force of attraction or repulsion between objects due to charge responsible for holding atoms together

Electric Charge & Force Electric force depends on charge and distance Balloon example electric force is inversely proportional to the square of the distance between two objects Huh ? For example, if the distance between two charged balloons is doubled, the electric force between them decreases to ¼ its original value quadrupled 1/16

Electric Charge & Force Electric force acts through a field Electric field the region around a charged object in which other charged objects experience an electric force one way to show an electric field is by drawing electric field lines electric field lines point outward on a positive charge electric field lines point inward on a negative charge

Electric Charge & Force field lines for two positive charges repel  could also be for negative charges field lines for a positive & a negative attract Number of field lines and their closeness determines strength of field

Current How do charges move through a light bulb? What causes the charges to move? Voltage & Current Electrical Potential Energy potential energy of a charged object due to its position in an electric field if two charged objects repel each other, the electrical potential energy will be greatest when the objects are near each other

Current Potential difference is measured in volts Potential difference/ voltage the change in the electrical potential energy per unit charge this change occurs as a charge moves from one place to another in an electric field SI unit for potential difference is the volt, V

Current There is a voltage across the terminals of a battery Most common batteries are an electric… Cell a device that is a source of electric current because of a potential difference, or voltage, between the terminals + - or a combination of connected electric cells – that convert chemical energy to electrical energy

Current “dc”

Current A voltage sets charges in motion How does a flashlight work? Current Continuous flow of electric charges through a material; the rate that electric charges move through a conductor SI unit of current is the ampere, Aaka. Amp this is equal to one coulomb of charge per second (C/s) a battery is a direct current source because the charges always move from one terminal to the other in the same direction it is the negatively charged electrons that move from atom to atom based on how tightly they are held by the atom

Current the wire moves… not the magnet when the loop of the wire is rotated within the magnetic field of this fixed permanent magnet, electrons shift according to its polarity again, because of the attachments of the ends of the wires of the loop, the process keeps repeating the result is an oscillation of electrons within the wire often referred to as “alternating current” “ac”

Current most appliances are designed for use w/what voltage? 120 V Resistance Opposition to the flow of charges in a circuit caused by internal friction (slows the movement of charges through a conducting material) Resistance can be calculated from current and voltage Resistance Equation (Ohm’s law) resistance = voltage current R = V I SI unit for resistance is the ohm

Current Factors that determine resistance: 1. Material 2. Length 3. Diameter 4. Temperature

Current Conductors have low resistance a good conductor is any material in which electrons flow easily Some materials become superconductors below a certain temperature certain metals & compounds have zero resistance when their temp falls below a certain temp called the… Critical temperature superconductors these types of materials are called superconductors Insulators have high resistance to charge movement insulating materials are used to prevent electric current from leaking

Current When is it important to provide a pathway for current to leave a charged object? conducting wires are often run between the charged object and the ground, thereby… grounding grounding the object Semiconductors are intermediate to conductors and insulators in their pure state, semiconductors are insulators specific atoms are added as impurities to increase their ability to carry a charge

Circuits What are circuits? An electric circuit is a path through which charges can be conducted Electric circuit an electrical device connected so that it provides one or more complete paths for the movement of charges closed circuitthe connecting path produced when the light bulb is connected across the battery’s terminals open circuitwithout a complete path, there is no charge flow and therefore no current the voltage source is always part of the conducting path of a closed circuit Why do light bulbs burn out?

Circuits What is a switch? switches are used to open and close circuits Schematic diagrams are used to represent circuits How would you describe the contents & connections in the photo above? Schematic diagram a graphic representation of an electric circuit or apparatus, w/standard symbols for the electrical devices

Circuits can be used to describe any circuit w/a battery & one or more bulbs standard symbols = can be read all over the world

Series & Parallel Circuits Series circuits have a single path for current Series describes a circuit or portion of a circuit that provides a single conducting path What happens when one element along the path is removed? Why is this a problem?

Series & Parallel Circuits Parallel circuits have multiple paths for current Parallel describes components in a circuit that are connected across common points, providing two or more separate conducting paths What happens when one element along the path is removed?

Electric Power & Electrical Energy Electrical energy the energy associated w/electrical charges, whether moving or at rest Electric power is the rate at which electrical energy is used in a circuit Does a charge in a circuit ever lose energy? electric poweris the rate at which electrical work is done Electric Power Equation Power = current x voltage P = IV SI unit for power is the watt (W)

Electric Power & Electrical Energy

Electric companies measure energy consumed in kilowatt-hours power companies charge for energy used in the home, not power one kilowatt-hour is the energy delivered in 1 hour Fuses & Circuit Breakers What happens when you have too many appliances, lights, CD players, tv’s, & other devices connected across a 120 V outlet? Q A The overall resistance of the circuit is lowered & the wires are carrying more than a safe level of current. OVERLOAD

Electric Power & Electrical Energy Worn insulation on wires can also be fire hazards, WHY? Two wires may touch creating an alternate pathway for the current. SHORT CIRCUIT Fuses melt to prevent circuit overloads Fuse an electrical device containing a metal strip that melts when current in the circuit becomes too great Circuit breakers open circuits with high current Circuit breaker a device that protects a circuit from current overloads uses a bimetallic strip, a strip w/two different metals welded together