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
Resistance
Amperes = Volts
Ohms
I= V
R
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
Parallel 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=5A
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?
R1=40 Ω
Given:
V = 120 V
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