Chapter 19: Chapter 19: Electric Charges and CurrentsElectric Charges and Currents

19-3: The Flow of Electricity19-3: The Flow of Electricity

Making Electric Charges MoveMaking Electric Charges Move• You must do work to move a charged particle

against an electric field

UnitUnit ofof ChargeCharge:: Coulomb (C)

ElectricElectric PotentialPotential DifferenceDifference (Voltage): (Voltage): work required per coulomb of charge between 2 points

UnitsUnits:: Volt (V)

BatteryBattery

• 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

ThermocouplesThermocouples• 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)

PhotocellsPhotocells

• 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 CurrentElectric Current

• CircuitCircuit:: complete path through which electric charge can flow

• CurrentCurrent ( (II):): amount of charge that passes a given point at a given timeUnitUnit ofof currentcurrent:: Ampere (A)

**Potential Difference is **Potential Difference is required to produce an required to produce an electric current**electric current**

ResistanceResistance

• ResistanceResistance (R): (R): opposition of the flow of electric charge

UnitUnit:: Ohm (Ω)- Different wires have

different resistances- All devices have some

resistance- Depends somewhat on

temperature: Resistance increases with temperature

Ohm’s LawOhm’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 ResistanceCalculating 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 CircuitsSeries and Parallel Circuits

Series CircuitParallel Circuit

CurrentCurrent

• DirectDirect CurrentCurrent (DC): (DC): current flow in the same direction (dry cells and batteries)

• AlternatingAlternating CurrentCurrent (AC): (AC): amount of current changes in time and reverses direction regularly

• Current in your home changes direction 120 times every second

Calculations with Current ElectricityCalculations 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 ElectricityCalculations 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:Given:

V = 120 V

R1=40 Ω R2=60 Ω R3=80 Ω

a) What current flows through each lamp?a) What current flows through each lamp?

Lamp 1: I = V/RI = 120V/40 Ω = 3.0 A

Lamp 2: I = V/RI = 120V/60 Ω = 2.0A

Lamp 3: I = V/RI = 120V/80 Ω = 1.5 A

b) What is the total resistance of the 3 lamps?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 = Total resistance = 18.518.5ΩΩ

c) What is the total current used by the 3 lamps?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