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UNIT 3: Electricity CHAPTER 8: Ohm’s law describes the Relationship of current, voltage, and resistance . RECALL: ENERGY (E) – the ability to do work There are many forms of Energy kinetic, mechanical, chemical, solar... Types of Energy KINETIC ENERGY Energy of motion POTENTIAL ENERGY stored Energy ex: spring, elastic band Electric Potential Energy and Voltage Section 8.1 When + and – charges are pulled apart there are attractive forces wanting to put them back together, this is a form of potential energy as it has the ability to do work! page 250 Electric Potential Energy Stored electrical energy such as that stored in a battery Electrochemical Cell (Battery) Converts: Chemical Energy Electrical Energy + and – charges are separated into terminals located at either end of the cell, when connected – charges (electrons) travel towards the + (positive) terminal as the – charges repel one another Batteries have electric potential Energy because the stored e-’s have the ability to do work, i.e. run your CD player! An Electrochemical Cell Requires: different electrodes (usually metals also Carbon) And an electrolyte (electrolytic solution) Electric Potential Difference (Voltage) Change in potential energy per coulomb of charge Measured in by a volts (V) voltmeter Analogy! Stairs = the voltage Backpack = amount of charge separated More work has been done in B, therefore it has a greater Potential Energy! Section 8.2 Electric Current page 260 Electric Circuit A complete pathway that allows electrons to flow Transforms electrical energy into other forms of Energy Interactive website Electric Circuit Analogy Parts of an Electric Circuit Source – source of electrical E Switch – turn circuit on or off Interactive Website on Switches Conductor – wire through which current flows Load – converts electrical E into other forms of E Circuit Diagram Symbols Interactive Website on Circuit Diagrams Sample Circuit Diagram Current Electricity THE CONTINUOUS FLOW OF CHARGE (ELECTRONS) IN A COMPLETE CIRCUIT Electric Current •the amount of charge passing a given point in a conductor per second •measured in amperes (A) by an ammeter or (mA) by a galvanometer Resistance & Ohm’s Law Section 8.3 page 270 Electrical Resistance: Slows down the flow of electrons and transforms electrical E into other forms of E Light Heat Sound Electrical E Motion etc RESISTANCE •Measured in ohms ( Ω) •Equal to the ratio of the voltage to the current R = V/I Resistance and Current Analogy: Current Dependent on Potential (voltage) Current Dependent on Resistance Factors Affecting the Amount of Resistance in a wire include: 1. Length 2. Diameter (thickness) 3. Type of wire 4. Temperature Resistance in a Light Bulb Long, Thin Tungsten wire Short, Thick copper wire Ohm’s Law The Mathematical Relationship comparing voltage (V), current (I) and resistance (R) V = I●R Interactive website Simulation Ohm’s Law – The VIR Tree V I R Various Forms of Ohm’s Law Where: V=I●R I=V R R=V I V = voltage measured in volts (V) I = current measured in amperes (A) R = resistance measured in ohms (Ω) Sample Problem: What voltage is used to run a flashlight with a 2 Ω resistor at a current of 0.5 A? STEP 1 : Read and reread the question. Step up variables with known information. V = ? I= 0.5A R= 2Ω Use a question mark for the unknown value you are trying to find. Be sure to include units (V,A,Ω) STEP 2: Determine the form of ohm’s law needed to solve for the unknown in the question. V = I●R STEP 3: Convert units if necessary, measurements must be in V for voltage, A for current and Ω for resistance. Fill in the information given in the question (including units). STEP 4: Multiple or divide to calculate the unknown as per the formula chosen in step 3. Be sure to include units in your answer, round digits if necessary. V =I●R V = (0.5A)(2.0 Ω) V= 1.0 V The flashlight would require a voltage of 1.0 V. Resistor Used to control C U R R E N T or potential difference ( V O L T A G E ) in a circuit Core lab Activity: RESISTANCE AND OHM’S LAW PAGE 278-9