Electricity

Electric Charge and Force Electric charge: an electrical property of matter that creates electric and magnetic forces and interactions. An object can have a negative charge, positive charge, or no charge. Like energy, electrical charges can never be created nor destroyed. Conservation of charge is one of the fundamental laws of nature.

Electric Charges and Forces Like charges repel, and opposite charges attract. Electric charge depends on an imbalance of protons and electrons. SI unit for electric charge is the coulomb (C). Because the amount of electric charge on an object depends on the number of protons and electrons, the net electric charge of an object is always a multiple of 1.6X10-19 C.

Transfer of Electric Charge When different materials are rubbed together, electrons can be transferred from one material to the other. Conductor: a material in which charges can move freely. Insulator: a material in which charges cannot move freely. Conductors allow charges to flow; insulators do not.

Transfer of Electric Charge Charges can move within uncharged objects. Objects can be charged by contact. Objects can be charged by friction Static electricity is caused by friction and damage sensitive electronics when it discharges quickly through them. A surface charge can be induced on insulators.

Electric Force Electric Force: the force of attraction or repulsion on a charge particle that is due to an electric field. The electric force at the atomic and molecular levels is responsible for most of the everyday forces that we observe, such as the force of a spring and the force of friction. Electric force depends on charge and distance.

Electric Force Electric force acts through a field. Electric field: the space around a charge object in which another charged object experiences an electric force. Electric field lines never cross one another.

Currents

Voltage and Current Electrical potential: the ability to move an electric charge from one point to another. Just as a ball will roll downhill, a negative charge will move away from another negative charge. The movement is the result of the 1st negative charge’s electric field.

Voltage and Current Potential difference is measured in volts. Potential difference: the voltage difference in potential between two points in a circuit. (the change in the electrical potential energy of a charged particle divided by its charge. SI unit for potential difference is volt (V) 1V = 1 J/C

Voltage and Current There is a voltage across the terminals of a battery. Terminals: two ends of a battery. One positive and one negative. Most batteries are electrochemical cells: a group of connected cells that convert chemical or radiant energy into electrical energy.

Voltage and Current Electrolyte: a solution that conducts electricity. Electrodes: various conducting materials Cells can be wet cells or dry cells Dry cells contain a pastelike electrolyte. Flashlight battery Wet cells contain a liquid electrolyte. Car battery

Voltage and Current Current is the rate of charge movement. SI unit is ampere (A). 1 A = 1 C moving past a point in 1 s. Direct current: charges always move from one terminal to the other in the same direction. Conventional current: positive charge that would have the same effect as the actual motion of charge in the material.

Electrical Resistance Resistance: the opposition presented to the current by a material or device. Resistance is caused by internal friction, which slows the movement of charges through a conducting material. Resistance can be calculated if current and voltage are known.

Electrical Resistance Resistance = voltage/current or R=V/I This equation is also called Ohm’s Law. SI unit for resistance is ohm (Ω) 1 Ω = 1V / A Resistor: a special type of conductor used to control current. Every resistor is designed to have a specific resistance.

Electrical Resistance Conductors have low resistance. Insulators have high resistance. Ground wire: It is a wire that is literally connected to the Earth. It conducts any excess charge into the ground, where it spreads safely over the planet.

Electrical Resistance Semiconductors conduct under certain conditions and have properties between insulators and conductors. Silicon and germanium are two common semiconductors.

Electrical Resistance Some materials can become superconductors: materials that have zero resistance when their temperature falls below a critical temperature. Critical temperature depends on the material and has a wide range. There is no know material that is a superconductor at room temperature.

Circuits

What are circuits? Electrical circuits: a set of electrical components connected such that they provide one of more complete paths for the movement of charges. The conducting path produced when a load, such as a string of light bulbs, is connected across a source of voltage is called a closed circuit. Open circuit: when there is no complete path.

What are circuits? Switches interrupt the flow of charges in a circuit. Schematic diagrams are used to represent circuits. Schematic diagram: a graphical representation of a circuit that uses lines to represent wires and different symbols to represent components.

Series and Parallel Circuits Electrical devices can be connected as a series circuit so that the voltage is divided among the devices. They can also be connected as a parallel circuit so that voltage is the same across each device. Series circuits have a single path for current. Parallel circuits have multiple paths for current.

Electrical Energy & Electrical Power Some of this energy is transformed into useful work, such as the turning of a motor, and some is lost as heat. Electric power: the rate at which electrical energy is converted into other forms of energy. Power = current X voltage or P = IV

Electrical Energy & Electrical Power SI Unit for power is the watt (W). Electric companies measure energy in kilowatt-hours (kW X hr) Energy ranges from $0.05 – $0.20 per kilowatt-hour.

Fuses and Circuit Breakers Overload: when electrical wires carry more than a safe level of current. High currents in overloaded circuits cause fires. Short circuit: when 2 wires have insulation worn down so that they touch, creating an alternate pathway for current. Short circuits can be very dangerous.

Fuses and Circuit Breakers Fuses melt to prevent circuit overloads. Fuse: an electrical device that contains a metal strip that melts when current in the circuit becomes too great. Circuits breakers open circuits with high current. Circuit breaker: a switch that opens a circuit automatically when the current exceeds a certain value.

Fuses and Circuit Breakers Bimetallic strip: a strip with two different metals welded together that responds to circuit overload by opening the circuit. Ground Fault Circuit Interrupter (GFCI): a special kind of electrical outlet that acts as a small circuit breaker , usually found near water sources like bathrooms or kitchens.