Download Name Date Class _ Please turn to the section titled Current. In this

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Please turn to the section titled Current.
In this section, you will examine batteries and see how a potential difference produces a
current in a conductor. You will also learn about resistance, and you will solve problems that
involve resistance, current, and voltage. Finally, you will distinguish between conductors,
superconductors, semiconductors, and insulators.
When you wake up in the morning, you reach up and turn on the light switch. The light
bulb is powered by moving charges. How do the charges move through a light bulb? And what
causes the charges to move?
To find answers these questions, let's examine the concepts of voltage and current.
Figure 8A shows that gravitational potential energy depends on the position of the ball.
As the ball rolls downhill, the ball moves from a position of higher gravitational potential energy
to a position of lower gravitational potential energy. An electric charge also has potential energy
that depends on its position relative to the source of an electric field. This potential energy is
called electrical potential energy.
Just as a ball will roll downhill, a negative charge will move away from another negative
charge. This happens because of the electric field produced by the first negative charge. Figure
8B shows that the electrical potential energy between two negative charges decreases as the
distance between the charges increases.
Please turn to the next page.
You can do work on a ball to move it uphill. This work will increase the ball’s
gravitational potential energy. In the same way, a force can do work on a charge by pushing it in
a direction opposite to an electric force. This push will increase the electrical potential energy of
the charge. Figure 9 shows how electrical potential energy depends on the distance between two
charged objects, and demonstrates this for both an attractive electric force and a repulsive
electric force.
Notice on the left side of Figure 9 that when two positively charged objects are close
together, the amount of electrical potential energy associated with their relative positions is
high. Conversely, when two positively charged objects are further apart, the amount of electrical
potential energy associated with their positions is low.
Now look at the right side of Figure 9. Because a negatively charged object and a
positively charged object attract one another, this situation is more like the gravitational
attraction between two masses. When the negatively charged object is near the positively
charged object, the electrical potential energy in the system is relatively low, just as the
gravitational potential energy is low when a ball is held just off the ground. Likewise, the
electrical potential energy is great when the oppositely charged objects are far apart, just as the
gravitational potential energy is high when a ball is held high above the ground.
Potential difference is measured in volts.
When studying electricity, it is more practical to consider the potential difference than
the electrical potential energy. Potential difference is the change in the electrical potential energy
of a charged particle divided by its charge. This change in electrical potential energy occurs as a
charge moves from one place to another in an electric field.
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The S-I unit for potential difference is the volt, represented by uppercase V. One volt is
equivalent to one joule per coulomb. Because potential difference is measured in volts, potential
difference is often simply called voltage.
There is a potential difference across the terminals of a battery.
The voltage across the two terminals of a battery can range from about 1.5 volts for a
small battery to about 12 volts for a car battery. Figure 10 shows a typical car battery. A
battery contains a positive terminal and a negative terminal. There is a potential difference, or
voltage, between these two terminals.
A device that is a source of electric current because of a potential difference is called a
cell. Table 1 on the next page lists a summary of various types of electric cells. Most common
batteries are electrochemical cells, which derive electrical energy from chemical energy. An
electrochemical cell contains an electrolyte, which is a solution that conducts electricity, and two
electrodes, each of which is a different conducting material.
Electrochemical cells can be either dry cells or wet cells. Dry cells contain a paste-like
electrolyte, while wet cells contain a liquid electrolyte. Flashlight batteries are an example of dry
cells. Wet cells are used in almost all car batteries. An average cell has a potential difference of
1.5 volts between the positive terminal and the negative terminal.
A voltage sets charges in motion.
When a flashlight is switched on, the terminals of the batteries are connected through the
light bulb. Electrons move from the negative terminal, through the light bulb, and then to the
positive terminal.
Look at the next page.
Charges can be accelerated by an electric field to move to a position of lower electrical
potential energy. When this happens, an electric current is produced. Electric current can be
measured as the rate that electric charges move through a conductor. The S-I unit of current is
the ampere, represented by uppercase A. One ampere, or amp, equals 1 coulomb of charge
moving past a certain point in 1 second.
A battery produces a direct current. That means that the charges always move from one
terminal to the other terminal in the same direction. The moving charges in a current can be
positive charges, negative charges, or a combination of both positive and negative charges.
Metals contain electrons that are free to move, so current in metals consists of negative charges
in motion. In gases and many chemical solutions, current is the result of both positive and
negative charges in motion.
In our bodies, current is mostly positive charge movement. Figure 11 shows how
positive ions move across the membrane of a nerve cell. The motion of ions across these
membranes creates current that sends signals throughout the nervous system.
Conventional current is defined as the movement of positive charge.
A negative charge moving in one direction has the same effect as a positive charge
moving in the opposite direction. Conventional current is defined as the current made of
positive charge that would have the same effect as the actual motion of charge in the material.
In this book, the direction of current will always be given as the direction of positive
charge movement that is equivalent to the actual motion of charges in the material. So, the
direction of current in a wire is opposite the direction that electrons move in the wire.
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For now, please skip the Science and the Consumer feature on the next page and turn to
the following page, where we consider electrical resistance.
Most electrical appliances you plug into an outlet are designed for the same voltage,
which is 120 volts. But light bulbs come in many varieties, from dim 40 watt bulbs to bright 100
watt bulbs. These bulbs shine differently because they have different amounts of current in
them. The difference in current between these bulbs is due to their resistance.
Resistance is caused by internal friction that slows the movement of charges through a
conducting material. Because it is difficult to measure this internal friction directly, resistance is
determined by examining the relationship between the voltage across a conductor and the current
that is passing through the conductor. Resistance is defined as the ratio of the voltage across a
conductor to the current it carries.
Figure 12 shows the inside of a light bulb. Examine Figure 12. What element is used to
make the filament used in a light bulb? If you said tungsten, you are correct. When charges pass
through the tungsten filament, internal friction in the bulb slows the movement of the charges,
and the filament releases energy as heat and light. The resistance of the filament determines how
bright the bulb is. The filament of a 40 W bulb has a higher resistance than the filament of a 100
W bulb.
Please turn to the next page.
Resistance can be calculated from current and voltage.
You have probably noticed that electrical devices such as televisions and stereos become
warm after they have been on for some time. Moving electrons collide with the atoms of the
materials that are used to make the electrical devices. Some of the kinetic energy of the electrons
is transferred to these atoms. This energy transfer causes the atoms to vibrate. As a result, the
materials warm up. In most materials, some of the kinetic energy of the electrons is lost as heat.
A conductor’s resistance indicates how much the motion of charges within the conductor
is resisted because of collisions. Resistance is found by dividing the voltage across the conductor
by the current.
The resistance equation is resistance, represented by uppercase R, equals voltage,
represented by uppercase V, divided by current, represented by uppercase I. R equals V over I.
The S-I unit of resistance is the ohm, represented by the capital Greek letter omega.
Ohms are equivalent to volts per ampere. If a voltage across a conductor of 1 volt produces a
current of 1 amp, then the resistance of the conductor is 1 ohm.
A resistor is a special type of conductor that is used to control current. Every resistor is
designed to have a specific resistance. For example, for any voltage, the current in a 10-ohm
resistor is half the current in a 5-ohm resistor.
Please look at the next page.
Let’s work through the Math Skills problem titled Resistance. The problem states that
the headlights of a typical car are powered by a 12-volt battery. What is the resistance of the
headlights if they draw 3.0 amps of current when turned on?
In step 1, you list the given and the unknown values. The given values include the
current, I, which equals 3.0 amps, and the voltage, V, which equals 12 volts. The unknown value
is the resistance in ohms.
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In step 2, you write the equation for resistance. Resistance equals voltage divided by
current, or R equals V over I.
In step 3, you insert the known values into the equation, and solve. R equals 12 volts
divided by 3.0 amps. Therefore, R equals 4.0 ohms.
Use the Math Skills Resistance problem as a model when working the practice
problems. The Practice Hint provides additional tips for the practice problems.
Conductors have low resistance.
Whether or not charges will move in a material depends partly on how tightly electrons
are held in the atoms that make up the material. A good conductor is any material in which
electrons can flow easily under the influence of an electric field.
Metals, like the copper used to make wire, are some of the best conductors. That’s
because electrons can move freely through metals. However, some metals are used to make
resistors. Conducting alloys and carbon are also used in resistors.
When you flip the switch on a flashlight, the light seems to come on immediately. But
electrons in a wire actually travel very slowly, due to collisions with atoms within the wire. So
how does the signal get to the bulb in the flashlight so quickly? When a voltage source is
connected to a conductor, the electric field is directed through the conductor at almost the speed
of light. Electrons everywhere throughout the conductor then experience a force due to the
electric field. All these electrons move at once in the opposite direction of the field lines. This is
why the light comes on so quickly when you turn on the flashlight.
Let's move on to the next page.
At low temperatures, some materials act as superconductors.
Certain metals and compounds have zero resistance when their temperatures fall below a
certain point called the critical temperature. These types of materials are called
superconductors. The critical temperature varies among materials, ranging from less than
negative 272 degrees Celsius to as high as negative 123 degrees Celsius.
Metals that can become superconductors below their respective critical temperatures
include niobium, tin, and mercury. Some metallic compounds that contain barium, copper, and
oxygen can also become superconductors. Superconductors have been used in electrical devices
such as filters, powerful magnets, and Maglev [MAG LEV] high-speed express trains.
Unlike conductors, insulators have high resistance.
Insulators have high resistance to charge movement. Therefore, insulating materials are
used to prevent electric current from leaking. For example, the plastic coating around the copper
wire of an electric cord keeps the current from escaping into the floor or your body.
Sometimes a pathway must be provided for the current to leave a charged object. In this
case, a conducting wire is run between the charged object and the ground. This is known as
grounding the object. Grounding is an important part of electrical safety.
Look at the next page.
Many electrical sockets are wired with three connections. Two of these connections are
current-carrying wires. The third connection is the ground wire. If there is any charge buildup,
the ground wire conducts the charge to Earth. Any excess charge can thus be spread over the
planet safely.
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Semiconductors belong to a third class of materials. This class of materials has electrical
properties between those of insulators and conductors. In their pure state, semiconductors are
insulators. The addition of a precise number of specific atoms of other materials dramatically
increases a semiconductor’s ability to conduct an electric charge. Silicon and germanium are two
common semiconductors. Figure 13 shows a computer board, which is a complex electrical
device that is made of conductors, insulators, and semiconductors.
Please turn your attention to the key concepts that are listed in the Summary.
A charged object has electric potential energy due to its position in an electric field.
Potential difference, or voltage, is the difference in electrical potential energy per unit
charge.
A voltage causes charges to move, producing a current.
Current is the rate of charge movement.
Electrical resistance can be calculated by dividing voltage by current.
Conductors are materials in which electrons flow easily.
Superconductors have no resistance below their critical temperature.
Insulators are materials with high resistance.
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Holt Science Spectrum
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Copyright  by Holt, Rinehart and Winston. All rights reserved.
Electricity p.10