Download psaa electromotive force worksheet

GENERATING AN
ELECTROMOTIVE FORCE
Student Learning Objectives Instruction in this lesson should result in
students achieving the following objectives:
1 Generate an electromotive force by means of chemicals, magnetism, heat, and
solar radiation.
2 Construct a primary cell and understand its function.
3 Explain the function of a voltmeter and construct a voltmeter circuit, which is
the same type as used in commercial voltmeters.
4 Properly connect a voltmeter to a circuit and measure voltage in a circuit.
Illinois Physical Science Applications in Agriculture Lesson B4–1 • Page 1
Anticipated Problem: What different methods can be used to generate an electromotive
force?
I. Electricity is a form of energy present when electrons move through a complete path.
Electrons are negatively charged atomic particles that normally revolve around the
nucleus of an atom. The force that causes the electrons to move through the path is
voltage or electromotive force. The path the electrons follow through easily is a
conductor.
A. The most popular commercial method of generating an electromotive force is by
means of magnetism. A coil of wire has a moving magnetic field passed through it. This
produces an electromotive force. This type of generation occurs in an alternator in an
automobile or in the large generators used to produce electricity for the home.
1. In 1820, Danish physicist, Hans Oersted discovered that a compass needle could be
deflected by an electric current.
2. Until that time, it was generally thought electricity and magnetism were completely
different.
B. In order for there to be a reading on a voltmeter, for example, a force must act upon
the charges that are moving along the magnetic field lines.
1. If the charge is not moving or is moving in the same direction as the magnetic fields,
then there is no force. But, if the charge is moving across the field lines, then the
charge has force acting upon it and an electrical current can be measured.
2. Hans Oersted showed that electricity can have an effect on magnetic fields, but it
wasn’t until 1832 that Michael Faraday showed that magnetic fields could have an
effect on electrical currents. This is called the complementary effect and is demonstrated
in this experiment.
C. There are additional methods used to generate electromotive force (voltage), some
more efficient than others. Emf (electromotive force) by chemical reaction was
discovered by Anastasio Volta who invented the first battery by discovering that certain
combinations of metals produced an electromotive force.
1. Emf by heat is demonstrated by the use of a thermocouple, a device that consists of
the junction of two dissimilar metallic conductors, such as, copper and iron. An emf
is induced when the conductors are maintained at different temperatures.
2. Emf by solar radiation is demonstrated by the photovoltaic cell. This solar cell has
the capacity to convert light directly into electricity without having a heat engine
(defined as any device that changes thermal energy into mechanical work) involved.
3. Emf by magnetism was discussed earlier in this lesson.
Anticipated Problem: What is a primary cell? How does a cell generate electromotive
force?
II. Chemical reaction is one of the six basic forms of energy (Others include friction,
magnetism, pressure, heat, and light). Chemical energy is converted into electrical energy
by a device known as a cell. Two or more cells connected form a battery.
A. Primary cells can be recharged using only new materials while secondary cells are
rechargeable without replacing materials used to make the cell. The lead-acid storage
battery is a type of secondary cell.
B. Each cell in the battery has two groups of coated lead plates known as electrodes. The
positive plate is made of lead peroxide and the negative plate is made of lead. The electrodes are separated from each other and immersed in a liquid solution called an
electrolyte.
Anticipated Problem: How does a voltmeter function and how can we construct a
voltmeter circuit?
III. A voltmeter circuit contains a d’Arsonval meter (similar to a galvanometer) and an
extremely high ohm resistor in series with the meter. A linear scale on the meter has
readings divided into equal portions. A nonlinear scale on the meter is marked off by
uneven spaces. Voltmeters indicate the amount of voltage present in the circuit,
ammeters measure current flow through the conductor, and ohmmeters indicate electrical
resistance.
A. Multimeters are capable of measuring more than one characteristic of a circuit, i.e.
voltage, amperage, and resistance. Multimeters usually show both linear and nonlinear
scales. A square-law scale is a type of nonlinear scale which increases by squares. Each
larger scale marking indicates that the measured value is multiplied by itself.
B. d’Arsonval meter movement is the portion of the meter consisting of a fixed
permanent magnet and a moving electromagnetic coil. The voltmeter is calibrated to
show the amount of voltage which would cause an amount of current to flow through the
carefully selected resistance of the voltmeter. The voltmeter is current sensitive. The
extent of the movement of the meter pointer is directly proportional to the amount of
current flowing through the meter.
Anticipated Problem: How is a voltmeter used to properly measure voltage in a circuit?
What is voltage drop?
IV. A voltmeter is always connected in parallel in a circuit. It is put across a resistor or
voltage source. The multiplier resistor allows for varying degrees of accuracy when using
a multimeter for ampere, ohms, and volt readings.
A. Voltage drops occur in power lines running from the transformer to the circuit box and
in the wiring inside a building. A small amount of power is always lost as electricity
flows through the wires.
B. The energy loss is measured as voltage drop. The voltage drop is calculated by
multiplying the current times the resistance.
GENERATING AN ELECTROMOTIVE
FORCE
Part One: Matching
Instructions: Match the word with the correct definition.
a. Cell
e. Multimeter
b. Electricity
f. Magnetic field
c. Conductor
g. Ammeter
d. Electromotive force h. Solar cell
_______1. A solid, liquid, or gas that permits the flow of electrons.
_______2. Cell that turns light energy into electrical energy.
_______3. A meter which is capable of measuring more than one characteristic of a circuit.
_______4. Device that produces an electromotive force by chemical reaction and is usually made
from two electrodes and an electrolyte.
_______5. Device used to measure current in a circuit.
_______6. Forces present around the ends of a magnet.
_______7. Form of energy present when electrons move through a complete path or circuit.
_______8. Force that causes electrons to move through a conductor, also referred to as voltage.
Part Two: Fill-in-the-Blank
Instructions: Complete the following statements.
1. Electrical resistance is measured in _________.
2. Emf and _________ are interchangeable terms and mean the same thing.
3. An electron is __________ charged.
4. A voltmeter is always connected __________.
5. Electrical current in a circuit flows along the path of _________ resistance.
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Part Three: Multiple Choice
Instructions: Write the letter of the correct answer.
_______1. The force that causes movement of electrons through a conductor is called:
a. ammeter
b. electricity
c. electromotive force
d. voltage
_______2. There is (a) _________ chance of a voltage drop over long distance.
a. large
b. small
c. no
d. slight
_______3. Electromotive force can be generated through which of the following materials?
a. chemical reaction
b. magnetism
c. heat
d. all of the above
_______4. The most popular commercial method of generating an emf is by means of ____?
a. solar cells
b. chemical reaction
c. acid
d. magnetism
_______5. A device used to indicate the amount and direction of current flow is a ________.
a. galvanometer
b. electrometer
c. diode meter
d. ohm meter
_______6. By Ohm’s Law, the amount of current is _________ to the voltage.
a. indirectly proportional
b. reversely proportional
c. directly proportional
d. not related
Part Four: Short Answer
Instructions: Answer the following questions.
1. Why are conductors able to conduct electricity?
2. Explain the difference between alternating current and direct current.
Illinois Physical Science Applications in Agriculture Lesson B4–1 • Page 9
Assessment
TS–A
Technical Supplement
GENERATING AN
ELECTROMOTIVE FORCE
1. What is electricity?
Electricity is the form of energy present when electrons move through a complete
path. Electricity is the flow of electrons. We live in a world of electricity. Electricity
provides us with sources of heat, light, energy to produce rotary motion in electric
motors, movement, for sensing devices, and for transportation. We are therefore,
dependent on electricity for food, clothing, shelter, entertainment and recreation.
2. How can electricity be produced?
There are two types of electric current: direct current (dc) and alternating current
(ac). Direct current travels in one direction only. Common sources of direct current
include batteries, solar photovoltaic cells, dc generators, rectifiers, and dc power
supplies used in electronic equipment. Alternating current (ac) travels first in one
direction, stops, then reverses and flows in the opposite direction. Alternating current
is produced by generators. The common source of generators is a power generating
plant. The power generating station is an energy conversion plant, meaning
one form of energy (i.e. coal, nuclear fission, oil, and natural gas ) is burned and
converted by mechanical means to electricity. Generators are connected to turbines or
other mechanical power units that turn them. The rotating action of the generators
produce the electricity that is ultimately used in homes, farms, and industry.
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3. How can electricity be stored?
Electricity can be stored in cells and batteries. There are various types of cells and
batteries. Some common sources include dry cells, alkaline cells, lead-acid accumulator
( common automobile battery ), standard cells, and voltaic cells.
4. What is a conductor?
Conductors are materials containing a large number of electrons which are free to
move. Conductors have low resistance to electron flow. Copper and aluminum are
the common metals used for electrical conductors.
5. How and why do elements differ in their ability to conduct electricity?
Elements differ in the ability to conduct electricity by the type of matter they are
made up of in their structure. Some materials allow electrons to move and other
materials have few electrons or no electrons free to move. Some materials can
become electrically charged when rubbed because electrons from the surface atoms
are transferred from one substance to the next, but the charge remains on the surface.
6. What is resistance in a circuit?
A complete circuit is necessary for the controlled flow of movement of electrons
along a conductor. A complete circuit is made up of a power source, a conductor,
and a consuming device. The flow of electrons through the consuming device produces
heat, light, or work. Resistance in a circuit can be defined as the tendency to
prevent electricity flow. More specifically, resistance is the impeding of movement
of electrons along a conductor. Conductors such as aluminum, copper, or silver
offer very little resistance to the flow of electric current. Examples of poor conductors
( insulators ) are glass, wood, and paper. In circuit design resistance is an important
tool. Materials that resist current give off heat. Resistance also makes it possible
to control electron flow and supply the correct amount of voltage to a device. The
resistance of an electric conductor depends on four factors: material, length of conductor,
cross section of conductor, and temperature. Materials differ in their ability
to conduct or resist electric currents. Most conductors are wires. The longer the
wire through which current must travel, the higher the resistance. The cross section
of a conductor is its thickness diameter. With wire conductors, the larger the diameter,
the lower the resistance for each foot of length. For most materials, resistance
increases as temperature rises. The higher the temperature, the greater the resistance.
Some electrical devices are built around this factor. They increase their resistance
as they are heated by the passage of current.
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On some devices, resistance is lowered as the temperature is increased. These
devices are called thermistors or surgistors. Thermistors are used to control current
and also as temperature sensors. Surgistors are used to prevent a large current in a
device when it is first turned on. Surgistors are often used as protective devices in
television receivers. Resistance is measured in ohms. One ohm of resistance is present
when one ampere of current flows under one volt of pressure. Thus, volts,
amperes and ohms are closely related. This relationship is called Ohm’s Law, discovered
in 1827 by George S Ohm. Ohm stated that “in any circuit where the only
opposition to the flow of electrons is resistance, there is a relationship between the
values of voltage, current and resistance.” The strength or intensity of their current
is directly proportional to the voltage and inversely proportional to the resistance.
Ohm’s Law is generally expressed as a formula. In the formula, E represents voltage
(EMF), I is the current, and R is the resistance. The formula is E=I×R (to find the
voltage). To find the current when the voltage and resistance are known, use I=E/R.
To find the resistance when voltage and current are known, use R=E/I.
The devices used to measure the basic elements are all forms of galvanometers using
the d’Arsonval meter movement. A multimeter measures more than one characteristic
of a circuit and has a number of scales. A voltmeter is a specific type of galvanometer
used to measure voltage in a circuit.
7. What is the function of a multiplier found in a voltmeter circuit?
In most cases, when reading voltage, you need to have a high range of voltage readings.
In order to have a voltmeter with a higher range, resistance must be added to
the circuit. This resistance is added in series with the meter movement. The resistor
used is called a voltmeter multiplier. Meter multipliers are usually precision-type
resistors. Their tolerance is typically +/- 1 percent. Such a resistor automatically
gives the circuit an additional limit of accuracy of +/- 1 percent.
8. How does a d’Arsonval meter movement function?
The d’Arsonval meter movement was developed by Arsene d’Arsonval. In this
movement there is a small, rectangular coil of wire suspended in a magnetic field
created by a permanent magnet. When a current is applied to the coil, an electromagnet
is produced. The coil then lines up with the poles of the permanent magnet.
The amount of current applied to the coil controls its movement. This is
d’Arsonval’s basic principle: An electromagnet that is free to move will align its axis
with the magnetic axis if a fixed magnet. The axis of an electromagnet is a straight
line between its poles.
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The coil in the meter is mounted on pivots that permit easy rotation. Two small
springs are mounted on top and bottom, offering slight resistance to the rotation of
the coil. These springs control the position of the coil when there is no current flowing.
When current flows, the magnetic change overcomes the force of the springs and
moves the coil which rotates a pointer on top of the coil to mark the amount of
movement. The greater the current through the coil, the more it turns and the further
the pointer moves. The pointer then stops in front of a marked scale on the face
of the meter, indicating the meter measurement.
9. How do linear and nonlinear scales differ?
A linear scale has readings divided into equal portions over its entire area. A non linear
scale is a meter scale which is marked off by uneven spaces.
10. What is a square-law scale?
A square-law scale is a type of nonlinear scale which increases by squares. Each
larger scale marking indicates that the measured value is multiplied by itself.
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