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Chapter 18
Electric Currents
Units of Chapter 18
• The Electric Battery
• Electric Current
• Ohm’s Law: Resistance and Resistors
• Resistivity
• Electric Power
Units of Chapter 18
• Power in Household Circuits
• Alternating Current
• Microscopic View of Electric Current
• Superconductivity
• Electrical Conduction in the Human Nervous
System
Objectives
After studying the material of this chapter, the student should be able to:
1. Explain how a simple battery can produce an electrical current.
2. Define current, ampere, emf, voltage, resistance, resistivity, and
temperature coefficient of resistance.
3. Write the symbols used for electromotive force, electric current,
resistance, resistivity, temperature coefficient of resistance and power
and state the unit associated with each quantity.
4. Distinguish between a) conventional current and electron current and
b) direct current and alternating current.
5. Know the symbols used to represent a source of emf, resistor,
voltmeter, and ammeter and how to interpret a simple circuit diagram.
6. Given the length, cross sectional area, resistivity, and temperature
coefficient of resistance, determine a wire's resistance at room
temperature and some higher or lower temperature.
7. Solve simple dc circuit problems using Ohm's law.
8. Use the equations for electric power to determine the power and energy
dissipated in a resistor and calculate the cost of this energy to the
consumer.
9. Distinguish between the rms and peak values for current and voltage
and apply these concepts in solving problems involving a simple ac
circuit.
Objectives
After studying the material of this chapter, the student should be able to:
1.Explain how a simple battery can
produce an electrical current.
2.Describe the concepts in an electrical
circuit including electric potential
energy, electric potential, voltage,
current, and resistance.
3.Describe conditions that create current
in an electric current.
Recap
• In ch. 16, Electrostatics, electric field must be 0
inside a conductor and charges did not move.
• When charges move in a conductor, there is
usually an electric field present.
• An electric field is needed to put charges in
motion.
• The flow of charge can be controlled using
electric fields and electric potential.
• Therefore, in order to have a current in a wire, a
potential difference is needed.
• That difference can be provided by a battery.
Mechanical Universe The Electric Battery
18.1 The Electric Battery
Volta discovered that
electricity could be
created if dissimilar
metals were
connected by a
conductive solution
called an electrolyte.
This is a simple
electric cell.
BATTERY BASICS
INTRODUCTION
• 1800 – Alessandro Volta
discovered the chemical battery by
creating a portable electricity
source known as a “Voltaic Pile”.
• A Voltaic Pile is a device using
pieces of silver and zinc separated
by moist cloth soaked in an
electrolyte (in Volta’s case, sea
water) solution.
• Humphry Davy later proved that
the electricity from voltaic piles
was caused by the chemical
reaction, and not the different
metals, as first assumed.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
BATTERY BASICS
Voltaic Pile
• In the lemon experiment,
the lemon juice allows the
metal plates to gain or lose
electrons. Then, those
electrons travel over to the
other plate (via the
electrolyte solution, lemon
juice), forming a redox
reaction.
• The electrolyte is
electrically the same on
both sides, but the reaction
creates a different electrical
potential on the two
different plates, so
connecting them shows a
voltage difference.
Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc.
18.1 The Electric Battery
A battery transforms chemical energy into
electrical energy.
Chemical reactions within the cell create a
potential difference between the terminals by
slowly dissolving them. This potential
difference can be maintained even if a current is
kept flowing, until one or the other terminal is
completely dissolved.
18.1 The Electric Battery
Several cells connected together make a
battery, although now we refer to a single cell
as a battery as well.
What does a battery do?
• Static discharge is the moving of electrons from
one atom to another.
• In order to keep the electrons moving through the
circuit, there has to be something that causes a
push, or a voltage difference- a battery does just
that…
How do batteries work?
• Batteries create an uneven level of electrons
which causes the electrons to move from a
high concentration to a low concentration…
• This is also known as the voltage difference..
Parts of the Dry Cell
•
•
•
•
•
•
Positive End
Plastic Insulator
Moist Paste
Carbon Rod
Zinc Container
Negative Terminal
How does a Dry Cell Work?
• When the circuit is closed, and the
battery is connected, a chemical
reaction starts the process.
• The chemical reaction with zinc and
several other chemicals occurs in the
moist paste.
How does a Dry Cell Work?
• The carbon rod acts as a conductor and transfers
electrons.
• The carbon rod is not part of the reaction
happening in the moist paste.
• But…the chemical reaction in the moist paste does
cause the carbon rod to become charged.
• This charge on the carbon rod creates a positive
end.
• The negative end is made by the Zinc.
How does a Dry Cell Work?
• The voltage difference between the positive
and negative ends causes the current to
flow.
• By connecting more batteries you increase
the voltage difference.
18.2 Electric Current
In order for current to flow, there must be a
path from one battery terminal, through the
circuit, and back to the other battery
terminal. Only one of these circuits will work:
18.2 Electric Current
By convention, current is defined as flowing
from + to -. Electrons actually flow in the
opposite direction, but not all currents consist
of electrons.
18.2 Electric Current
Electric current is the rate of flow of charge
through a conductor:
(18-1)
Unit of electric current: the ampere, A.
1 A = 1 C/s.
18.2 Electric Current
A complete circuit is one where current can
flow all the way around. Note that the
schematic drawing doesn’t look much like the
physical circuit!
Electric Circuit
Electric Circuit –
continuous conducting
path
Electric Current – flow
of charge from one
terminal to the other
Diagram of Electric Circuit
Remember: Electric Potential EnergyTwo Unlike Charges
Higher Potential
Energy
+
Lower Potential
Energy
-
•To cause movement of a charge,
there must be a potential difference.
While the switch is open:
• Free electrons (conducting electrons) are
always moving in random motion.
• The random speeds are at an order of
106 m/s.
• There is no net movement of charge across a
cross section of a wire.
What occurs in a wire when the
circuit switch is closed?
http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/imgele/micohm.gif
What occurs in a wire when the
circuit switch is closed?
• An electric field is established
instantaneously (at almost the speed of
light, 3x108 m/s).
• Free electrons, while still randomly moving,
immediately begin drifting due to the
electric field, resulting in a net flow of
charge.
• Average drift velocity is about 0.01cm/s.
Closing the switch establishes a potential difference
(voltage) and an electric field in the circuit.
• Electrons
flow in a
net
direction
away from
the (-)
terminal
towards
the (+)
terminal.
Low
Potential
High
Potential
Conventional Current
• By tradition,
direction in
which
“positive
charges”
would flow.
• Direction is
opposite of
electron
flow.
Question:
What is required in order to have an
electric current flow in a circuit?
Answer:
1. A voltage source.
2. The circuit must be closed.
Homework
• Questions p. 514 2-5
Due by Friday – collected and graded