Chapter 19: Electric Charges and Currents 19-3: The Flow of Electricity

Making Electric Charges Move You must do work to move a charged particle against an electric field Unit of Charge: Coulomb (C) Electric Potential Difference (Voltage): work required per coulomb of charge between 2 points Units: Volt (V)

Battery Produces electricity by converting chemical energy into electrical energy Made up of electrochemical cells which are made from materials called electrodes and electrolyte Electric cells can be dry (paste-like) or wet (liquid) cells

Thermocouples Produces electrical energy from thermal energy Releases electric charges as a result of temperature differences Used in thermometers in cars to show engine temp. Engine gets warmer, increases flow of charge, moving charge operate gauge (also in ovens and gas furnaces)

Photocells When light with a certain amount of energy shines on a metal surface, electrons are emitted from the surface, electron routed through a wire to create a constant flow of electric charge

Electric Current Circuit: complete path through which electric charge can flow Current (I): amount of charge that passes a given point at a given time Unit of current: Ampere (A) **Potential Difference is required to produce an electric current**

Resistance Resistance (R): opposition of the flow of electric charge Unit: Ohm (Ω) - Different wires have different resistances - All devices have some resistance - Depends somewhat on temperature: Resistance increases with temperature

Ohm’s Law The current in a wire (I) is equal to the voltage (V) divided by the resistance (R) Current = Voltage I = V Resistance R Amperes = Volts Ohms

Calculating Resistance In a Series circuit, total resistance is calculated by the equation: R = R1 + R2 + R3… In a Parallel circuit, total resistance is calculated by the equation:

Series and Parallel Circuits Series Circuit

Current Direct Current (DC): current flow in the same direction (dry cells and batteries) Alternating Current (AC): amount of current changes in time and reverses direction regularly Current in your home changes direction 120 times every second

Calculations with Current Electricity 1. There is a 22-ohm resistance in the heating element of a coffee pot. It is plugged into a 110 Volt circuit. How much current passes? R= 22Ω I = V V = 110V R I=? I = 110 V 22 Ω I = 5 A

Calculations with Current Electricity 2. Three lamps with 40,60, and 80 ohms of resistance are connected in parallel to a 120 volt circuit. a) What current flows through each lamp? b) What is the total resistance of the 3 lamps? c) What is the total current used by the 3 lamps? Given: V = 120 V R1=40 Ω R2=60 Ω R3=80 Ω

a) What current flows through each lamp? Lamp 1: I = V/R I = 120V/40 Ω = 3.0 A Lamp 2: I = V/R I = 120V/60 Ω = 2.0A Lamp 3: I = V/R I = 120V/80 Ω = 1.5 A

b) What is the total resistance of the 3 lamps? Given: R1=40 Ω R2=60 Ω R3=80 Ω Since the circuit is in parallel, we use the equation: 1/R1+1/R2+1/R3 = 1/40Ω + 1/60Ω +1/80Ω = 0.054Ω Then divide: 1/0.054Ω = 18.5Ω Total resistance = 18.5Ω

c) What is the total current used by the 3 lamps? Given: V = 120 V RT= 18.5Ω I = V/RT I = 120V/18.5Ω I = 6.5 A