Download 18-1through18-3_Battery_Current_Ohm`s_Law

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
Question 2
In the circuit (not in the battery), electrons flow
from high potential energy (at the negative
terminal) to low potential energy (at the positive
terminal). Inside the battery, the chemical
reaction does work on the electrons to take
them from low potential energy to high potential
energy (to the negative terminal). A chemical
description could say that the chemical reaction
taking place at the negative electrode leaves
electrons behind on the terminal, and the
positive ions created at the negative electrode
pull electrons off the positive electrode.
Question 3
Battery energy is what is being “used up.” As
charges leave the battery terminal, they have a
relatively high potential energy. Then as the
charges move through the flashlight bulb, they
lose potential energy. The battery uses a
chemical reaction to replace the potential
energy of the charges, by lowering the
battery’s chemical potential energy. When a
battery is “used up,” it is unable to give
potential energy to charges.
Question 4
The terminal of the battery (usually the
negative one) is connected to the metal
chassis, frame, and engine block of the
car. This means that all voltages used for
electrical devices in the car are measured
with respect to the car’s frame. Also,
since the frame is a large mass of metal, it
can supply charges for current without
significantly changing its electric potential.
Question 5
Water flows immediately from the spout because there is
already water in the spout when you turn on the
faucet. Since water is (essentially) incompressible, a
push on the water from some distant location (at the
valve or pump) causes all the water to move almost
immediately, pushing the water out of the spout. In a
battery, a similar phenomenon exists in that the wires
are “full” of charges (free electrons). The voltage
“push” from the battery pushes all the electrons along
the wire almost immediately, and the first ones at the
other end of the wire are the beginnings of the electric
current.
Ohm’s Law explains the
relationship between voltage (V),
current (I) and resistance (R)
 Used by electricians, automotive
technicians, stereo installers



It is the push or pressure behind
current flow through a circuit,
and is measured in (V) volts.

Current refers to the
quantity/volume of electrical
flow. Measured in Amps (A)

Resistance to the flow of the
current. Measured in Ohms
18.3 Ohm’s Law: Resistance and
Resistors
Experimentally, it is found that the current in
a wire is proportional to the potential
difference between its ends:
Hewitt- Ohm’s Law
1. Assuming the resistance does not
change:
As voltage increases, current increases.
as voltage decreases, current decreases.




2. Assuming the voltage does not change:
As resistance increases, current decreases.
As resistance decreases, current increases.
18.3 Ohm’s Law: Resistance and
Resistors
The ratio of voltage to current is called the
resistance:
(18-2a)
(18-2b)


V (E) = I x R
I=V
R
R=V
I
18.3 Ohm’s Law: Resistance and
Resistors
In many conductors, the
resistance is independent
of the voltage; this
relationship is called
Ohm’s law (a). Materials
that do not follow Ohm’s
law are called nonohmic
(b). (semiconductor diode)
Unit of resistance: the ohm, Ω.
1 Ω = 1 V/A.
What can we use to measure?
Multi meters
Measures
voltage,
current, and
resistance.



Why do we need resistors
To decrease the amount of voltage
applied to a component
The value of the resistor is marked on the
body using colored rings
18.3 Ohm’s Law: Resistance and
Resistors
Standard resistors are
manufactured for use
in electric circuits;
they are color-coded
to indicate their value
and precision.
18.3 Ohm’s Law: Resistance and
Resistors
Ohm’s Law
Putting It All Together…
a.) The gumballs in this demonstration represent the
flow of ___________ through the pipe.
Putting It All Together…
a.) The gumballs in this demonstration represent the
flow of electrons through the pipe.
Putting It All Together…
b.) The height the pipe is raised above the desk
represents potential difference, or
_____________.
Putting It All Together…
b.) The height the pipe is raised above the desk
represents potential difference, or
voltage.
Putting It All Together…
c.) The nails placed in the pipe represent _________
and slow down the flow of the gumballs.
Putting It All Together…
c.) The nails placed in the pipe represent resistance
and slow down the flow of the gumballs.
Putting It All Together…
d.) The higher we raise the pipe, the ________ the
gumballs flow; the more nails we place in
the pipe, the ________ the gumballs flow.
Putting It All Together…
d.) The higher we raise the pipe, the faster the
gumballs flow; the more nails we place in
the pipe, the slower the gumballs flow.
18.3 Ohm’s Law: Resistance and
Resistors
Some clarifications:
• Batteries maintain a (nearly) constant
potential difference; the current varies.
• Resistance is a property of a material or
device.
• Current is not a vector but it does have a
direction.
• Current and charge do not get used up.
Whatever charge goes in one end of a circuit
comes out the other end.
Hewitt – Water and Electron Circuits
Question: Why is the bird on the
wire safe?
Answer
• When a bird is perched on a single wire, its two
feet are at the same electrical potential, so the
electrons in the wires have no motivation to
travel through the bird’s body.
• No moving electrons means no electric current.
• If that bird stretches out a wing or a leg and
touches a second wire, especially one with a
different electrical potential, it will open a path
for the electrons—right through the bird’s
body.
Source: http://engineering.mit.edu/ask/how-do-birds-sit-high-voltage-power-lines-without-getting-electrocuted
Question
Why do electricians work with one hand
behind their back?
Answer
Why do electricians work with one hand behind their
back?
•You're harmed only if current passes through part of
your body.
•In order for that to happen,
you must be touching two places that have a voltage
difference between them.
•The experienced electrician is constantly aware that he
has two big efficient electrodes hanging from his
shoulders. To avoid setting up a conducting path
through his body, he keeps one electrode behind his
back, or in his pocket.
Source: http://www.answers.com/Q/Why_do_electricians_work_with_one_hand_behind_their_back
Question:
Why is the ground prong longer
than the other two in a plug?
Question:
Why is the ground prong longer
than the other two in a plug?
• If there is a fault current (leakage) inside the
appliance, the whole metal case becomes live. If
you happen to touch it, a current will flow
through you to the earth (ie. electric shock).
• It is longer so it is connected before the live
prongs.
• This way if there is a short, you know you are
grounded before there is a circuit.
Practice Question 1
20 Coulombs of charge pass a given point in
a conductor in 4.0 seconds. What is the
current in the conductor?
Practice Question 1
20 Coulombs of charge pass a given point in
a conductor in 4.0 seconds. What is the
current in the conductor?
Practice Problem 2
In a circuit with a battery supplying 12 volts,
the current is 10 amperes.
(a) How much charge flows through the
circuit in 2.0 minutes?
(a) How many electrons were transferred
during these 2.0 minutes?
Practice Problem 2
In a circuit with a battery supplying 12 volts,
the current is 10 amperes.
(a) How much charge flows through the
circuit in 2.0 minutes?
Practice Problem 2
In a circuit with a battery supplying 12 volts,
the current is 10 amperes.
(b) How many electrons were transferred
during these 2.0 minutes?
Practice Problem 3
• What voltage is used by an electrical
appliance that draws 0.4 amps of current
and has a resistance of 3 ohms?
Practice Problem 3
• What voltage is used by an electrical
appliance that draws 0.4 amps of current
and has a resistance of 3 ohms?
Practice problem 4
Calculate the current used by a television
that runs on 240 volts and has a resistance
of 600 ohms.
Practice problem 4
Calculate the current used by a television
that runs on 240 volts and has a resistance
of 600 ohms.
Practice Problem 5
A 20 ohm resistor has 40 Coulombs of
charge passing through it in 5 s. What is the
potential difference across this resistor?
Practice Problem 5
A 20 ohm resistor has 40 Coulombs of
charge passing through it in 5 s. What is the
potential difference across this resistor?
Practice Problem 6
• A wire carries a current of 2.0-amperes.
How many electrons pass a given point in
this wire in 1.0 second?
Practice Problem 6
• A wire carries a current of 2.0-amperes.
How many electrons pass a given point in
this wire in 1.0 second?
phET activity
• Ohm’s Law
Homework
• Problems p. 515 1, 2, 6, 7, 9
Kahoot 18-1 through 18-3