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 e-

ready to be moved by an outside influence

If a single marble is suddenly inserted into this full tube on the left-

hand 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