Download CHAPTER 8 OHM`S LAW 8.1 Electric Potential Energy and Voltage

CHAPTER 8 OHM’S LAW
8.1 Electric Potential Energy and Voltage
Electric Potential Energy
• Energy = the ability to do work
• Electric energy that is stored is potential energy*
• Electric energy that is moving is kinetic energy.
*This is similar to gravitational potential energy.
• If a ball is lifted and held above the ground, the ball has energy
because, if released, the ball can do work.
• But if the ball is not released, then it is said to have potential
energy.
• Electrons separated from the positive nucleus “want” to return to
their original location, just like the ball “wants” to return to the
ground.
• If the electrons are held separated, then these electrons have
electric potential energy.
• The amount of energy the separated electrons possess is
dependent upon (1)how far they have been separated and
(2)how many electrons have been separated.
Electrochemical cells
• convert chemical energy into electrical energy.
• chemical energy separates the positive from the negative
charges
Battery
• Connecting cells together forms batteries.
• It is now accepted language to refer to all electrochemical cells
as batteries, regardless of the number of cells involved.
• The ends of batteries are called terminals
• Terminals are where we make a connection to the battery
• Extra e- accumulate on one terminal making it negatively
charged
• e- left from the other terminal to accumulate, leaving that
terminal positively charged
• when the battery is connected to an electrical device, e- can
flow through the connecting wires
• The electrical energy is transformed into other forms of energy
(eg. Sound, heat, light…)
• Batteries change chemical E into electrical E
Electric Potential Energy
• Electric energy can do work
Electric Potential Difference
• Potential difference, or voltage as it is more commonly called, is
proportional to the distance that the charges have been
separated.
• The actual potential energy is the product of both the voltage
and the amount of charge
(Energy =Voltage × Charge).
• The unit of electric charge is the coulomb (C)
• One coulomb = 6.25 x 1018 e- (gained or lost)
• The amount of electric potential energy per coulomb of charge
is called the potential difference or voltage.
• The unit of voltage = volt (V)
• Voltmeter measures the amount of potential difference between
two locations
Producing Voltage
Batteries come in 2 basic types:
1. Dry Cells: batteries in
flashlights, watches, etc.
2. Wet Cells: car batteries
• Two terminals on a battery are called electrodes
• Usually made of 2 different metals or a metal + some other
material
• Electrodes are in an electrolyte  substance that conducts
electricity
Electrolyte
In a dry cell
In a wet cell
Moist paste
liquid
Electrochemical Cell:
• Amount of voltage produced depends on the types of
electrodes used, and the electrolyte
Other Source of Electricity
• Batteries change chemical E  electrical E by separating
charge
• Other forms of energy can also be used to separate charge and
provide electrical E
1. Friction
2. Piezoelectric Crystals
3. Photo-electrochemical cells
How do solar cells work?
• convert the sun’s energy into electricity.
• rely on the the photoelectric effect: the ability of matter to emit
electrons when a light is shone on it.
• Silicon is a semi-conductor, meaning that it shares some of the
properties of metals and some of those of an electrical insulator
• Photons are tiny particles which radiate from the sun.
• photons hit the silicon atoms of the solar cell and transfer their
energy to e-, knocking them right off the atoms.
• Freeing up e- is only half the work of a solar cell: it then needs to
herd these stray e- into an electric current.
• This involves creating an electrical imbalance within the cell,
which acts a bit like a slope down which the electrons will flow in
the same direction.
• imbalance is made possible by the internal organization of Si.
• By squeezing small quantities of other elements into the silicon
structure, two different types of silicon are created:
n-type (negative), which has spare electrons,
p-type(positive), which is missing electrons,
leaving ‘holes’ in their place.
• When these two materials are placed side by side inside a solar
cell, the n-type silicon’s spare electrons jump over to fill the gaps
in the p-type silicon.
• This means that the n-type silicon becomes positively charged,
and the p-type silicon is negatively charged, creating an electric
field across the cell.
• Because silicon is a semi-conductor, it can act like an insulator,
maintaining this imbalance.
• As the photons smash the electrons off the silicon atoms, this field
drives them along in an orderly manner, providing the electric
current to power calculators, satellites & everything in between.
4. Thermocouples
• Possible to generate electricity from heat in a simple way that
has no moving parts: this usually involves thermocouples.
• Thermocouples take advantage of an electrical effect that
occurs at junctions between different metals.
• For example, take two iron wires and one copper wire. Twist one
end of the copper wire and one end of one of the iron wires
together. Do the same with the other end of the copper wire
and the other iron wire.
• If you heat one of the twisted junctions with a match and attach
the two free ends to a voltmeter, you will be able to measure a
voltage.
5. Generators
Reading Check p. 275
Check your Understanding p. 279
Workbook pp.110-115
8.2 Electric Current
Current Electricity: flow of charged particles in a complete circuit
 Amount of charge passing a point in a conductor every second
Ampere(A): unit for measuring current defined as: coulomb/second
Ammeter: device used to measure current
Electric circuit electric circuit is any complete pathway that allows
electrons to leave a source and eventually return to that source
 Must contain at least one source of voltage
• Electrons flow through devices (loads) in the circuit that convert
electricity to other forms of energy.
• Loads are things like: light bulb, motor, heater, etc.
• Chemical E in the battery gives e- on the negative terminal of
the battery potential energy.
• e- are attracted to the positive terminal
• the wire is the pathway they e- can travel through
• e- leave the (-)terminal & are pushed by the E of the battery
(voltage)
• electrical E is converted into light energy in the light bulb (load)
• e- complete the circuit by travelling the rest of the way back to
the positive terminal of the battery
• Upon returning to the source, all of the electric potential energy
in the charge must be converted to other forms of energy
A flashlight is a good example of a circuit
Water is a common analogy for current electricity.
Look at p. 281 figure 8.9
Basic Circuit Components and Diagrams
Four basic components:
1. Source: source of energy
2. Conductor: wire where current flows
3. Electric Load: turns electricity into other forms
4. Switch: turns circuit on or off
• All circuit diagrams should be drawn using a ruler.
• All turns in the circuit should be drawn at 90° angles.
• Not all circuit diagrams will be identical
 the size & spacing of the components is not important, but the
components should be in the same order as the illustration.
• The battery in a circuit may be symbolized as either a battery or a
cell. Modern convention is to use the cell symbol to represent both
cells and batteries
OOPS! We know that electrons move
through the wire toward the positive
terminal, but this diagram shows positive
flowing to negative!
Called “conventional Current”
An unfortunate and confusing
convention!
Conventional Current
 Ben Franklin originally named charges positive(+) and
negative(-) when he was studying static electricity.
 Franklin assumed that an electric “fluid” flowed from a positive
object into a negative object, & that became the “convention”.
 This was before electrons were discovered!
 For this reason, conventional current describes Franklin’s original
ideas of positive flow...which is backwards!
 The actual flow of electricity is from negative to positive (the flow
of electrons).
 So, when we use the term current, we are describing electron
flow, which is from negative to positive.
 When we use the term conventional current, we are describing
reverse electron flow, which is from positive to negative.
It is confusing, but once a convention is made and other principles
are based upon it, it is difficult to correct it!
Electrons are so Pushy!
As each e- moves through a conductor, it pushes on the one ahead
of it  all the e- move together as a group
The starting and stopping of e- flow is virtually instant
Imagine a tube filled end-to-end with marbles:
 The tube is full of marbles just as a conductor is full of free eready to be moved by an outside influence
 If a single marble is suddenly inserted into this full tube on the lefthand side, another marble will immediately try to exit the tube
on the right
 Even though each marble only traveled a short distance, the
transfer of motion through a tube is instantaneous from the left
end to the right end, no matter how long the tube is
 With electricity, the overall effect from one end of a conductor
to the other happens at the speed of light…a quick 299 792 458
meters per second!
Reading Check pp. 282 & 285
Check you understanding p. 289
Workbook pp. 116 - 121
8.3 Resistance and Ohm’s Law
• Resistance is the property of any material that slows down the
flow of e-, and converts electrical energy into other forms.
Ohm’s Law
How voltage, current, and resistance relate
Current
 formed when a conductive path is created to allow free
electrons to continuously move
 This continuous movement is called a current, & it is often
referred to in terms of "flow," just like the flow of a liquid through a
hollow pipe.
Voltage
 force motivating electrons to "flow" in a circuit
 specific measure of potential energy that is always relative
between two points.
 When we speak of a certain amount of voltage being present in
a circuit, we are referring to the measurement of how much
potential energy exists to move electrons from one particular
point in that circuit to another particular point. Without reference
to two particular points, the term "voltage" has no meaning.
Resistance
 e- tend to move through conductors with some degree of
friction, or opposition to motion.
 called resistance
 the amount of current in a circuit depends on the amount of
voltage available to motivate the electrons, and also the
amount of resistance in the circuit to oppose electron flow.
 Just like voltage, resistance is a quantity relative between two
points.
 For this reason, the quantities of voltage and resistance are often
stated as being "between" or "across" two points in a circuit.
Quantity
Symbol
Unit
Unit Abbrev.
Current
Ampere or amp
A
Voltage
V
Volt
V
Resistance
R
Ohm
Ω
Ohm’s Law
In this algebraic expression, voltage (V) is equal to current (I)
multiplied by resistance (R). Using algebra techniques, we can
manipulate this equation into two variations, solving for I and for R,
respectively:
Let's see how these equations might work to help us analyze simple
circuits:
In the above circuit, there is
 one source of voltage (the battery, on the left)
 one source of resistance to current (the lamp, on the right).
If we know the values of any two of the three quantities (V, and R)
in this circuit, we can use Ohm's Law to determine the third.
Example: What is the amount of current in this circuit?
Ω
Example: What is the amount of resistance (R) offered by the lamp?
R = V = 36 V = 9 Ω
Example: What is the amount of voltage provided by the battery?
Ω) = 14 V
Try Practice Problems p. 293 and 294
Reading Check p. 297
Check you understanding p. 301
Workbook pp. 122 - 127