Electric currents. Chapter 18. Electric Battery . Made of two or more plates or rods called electrodes. Electrodes are made of dissimilar metals Electrodes are immersed in an electrolyte
Electric currents Chapter 18
Electric Battery • Made of two or more plates or rods called electrodes. • Electrodes are made of dissimilar metals • Electrodes are immersed in an electrolyte • The electrolyte will react with one metal in a way to dissolve positive ions into the solution thus becoming negatively charged. The other metal loses electrons to the electrolyte becoming positively charged and a potential difference (pressure difference) is created. • The electrodes in an electrolyte is a cell • Cells put together form a battery.
Electrical potential and Voltage • Electric potential (V)– the potential energy per unit charge • This is caused by the electrical force of repulsion. • Voltage- a difference in electrical potential • Considered electrical pressure • This is measured in volts • Flashlight batteries are 1.5V, Household outlets are 110V or 220V
Electric current • Current is the flow of charge through a circuit. • I = ΔQ/Δ t • Rate at which charge flows • Measured in Amperes • 1A = 1Coulomb/second • Electron Current • Negative electron flows from (-) to (+) circuit simulation • Conventional current • Positive charge flows from (+) to (-)
Ohm’s Law/ Resistance • For a current (flow of charge) to occur there must be a Voltage (electric pressure) difference. This can be produced by a battery. • How much current there is depends on how easily the charge can move through a circuit (resistance). • Current, Voltage, and resistance are all related by Ohm’s Law
RESISTORS • Electrical resistance is the opposition of the flow of electricity by some object or substance. • Resistors are used to control the flow of electricity in a circuit. • Every conductor has some resistance depending on the material. • resistance simulation • Resistance is measured in Ohms ( ) • 1 = 1V/1A • The bands on the resistor show how much resistance the resistor has • Uses for resistance • Electronic circuits (control flow of electricity) • Toaster, heating element, variable resistors • Lightbulbs
Ohm’s Law • Current, Voltage, and resistance are all related by Ohm’s Law • V = I *R • If you graph the relationship between voltage and current, the slope represents the resistance. • Current is consistent through a circuit but voltage changes.
Practice Ohm’s Law • A small flashlight bulb draws 300mA from its 1.5V battery. • What is the resistance of the bulb? • If the voltage dropped to 1.2V, how would the current change. • How many electrons would pass through the bulb in one second?
Resistivity • How resistive a material is • ρ = constant of proportionality/resistivity • Measured in Ohms/meter • Temperature dependent (α) • ρT = ρo[1+ α(T-To)] • Measure of the resistance in a wire. • Depends on? • Length, Cross-sectional area, resistivity • R= ρ(L/A)
Resistors in series and parallel (19.1) • When resistors are connected end to end they are said to be in series • Resistors can be bulbs, heating elements, …etc. • Voltage = I Req • For a series Circuit: Req = R1+R2+R3… • When resistors are on their own wire they are in parallel. • Voltage = I Req • For a series Circuit: 1/Req = 1/R1+1/R2+1/R3…
Practice • Find the equivalent resistance. R = 8 ohms Req= 4 ohms solution Req=11.1 ohms
Electric Power • Electric energy is useful because it can be transformed into mechanical energy to do work. • To find the power transformed by an electrical device: Power = energy transformed/time • Measured in Watts • Electric companies charge in kW*hrs
Practice • Your parents yell at you for leaving the lights on in the living room overnight. You want to find out how much you cost them. If there were 2 75Watt bulbs running for 7 hours and the electric company charges $0.09 per kW*hr, how much do you owe them?