Unit 4-Electrical Principles and Technologies

Lesson 1 – Electrical Charges

  • Materials that attract and repel other materials are said to be charged, or carry an electric charge.
  • A neutral object has the same number of electrons as protons. When a neutral object gains electrons, it becomes negatively charged; when it loses electrons, it becomes positively charged.
  • Note: only electrons move. Protons do not because they are inside the nucleus.
  • Static electricity means electricity “at rest”.
  • Law of Charges:
  • Unlike charges attract.
  • Like charges repel.
  • Charged objects attract neutral objects.

Conductors, Insulators, and In-Between

Materialsthat allow charges to move are called

conductors– metals are good conductors.

Materials, which do not conduct electricity well, are called insulators– rubber, non-metals are good insulators.

  • Semiconductors are materials with higher conductivity than insulators but with lower conductivity than metals. Ex. Silicon.
  • Superconductors are materials that offer minimal resistance to the flow of electrons. However, most of these materials must be cooled to near absolute zero

(- 273.150C) to become superconductors.

Ex. Alloys and ceramics.

Neutralizing Unbalanced Charges

  • One practical way to neutralize charged objects is by grounding. Grounding allows the excess electrons to flow into the earth.
  • We ground the electrical panel in the basement to the water main, which is buried in the earth to maintain electrical neutrality within the home.

Lesson 2 – Electricity within a Circuit

  • An electric circuitprovides a continuous path for electrons (charges) to move.
  • All circuits have four basic components:
  • Source: produces electricity. Ex. a battery
  • Conductor: wires.
  • Load: uses the electricity. Ex. light bulb
  • Control: switches.

A cell is a device for producing electricity from chemical or solar energy. A battery is a combination of cells.

Common electrical symbols

Conductor (wire)

Cell

Battery

Lamp

Switch

Resistor

Variable Resistor

(Rheostat)

Current and Voltage

  • Electric current refers to the flow of electrons (charges). It is measured in amperes (A).
  • An instrument used to measure very weak currents is called a galvanometer. Larger currents are measured with an ammeter.
  • The energy for pushing electrons through a circuit comes from a battery or generator. For the electrons to move they must have more energy at one terminal than the other. This difference in energy is called potential differenceorvoltage.
  • Voltage is measured in volts(V). A voltmeter is used to measure voltage.

Lesson 3 – Resisting the Movement of Charge

-Resistance is a measure of how difficult it is for electrons to flow through a conductor

-If the resistance is high the energy of the electrons (voltage) gets converted to heat and light - like in a filament of a light bulb.

-Current in a circuit can be either controlled by adjusting the voltage or resistance

-The unit for resistance is the ohm.

-Symbol is Ω, omega.

Ex. 5 Ω means “resistance is 5 ohms”.

-We can measure resistance using an ohmmeter or a multimeter.

-Variable resistors or rheostats are used to control the amount of current in a circuit. Ex dimmer switches, volume controls on stereos.

Calculating Resistance

-Electrical resistance is calculated using the formula:

R = VV = voltage in V

II = current in A

R = resistance in Ω

-This relationship is known as Ohm’sLaw.

-Rearranging the formula to:

  • Find Voltage use:

V = IR

  • Find Current use:

I = V

R

Examples of Problems

  1. A current of 0.5 A flows through a lamp when it is connected to a 120 V source. What is the resistance of the lamp?

Given: I = 0.5 A

V = 120 V

Find: R = ?R = V = 120

I 0.5

R = 240 Ω

  1. A resistance of 30 Ω is placed across a 90 V battery. What current flows in the circuit?

Given: R = 30 Ω

V = 90 V

Find: I = ?I = V = 90

R 30

I = 3 A

  1. A motor with an operating resistance of 15 Ω is connected to a power source. If the current in the circuit is 4 A, what is the voltage of the source?

Given: R = 15 Ω

I = 4 A

Find: V = ?V = IR = (4)(15)

V = 60 V

Factors Affecting Resistance of Wire

  1. Length – the longer the wire the higher the resistance. Electrons have to travel longer, thus more chances for them to bump into each other.
  1. Cross-sectional Area – the greater the area the lower the resistance – more room for electrons to move. This “thickness” is commonly described by an American Wire Gauge (AWG) number.
  1. Temperature – the higher the temperature the higher the resistance – temperature increases the kinetic energy of electrons.
  1. Material – some materials allow electrons to move more freely than others.

Assignment: pg 291 #1 - 8

Assignment 3 : Ohm’s Law - Show your work

  1. What current flows between a potential difference of 120 V through

a resistance of 30 ?

  1. A voltage of 75 V is placed across a 15  resistor. What current

flows through the resistor?

  1. A current of 0.5 A flows through a lamp when it is connected

to a 120 V source. What is the resistance of the lamp?

  1. A resistance of 60  allows 0.4 A of current to flow when it is connected

to the terminals of a battery. What is the voltage of the battery?

  1. A radio uses 0.2 A of current when it is operated by a 3 V battery.

What is the resistance of the radio circuit?

  1. A diagram shows a circuit that indicates a 90 V battery, an ammeter, and a resistance

of 12.5 . What does the ammeter read? (Find current).

  1. A circuit diagram has a 16  resistor, a battery, with an ammeter

that reads 1.75 A. Find the voltage of the battery?

  1. What voltage is applied to a 4  resistor if the current is 1.5 A?
  1. A 20  resistor is connected to a 30 V battery. What current flows through the resistor?
  1. A lamp having a resistance of 10  is connected across a 15 V battery. What current flows through the lamp?
  1. A bulb of 15  is in a circuit across a 4.5 V battery. What is the current in this circuit?
  1. A heater draws 10 A from a 120 V source. What is the heater’s resistance?
  1. p. 282 #1 – 4 (do on loose leaf)

  1. Lesson 5 – Voltage, Current and Resistance Lab

p. 284

Lesson 4 – Electric Circuits

  • There are two types of circuits: seriesandparallel.
  • Series circuits have only one current path – the current is the same at all points along the wire.

The electrons flow from the battery along a single pathway and back to the battery. The more devices or lights in a circuit the weaker each becomes as the energy available from the electrons is being shared. If one light bulb is removed none of the lights will work because the circuit is broken and the electrons cannot flow.

e

-

+

  • Electrons flow from the negative to the positive terminal.
  • Parallel circuits have several current paths – several branches connected side by side. The electrons come from the battery and travel along alternate pathways and back to the battery. Only some of the electrons will pass through each device in the circuit. If one light bulb is removed the others continue to work. The number of lights in the circuit does not affect the brightness of the lights.

You can keep

adding resistors.

  • House wiring uses parallel circuits – if one resistor (light) burns out the current in the others is not stopped.
  • To guard against electrical fires, which may occur if wires become hot enough, household circuits have fuses or circuit breakers.

Factors Affecting Resistance of Wire

  1. Length – the longer the wire the higher the resistance. Electrons have to travel longer, thus more chances for them to bump into each other.
  1. Cross-sectional Area – the greater the area the lower the resistance – more room for electrons to move. An American Wire Gauge (AWG) number commonly describes this “thickness”.
  1. Temperature– the higher the temperature the higher the resistance – temperature increases the kinetic energy of electrons.
  1. Material– some materials allow electrons to move more freely than others.

Lab - Lamps in a Series and Parallel Circuit

(reference p. 287)

QUESTION How does adding extra bulbs

the other bulbs in a series or parallel circuit

*** Fill in predicted values before doing the lab.

Series circuit layout / Predicted brightness / Observed brightness / Current (A) / Seen by Teacher
1 bulb / omit
2 bulbs / omit
3 bulbs / omit
3 bulbs,
1unscrewed / omit
Parallel circuit layout / Predicted brightness / Observed brightness /

Current

(A) / Seen by Teacher
2 bulbs / omit
3 bulbs / omit
3 bulbs
1unscrewed / omit

Analysis

  1. How does the brightness of the bulbs change as more bulbs are added to the series circuit? How did the electric current as measured by the ammeter change?
  1. How does the brightness of the bulbs change as more bulbs are added to the parallel circuit? How did the electric current as measured by the ammeter change?
  1. Using your results, explain why the brightness of the bulbs changes in the series circuit. Use your knowledge of Ohm’s Law and resistance to answer the question.
  1. Using your results from the parallel circuit explain any changes or lack of changes in the brightness of the bulbs.
  1. What happened to the series circuit when one of the bulbs was unscrewed? Explain why.
  1. What happened when one of the bulbs was unscrewed in the parallel circuit? Explain why.

Lesson 5: Electricity and Heat

  • Heat can be converted directly to electric energy using a device called a thermocouple.
  • A thermocouple is a loop of two wires made of different types of metals. The wires are wrapped together at both ends, called “junctions”.

(See p. 294)

When one junction is heated, a small electric current is produced. If the temperature is increased, the current increases. Thomas Seebeck discovered this principle in 1821. This is called the Seebeck Effect.

  • A thermo-electric generator is a device based on a thermocouple that converts heat directly into electricity.
  • Heat from a gas burner moves through several thermocouples connected in series, called a thermopile, creating a potential difference.

Electricity and Motion

  • Some crystals such as quartz will produce a sound when an electric current passes through them. This phenomenon is called the piezoelectric effect.

Ex. Sound produced by tiny electric watches or greeting cards.

  • A barbecue “spark”lighter uses the piezoelectric effect in reverse. When the crystal is being compressed an electric current is produced.

Electricity and Light

  • One of the most common uses of electricity is to produce light - using light to produce electricity might be the way of the future (no pollution).
  • Photovoltaic(PV) cells, or solar cells, are used to convert light energy to electrical energy.

Electrochemical Cells

  • All chemical cells have two features in common. One is the presence of two differentmetals called electrodes. The other is the separation of the metals by a substance called electrolyte.
  • The electrolyte can be a liquid, a “wetcell”, or a solid (most likely a paste), a “drycell”.
  • The chemical reactions in a cell determine the voltage that the cell can create. Single cells usually produce a maximum of 2 V. To get higher voltages several cells connected in series are used (battery).
  • If a cell cannot be recharged, a primary cell, the amount of chemicals it contains determines the energy the cell can produce.
  • Rechargeable secondary cells use chemical reactions, which can be reversed. In a re-charger, electricity is forced through the “dead” cell, rebuilding the original chemicals and allowing the cell to be reused.

Lesson 6 -Generators and Motors

  • The principle behind generators and motors is the relationship that exists between electricity and magnetism.
  • Oersted and Ampere made the discovery of this relationship in 1820. Oersted observed that a magnetic (compass) needle turned when it was near a current carrying wire.
  • An electric generator is a device that converts mechanical energy intoelectrical energy. A generator produces electricity by rotating loops of wire, called armature, in a magnet.
  • Most large generators use electromagnets instead of permanent magnets. An electromagnetis a wire wrapped around a soft core. When a current is passed through the wire a temporary magnet is produced.
  • As the wires rotate in a generator the electrons (electricity) begin to move along the wire in one direction. As the coil moves through the other pole of the magnet the electrons start moving in the other direction. Thus, the direction of the current flowing from the generator changes twice with each revolution.
  • Electricity produced by this type of generator is called alternatingcurrent (AC) because it changes direction, or “alternates”.
  • In North America the cycle is set at 60 cycles/second

or 60 Hertz (Hz).

  • Why AC current instead of DC? One reason is that it is easy to increase or decrease the voltage of AC current. In order to travel long distances efficiently through transmission lines, the voltage is increased. For consumer use, it is then decreased.

DC Generators

  • Directcurrent(DC), or current that flows in only one direction, can also be produced by generators. A generator that produces direct current is often called a dynamo.
  • In a dynamo, the armature is connected to the outside circuit by a split-ring commutator. (See Fig. 4.38A p. 314)

Electric Motors

  • An electric motorconverts electric energy to mechanical energy. Same basic design as a generator.
  • When the armature is connected to a source of energy it turns into an electromagnet, which is rotated by magnetic forces from the permanent magnet. The fundamental law of

magnets – opposite poles attract and like poles repel – is the

basis upon which electric motors function.

DC Motors

  • DC motors use a split-ring commutator, which acts as a switch, cutting off and then reversing the direction of current flow to keep the armature turning. (See p. 315)

AC Motors

  • AC motors have a rotating core, or rotor, made up of a ring of non-magnetic conducting wires. Surrounding the rotor is a stationary component called a stator. The stator is a two-pole (north and south) electromagnet.
  • When an AC motor is turned on, the attraction and repulsion between the magnetic poles of the stator and the rotor causes the rotor to spin. (Fig. 4.40 p. 317).

Lesson 7 - Electricity in the Home

  • Large electric generators in power stations produce AC current for use in homes and industry.
  • Transformers are used to “step up” the voltage for efficient transmission over long distances. At the destination, other transformers “step down” the voltage to the 120/240 V used in homes and factories.
  • In our homes the lines first go through a meter, then through a circuit breaker (older homes might have a fusebox instead). Fuses are common in cars and electric stoves.
  • Cables that contain three wires connect the breakers, plugs, lights, and switches in each branch circuit. There are two “live” wires – a white insulated wire (usually called the neutral wire) and a black insulated wire (usually called the hot wire). The third wire, the ground wire, is either bare copper or covered with green insulation.
  • The black wire carries high-energy electricity through the circuit. The white wire returns low-energy electricity back to the breaker panel. The ground wire reduces shock hazards.
  • Home wiring follows strict regulations and must meet a set of standards called the electrical code.

Measuring Electric Power

  • Poweris defined as energy per unit time.

Power (in watts) = Energy (in joules)

Time (in seconds)

  • The units of power are joules per second. One joule per second is equal to one watt(W) in honor of

James Watt (1736-1819).

  • However, in electricity the formula for power is:

P = VIV = Voltage in V

I = Current in A

P = Power in W

Rearranging we can find:

V = PI = P

I V

Examples:

  1. A current of 13.6 A passes through an electric baseboard heater when it is connected to a 110 V wall outlet. What is the power of the heater?

I = 13.6 A

V = 110 VP = VI

P = ?P = (13.6)(110)

P = 1496 W

  1. A 990 W oven requires 8.7 A of current to run. What is the voltage of the circuit?

P = 990 W

I = 8.7 AV = P = 990

V = ? I 8.7

V = 114 V

  1. A 100 W light bulb requires 120 V to operate. What is

the current?

P = 100 W

V = 120 V

I = ?I = P = 100

V120

I = 0.83 A

Measurement and Cost of Electrical Energy

-The unit of energy is the Joule (J)

-If a device uses energy at a rate of one Joule persecond that’s equal to one Watt.

1 J/s = 1 watt (W)

-The Joule is a very small unit for measuring energy, so instead we use the kilowatt-hour (kW-h) to measure

large amounts of energy

1 kW-h = 3 600 000 J

-Power companies charge users by the kW-h

-Example: $ 0.09/kW-h or 9 cents per kW-h

Four steps to follow to solve problems

  1. Always convert watts to kilowatts

(W to kW divide by 1000)

  1. Convert time to hours (60 min =1 h)
  2. Calculate kW-h (multiply the kW by the hours)
  3. Calculate the total cost.

Total Cost = # of kW-h • cost/kW-h

Examples:

  1. A family uses 3000 kW-h of energy in a two-month period. If the energy costs 11.0 cents per kilowatt-hour, what is the electric bill for the period?

Total Cost = # of kW-h • cost/kW-h

= 3000 x 11 = 33,000 cents

= $ 330.00

  1. A 60 W light bulb is left on for 5 hour and 40 minutes. If the cost of electricity is 13 cents per kW-h what did leaving this light on cost?

First change the W to kW 60 W = 0.06 kW

Next find the time in hours 5 h and 40 min. = 5.67 h

Then find the # of kW-h

# of kW-h = 0.06 kW • 5.67 h

= 0.34 kW-h

Therefore,

Total Cost = 0.34 x 13 = 4.4 cents

  1. A toaster is used an average of 5 hper month. The toaster

draws 8 A of current from a 110 V outlet. If electricity costs

8 cents per kW h how much will it cost to operate the toaster

for one year?

Step 1 – Find power (the watts)

P = IV

= 8 x 110

= 880 W = 0.88 kW

Step 2 – Find the energy in kW-h

= 0.88 kW x 60 h

= 52.8 kW-h

Step 3 – Find the cost.

Cost = kWh x $ 0.08/kWh

= 52.8 x 8

= 422 cents = $4.22

Lesson 8: Power Rating

  • Many electric appliances have a power in watts marked on them. This rating tells you how many joules of energy the device uses every second of operation (1 W = 1 J/s).

Ex. A 100 W light bulb uses 100 J of energy every second.

  • If a light bulb were perfect, all of the electric energy it took in (input energy) would be converted into light (useful output energy).
  • Since no device is perfect some energy is always wasted, that is no device is perfect or 100% efficient.

To find efficiency we use the formula:

Efficiency = energy output x 100%

energy input

or:

Efficiency = output x 100%

input

If power and time are given to find energy use:

Energy = PtP = Power in W

E = Ptt = time in s

  • Incandescent bulbs are about 5% efficient, halogen bulbs 15% and fluorescent 20%.

Note – Efficiency cannot ever be greater than 100%.

Ex. Problem

A 1000 W kettle takes 4 minutes to boil some water. If it takes 1.96 x 105J of energy to heat the water, what is the efficiency of the kettle?