chapter 11: current electricity. current electricity to operate electrical devices, you need a...

Chapter 11: Current Electricity

Current Electricity

• To operate electrical devices, you need a steady flow of electrons.

• Unlike static electricity, a flow of electrons move continuously as long as 2 conditions are met.

1. The flow of electrons requires an energy source2. The electrons will not flow unless they have a

complete path to flow through (electrical circuit)• Current electricity is the continuous flow of electrons

in a circuit.

Electric Circuits

An electric circuit must include:

1. An energy source

2. A conducting wire

3. A load which is a device that converts electrical energy to another form of energy; ex. Light bulb

Many electric circuits also include a switch that turns the circuit on or off by closing or opening the circuit.

Electric Current

• Electric Current is the measure of the rate of electron flow past a given point in a circuit; measured in amperes (A).

Examples of Loads

Examples of Loads

Converts electricity to…

light bulb light or heat energy

fan mechanical energy

doorbell sound energy

Electric Current, con’t

• Think of it in terms of a waterfall. If you could count the number of water molecules that drop over the edge every second, you could get the rate at which water is flowing past a particular point.

• Now think of the water molecules as electrons in a wire running past a particular point in a specific amount of time and you have the concept of current.

Measuring Current• Current in a circuit is measured using an ammeter. • The unit of electric current is the ampere (A). • An ampere is a measure of the amount of charge moving

past a point in the circuit every second.

Measuring Current, con’t• One ampere of charge in a circuit means that

6.2 x 1018 electrons pass a particular point every second.

Measuring Current, con’t• When you connect an ammeter or voltmeter to a

circuit, they must be on the right terminals. • There are two terminals on a meter that you use to

connect to a circuit. • The negative (–) terminal is often black, • The positive (+) terminal is often red.

Measuring Current, con’t

• Always connect the positive terminal of the meter to the positive terminal of the electrical source.

• Connect the negative terminal of the meter to the negative terminal of the electrical source.

Calculating Current

I

Q

t

Quantity Symbol Units of measurement

Charge Q C (coulomb)

Current I A (amperes) amps

Time t s (seconds)

Calculating Current, con’t

I

Q

tRearranging formulas- The magic triangle

Therefore, 1 Ampere = 1 Coulomb / s

Calculating Current, con’t

Sample Question 1

• If 310 C of charge passes a point in a conductor in 10 minutes, what is the current through that point in the conductor? Use GRASP and sig. digits.

Sample Question 2

• A 6.5 amp vacuum cleaner is used for 34 minutes, how much charge would pass through the vacuum during this time? Use GRASP and sig. digits.

Sample Question 3

• A 5.3 amp drill has 20 coulombs of charge pass through it, how long was the drill used for? Use GRASP and sig. digits.

Potential Difference

– Example: An apple hanging from a low. The apple has potential energy because of its position above the ground. If the apple falls down, it will convert its stored energy, or potential energy, into motion. Suppose an apple at a higher branch. It would have even more potential energy to convert.

Each electron has electric potential energy.

•Potential energy is the energy stored in an object.

Potential Difference, con’t

A battery has chemical potential energy in the electrolyte in its electrochemical cells.

• The chemicals in the electrolyte react with the electrodes.

• This causes a difference in the amount of electrons between the two terminals.

Potential Difference, con’t– One terminal in a battery has

mainly negative charges (electrons). – The other terminal has mainly

positive charges.

• The negative charges are electrons, which can move, are attracted to the positive charges at the positive terminal.


• If a conductor, such as a copper wire, is connected to both terminals, then the electrons flow from the negative terminal to the positive terminal.

• The difference in electric

potential energy between

two points in a circuit is

called the potential difference

or voltage (V).


• This difference causes current to flow in a closed circuit. The higher the potential difference in a circuit, the greater the potential energy of each electron.

Measuring Potential Difference

• The potential difference between two locations in a circuit is measured with a voltmeter.

Measuring Potential Difference, con’t

• Example: Connecting wires from a negative terminal on a battery across a voltmeter and then back to the positive terminal. The voltmeter would then display the potential difference of the battery.

• The SI unit for measuring potential difference is the volt (V).

Measuring Potential Difference, con’t

• A voltmeter must be connected in parallel with a load in the circuit in order to compare the potential before and after the load.

Electrochemical Cells – Batteries

• A battery is a combination of electrochemical cells. Each electrochemical cell is a package of chemicals that converts chemical energy into electrical energy that is stored in charged particles.

• A simple electrochemical cell includes an electrolyte and two electrodes.

Electrochemical Cells – Batteries; con’t

• An electrolyte is a liquid or paste that conducts electricity because it contains chemicals that form ions. – An ion is an atom or a group of atoms that has

become electrically charged by losing or gaining electrons. Sulphuric acid is an example of an electrolyte.

Electrochemical Cells – Batteries; con’t

• Electrodes are metal strips that react with the electrolyte. Two different electrodes, such as zinc and copper, are used in a battery.

• The electrodes and electrolyte

react causing one electrode to

collect and the other to lose

electrons.

Wet Cells and Dry Cells

An electrochemical cell that has a liquid electrolyte is called a wet cell.

• Wet cells are often

used as an energy

source for cars

and other motorized

vehicles.

Wet Cells and Dry Cells, con’t• An electrochemical cell that uses a

paste instead of a liquid electrolyte is called a dry cell.

• Dry cells are used in flashlights, hand-held video game devices, cameras, and watches. Each electrode in a dry cell or battery can also be called a terminal. Terminals are the end points in a cell or battery where we make a connection

Fuel Cells• A fuel cell is an electrochemical cell that generates

electricity directly from a chemical reaction with a fuel, such as hydrogen. Fuel cells are used in electric vehicles.

• The cell is not used up like an ordinary cell would be because as the electricity is produced, more fuel is added.

Homework

• Read 434-438

• Complete #1-4, page 436 and #1-5, page 438

Recall….

• Electric current can be thought of as the number of electric charges (electrons) passing a point in the circuit per second.

• Can also be thought of as a speed of electron flow.

Electric Potential (Voltage)

• Voltage can be thought of as the energy that each electron in the circuit contains.

• Electric potential refers to the amount of energy that electrons possess in a circuit.

Electric Potential, con’t• A load (ex. light bulb) converts electrical energy into another form of

energy (ex. light energy and heat energy).

• You can compare this to the water flowing past a water wheel. The wheel converts some of the energy of the water into motion. The water has more energy before the wheel than after the wheel.

• Therefore, there is higher voltage before the load than after the load.

Calculating Electric Potential (Voltage)

• The electrical potential energy for each coulomb of charge in a circuit is called the electric potential (V) aka voltage.

V

E

Q

Calculating Potential Difference (Voltage), con`t


Charge Q C (coulomb)

Energy E J (Joules)

Voltage V V (Volts)

• Where E is the energy required to increase the electric potential (V) of a charge, Q.

• Potential difference is often called voltage.


… and the formula can be rearranged

Energy

Charge V

E

Q


Therefore:

• One volt (V) is the electric potential difference between two points if one joule of work (J) is required to move one coulomb (C) of charge between the points.

• Volts = Joules ∕Coulombs

Sample Question 1:

• What is the potential difference across an air conditioner if 72 C of charge transfers 8.5 x 103 J of energy to the fan and compressor? Use GRASP and sig. digits.

Sample Question 2:

• A static electric shock delivered to a student from a “friend” transfers 1.5 x 101 J of electric energy through a potential difference of 500 V. What is the quantity charge transferred in the spark? Use GRASP and sig. digits.

Sample Question 3

• How many joules of energy are produced if a gas powered generator produces 120 V with a charge of 60 C at the negative terminal? Use GRASP and sig. digits.

Resistance

• The degree to which a substance opposes the flow of electric current through it.

• All substances resist electron flow to some extent.

Resistance, con’t

• Conductors, such as metals, allow electrons to flow freely through them and have low resistance values.

• Insulators resist electron flow greatly and have high resistance values.

Resistance, con’t

• Resistance is measured in ohms (Ω) using an ohmmeter.

• An ohmmeter is a device for measuring resistance.

Resistance, con’t

• When a substance resists the flow of electrons, it slows down the current and converts the electrical energy into other forms of energy.

• The more resistance a substance has, the more energy gained by the substance is radiated to its surroundings as heat and/or light energy.

Resistance in a Circuit

• A resistor is any material that can slow current flow.

• In a light bulb, the filament’s high resistance to the electron’s electrical energy causes it to heat up and produce light.

Resistors and Potential Difference• Resistors can be used to control electric current or

electric potential in a circuit. • In a circuit, electrons have a higher potential difference as

they enter a resistor compared to when they leave the resistor because they use up some energy in passing through the resistor.

Example:

• Imagine electrons entering a resistor as being at the high end of a ramp, where they have a lot of potential energy. In this analogy, electrons leaving the resistor are at the bottom end of the ramp, where their potential energy has been converted to another form of energy.

Wire-wound Resistors

• Resistors can be made with a number

of techniques and materials.• One common type is the wire-wound resistor.• A wire-wound resistor has a wire made of heat-resistant

metal wrapped around an insulating core. The longer and thinner the wire, the higher the resistance. They are available with values from 0.1Ω up to 200 k Ω.

Resistance in a Wire –comparing water to electricity

• Longer thinner pipes have more

resistance to the flow of water than

pipes with a larger diameter.

The same idea applies to electricity.

• The more resistance that you have in a circuit, the more it will decrease current at a given potential difference.

• Larger, shorter wires provide less resistance for electrons to travel.

• Temperature and material can also affect resistance.

Ohm`s Law

• Ohm’s law established the relationship between potential difference (V) and current (I).

• Ohm`s law refers to the amount of resistance in a circuit.

• The symbol for resistance is called the ohm (Ω) in honour of Georg Ohm’s work in this field.

Ohm`s Law

• According to Ohm`s law, the amount of voltage (or energy) in a circuit is equal to the current multiplied by the resistance.

• Ohm’s law states that, as long as temperature stays the same, V = IR

• if the resistance of a conductor remains constant, then

the current is directly proportional to the voltage.

Ohm`s Law and Temperature

• Ohm’s law works for most circuits. However, temperature affects resistance. Generally, resistance is lower when a conductor is cooler. As the temperature increases, resistance increases.

• For example, a filament in an incandescent light bulb often has 10 times more resistance when the bulb is warm.

Calculating Resistance

• Resistance = voltage current

V

I R

… and the formula can be rearranged

Voltage

Current

R = V I

I = V R

V = I x R

Calculating Resistance, con`t


Resistance R Ω (ohm)

Current I A (Amphere)

Voltage V V (Volts)

Sample Question 1

• A current of 4.0 A flows through a 40 Ω resistor in a circuit. What is the voltage? Use GRASP and sig. digits.

Sample Question 2

• A 30 V battery generates a current through a 15 Ω resistor. How much current does the battery generate? Use GRASP and sig. digits.

Sample Question 3

• An electric stove is connected to a 240-V outlet. If the current flowing through the stove is 20 A, what is the resistance of the heating element? Use GRASP and sig. digits.

Circuit DiagramsCircuit Diagrams

• Engineers and designers of electrical circuits use special symbols that show the components and connections in a circuit.

• A drawing made with these symbols is called a circuit diagram.

Circuit Diagrams, con’tCircuit Diagrams, con’t

• Follow these rules when you draw circuit diagrams.

1. Always use a ruler to draw straight lines for the conducting wires.

2. Make right-angle corners so that your finished diagram is a rectangle.


• Conductor or wire– To pass current very easily from one part of a

circuit to another.


• Cell-Supplies electrical energy-The positive end is bigger than the negative end.


• 2 Cells

• Note: every time a cell is added you need to draw another cell.


• Ground – – A connection to earth


• Switch - An on-off switch allows current to flow only when it is in the closed (on) position


• Lamp– A transducer which converts electrical energy

to light


• Resistor– A resistor restricts the flow of current


• Ammeter – -Device that measures current

A


• Voltmeter

• -Device that measures voltage

V


Motor -electrical load that converts electrical energy into movement

M

Series CircuitsSeries Circuits

• Electric circuit in which the components are arranged one after another in series.

• A series circuit has only one path along which electrons can flow.

• If that pathway is interrupted, the whole circuit cannot function.

Series Circuits, con’tSeries Circuits, con’t

• The amount of current is the same in all parts of a series circuit.

• If more resistors are added, it will increase the total resistance of the circuit.

• This decreases the current.

Series Circuits, con’tSeries Circuits, con’t

• Example: Adding an extra bulb to a series string of lights makes all the bulbs dimmer.

• Electrons use up all their potential difference going around a series circuit no matter how many loads are in the circuit. Each load will use part of the total potential difference, depending on how much it resists the flow of electrons.

Parallel CircuitsParallel Circuits

• A parallel circuit is an electric circuit in which the parts are arranged so that electrons can flow along more than one path. The points where a circuit divides into different paths or where paths combine are called junction points

• An interruption or break in one pathway does not affect the other pathways in the circuit.

Parallel Circuits, con’tParallel Circuits, con’t

• Similarly, adding a new pathway with more resistors does not affect the resistance in any of the other pathways.

• Adding extra resistors in parallel decreases the total resistance of the circuit.


• Each electron has the same amount of energy, and electrons must expend all their energy on the path they are on.

• This is why the potential difference across parallel resistors will always be the same, even though the resistors themselves are of different values.


• Loads connected in parallel circuits have different currents (paths).

Summary of Current, potential difference, Summary of Current, potential difference, and resistance in series and parallel circuits.and resistance in series and parallel circuits.

Circuit Potential Difference

Series circuit

Each load uses a portion of the total potential differences supplies by the

battery. VT = V1 + V2 + V3

Parallel circuit

Each load uses all the potential difference supplied by the battery.

VT = V1 = V2 = V3

Circuit Current

Series circuit

The current is the same throughout a series circuit.

Itotal = I1 = I2 = I3

Parallel circuit

The current divides into different paths. A pathway with less resistance will have a greater

current. Itotal = I1 + I2 + I3

Circuit Resistance

Series circuit

The current decreases when more resistors are added.

RT = R1 + R2 + R3

Parallel circuit

Adding resistors in parallel decreases the total resistance of the circuit.

Sample Question 1: In Series

1. Consider 2 and 4 resistors in series with a 12 V battery. Assuming ideal ammeters and voltmeters, what will be: I1, I2, V1, V2.

Recall: For resistors in series, the total resistance, RT,

is given by: RT = R1 + R2

Solution

• Current, I1

• First, calculate the total resistance of circuit:

• RT = R1 + R2 = 2 + 4= 6

• Now use Ohm's law to calculate current:• V= I R

• So I = V / R= 12 V / 6= 2 A

• The current in the wire to the left of the 2 resistor = 2 A• Current, I2

• In a series circuit, the current is always the same, so the current in the wire to the right of the 4 resistor is also 2 A.

• Voltage, V1

• Use Ohm's law to calculate voltage across the 2 resistor:

• V = IR= 2 A × 2= 4 V

• The voltage V1 = 4 V

• Voltage, V2

• Similarly for the 4 resistor:

• V = IR= 2 A × 4= 8 V


• Check: • VT = V1 + V2

• 12V = 4V + 8V

Sample Question 2: In Parallel

Now consider two resistors in parallel with a 6 V battery. For resistors in parallel, the total resistance is given by:

Using the values displayed on the graph, calculate V1, V2, I4, I5.

• Voltage, V1

• First, calculate the equivalent resistance of the two resistors:

• 1 / R = 1 / R1 + 1 / R2 = (1 / 3) + (1 / 4)= (7 / 12

• So R = (12 / 7)

Solution

• Now use Ohm's law to calculate voltage V1:

• V1 = I1 × R

= 3.5 A × (12 / 7) = 6 V

• i.e. the battery voltage, V1 = 6 V


• Voltage, V2

• Use Ohm's law to calculate voltage across the 4 resistor:

• V2 = I3 × R2

= 1.5 A × 4= 6 V


• Note that for parallel circuits the voltage is the same across all resistors.

• Current, I4

• This must be identical to I3, i.e. 1.5 A

• The current I4 = 1.5 A

• Current, I5

• This must be identical to I1, i.e. 3.5 A

• The current I5 = 3.5 A

Short Circuits

• A path that allows electrons to flow along a different path than the one intended.

• A short circuit is an accidental low-resistance connection between two points in a circuit, often causing excess current flow.

Short Circuits, con’t

• This will cause your electrical device to stop working.

• Due to the low resistance, the current increases and the conducting wires can quickly become hot and can start a fire.

• Short circuits can also kill if a person becomes the ground (electricity will take a path through your body).

Electrical Safety and Safety Devices

• All electrical appliances present a risk of electric shock.

• Some electronic devices, such as computers, retain electric charge even when they are unplugged. This is why many electrical devices have a “Do Not Open” warning printed on them.

Fuses and Circuit Breakers

• A fuse is a safety device in an electric circuit that has a metallic conductor with a low melting point compared to the circuit’s wires. If the current gets too high, the metal in the fuse melts and the current flow stops.

• This prevents further problems, such as damage to your electrical components or a possible fire. A blown fuse must be physically replaced as it can work only once.

• This symbol represents a fuse in a circuit diagram.

Fuses and Circuit Breakers, con’t

• A circuit breaker does the same job as a fuse except that the wire inside does not melt. Instead, the wire heats up and bends, which triggers a spring mechanism that turns off the flow of electricity. Once the breaker has cooled, it can be reset.

Three-Prong Plug

• The third prong of a three-prong electrical plug connects the device to the ground wire of the building.

• The ground wire sends any unwanted current flow directly to the ground.

• Instead of electricity travelling to the metal body of the device and shocking a person using the device, the current is directed to the ground.

Ground Fault Circuit Interrupter (GFCI)

• A ground fault circuit interrupter (GFCI) or residual current device is a device that detects a change in current and opens the circuit, stopping current flow.

• For example, if an appliance gets wet while you are handling it and some current starts to flow through the water, the GFCI opens the circuit so there is less chance of injury to you.

Homework

• Read pages 462-464

• Complete # 1-12, pg 467

Chapter 12:

Local Solutions to Generating Electricity

• The electricity used in most homes in Ontario is usually generated quite some distance away at some large scale electricity generation facility.

• The facilities are generally large scale products built by the government or businesses.

Local Solutions to Generating Electricity, con’t

The production of energy can be classified into two categories:

1. Non-renewable resources. – A resource that cannot be replaced once it is

used up.

2. Renewable resources. – A resource that can be reused or replaced.

Resources

Non-renewable

Fossil fuels

Nuclear energy

Renewable

Water power (hydroelectric)

Biomass

Geothermal energy

Solar energy

Wind energy

Tidal energy

• In 1831, Michael Faraday, an English chemist and physicist, demonstrated that an electric current can be generated by moving a conducting wire through a magnetic field, a process called electromagnetic induction.

Generating Electricity

Generating Electricity, con’t

• Electromagnetic induction is used today to generate electricity in large-scale generators. – An apparatus that transforms the energy of motion

into an electric current. – Magnets inside the generator are rotated by a

turbine. The magnets spin coils of copper wires. This pulls electrons away from their atoms and creates a current in the copper wire.

Generating Electricity, con’t• The current is sent through transmission lines to

cities and towns.

• The transmission lines and substations are known as an energy grid.

Electrical Power and Power

• Electrical power (measured in watts, W) is the rate at which work is done on a charge (by a battery) or on an electrical device (by the charge).

• In terms of an equation, P = VI, but since V = IR (Ohm’s Law), then P = I2R or P = V2

R

Electrical Energy and Power, con’t

• Electrical energy (measures in watt-hours, W·h) is the ability to do work with the power provided by moving electrons in a circuit.

• The equation for electrical energy is E = PΔt, but since P = VI, then E = VI Δt, where Δt is the time interval.

Sample Problem 1

• The T1-84 calculator uses four 1.5 V batteries and has a power of 0.0008 W. What is the current?

Sample Problem 2

• Calculate the resistance of a toaster oven if its power is 800 W when connected to a 110 V outlet.

Sample Problem 3

• A. Calculate the power rating of the light bulb.

• B. Calculate the energy consumed after 15 continuous hours of operation.

A I = 2 A

12 V

Percent Efficiency

• The efficiency of a device is the ratio of the useful energy that comes out of the device to the total energy that went in.

• The more input energy that a device converts into usable output energy, the more efficient the device is.

• Efficiency is usually calculated as a percentage:

Percent Efficiency, con’t

• An incandescent light produces a lot of heat. In fact, only about 5% of the energy that goes into the bulb is used as light, useful energy.

• A compact fluorescent bulb in comparison is about 20% efficient.

Sample Question

• Suppose a light bulb uses 780 J of input energy to produce 31 J of light energy. What is its percent efficiency?

% Efficiency, con’t

• By comparing the efficiency of different devices, we can judge both their energy cost and their environmental impact.

• A mini refrigerator from the 1970s uses the same energy as a modern full-size refrigerator as a result of improvements in insulation and efficiency of the compressor motor.

EnerGuide

• All large appliances have an EnerGuide label, which tells you how much energy the appliance will use in a month or a year of average use.

Energy Star

• The Energy Star symbol is used to indicate that a product meets specific criteria of energy consumption set by the EPA (Environmental Protection Agency).

• The Energy Star label was established to reduce greenhouse gas emissions from inefficient use of energy and to help consumers purchase energy efficient products, which help lower energy bills.

chapter 11: current electricity. current electricity to operate electrical devices, you need a...

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