Electricity and Magnetism

Chapter 5

1. What is electricity?

Many natural phenomena are electrical in nature.

Nerve impulses Bolts of lightning Chemical reaction

Electricity is one of the many forms of energy used in today’s society

Electricity describes all the phenomena caused by positive and negative electric charges

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1.1 Electrical Charges

Electrical charge is a property of protons and electrons. A proton carries a positive charge, while an electron carries a negative charge.

A negatively charged body contains more electrons than protons.

A positively charged body contains fewer electrons than protons.

Remember, only the electrons will move to create charges.

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Electrical Forces of Attraction and Repulsion Like charges repel Opposite charges attract

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The elementary charge is the charge carried by a single electron or proton.

The coulomb (C) is the unit of measurement for the quantity of electrical charge. One coulomb is equal to the charge of 6.25 x 1018 electrons or protons.

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1.2 Conductors and Insulators Most objects are electrically neutral – they

have the same number of protons and electrons.

However, by transferring electrons from one atom to another, some objects can acquire a charge.

Charging an object consists of creating an imbalance in the electrical charge of that object

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A conductor is a substance that permits the free flow of electrical charges.At the atomic level, the attraction between the

nucleus and the valence electrons is relatively weak. Therefore the valence electrons can easily pass from one atom to another.

Metals and electrolytic solutions are usually conductors

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An insulator is a substance that impedes the free flow of electrical chargesThe nuclei of insulators hold tight to their

valence electrons.Nonmetals are usually insulators.E.g. wood, plastic, glass, ceramics, rubber,

silk, paper, air

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Semiconductors exhibit a variable conductivity, depending on different factors.Metalloids and carbon are semiconductorsThey are widely used in electronics.

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2. Static Electricity Static electricity describes all the phenomena

related to electrical charges at rest. The static charge on an object will gradually

diminish due to the slow transfer of electrons to the water molecules in the air.

Or, the transfer can be rapid if two objects come close to each other or touch. This is called an electrostatic discharge and is often accompanied by a spark as the electrons pass through the air and heat it up making it light up.

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2.1 Charging an Object A object can be charged in various ways:

Friction Conduction’ Induction

Charging by Friction When two neutral object are rubbed together, the

atoms of one of the bodies may pull electrons away from the atoms of the other body.

The Triboelectric Series can be used to determine which way the electrons will flow …

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Plastic is rubbed with silkGlass is rubbed with woolWhat will happen if the glass is then brought close to the plastic?

Charging by ConductionAn object is put in contact with another object

that is already chargedThe two objects will share the charge between

them resulting in both having the same charge, but weaker than that of the original object.

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Charging by InductionThe charge is created without direct contact of

the two objects.The charged object will acquire the opposite

charge to the original one

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3. Dynamic Electricity Dynamic electricity describes all of the

phenomena related to electrical charges in motion.

The electrical charges part of a circuit, meaning they can flow in a loop.

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3.1 Electric Current As soon as current is generated at one

point in a circuit, all of the electrons in the loop are set in motion. This is due to the force of repulsion between the negative electrons. The effect is almost instantaneous.

Electric current is the orderly flow of negative charges carried by electrons.

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The conventional current direction is the direction in which a positive particle would flow in an electrical circuit. For this reason, the direction goes from the positive terminal of the power supply to its negative terminal.

BUT in reality, the electrons flow the opposite way (from negative to positive). This is called “electron flow”.

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Current IntensityCurrent intensity is the number of charges

that flow past a given point in an electrical circuit every second.

The formula for current intensity is:

An ammeter is an instrument that is used to measure the current intensity

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t

qI

Where: I is the current intensity, in amperes (A)q is the quantity of charge, in coulombs (C)Δt is the time, in seconds (s)

Sample Problem 21.875 X 1018 electrons flow through a resistor in 0.50

seconds Determine the current flowing through the resistor.

# e- = 21.875 X 1018

Δt = 0.5 s

I = ?C 3.5

10256

108752118

18

.

.q

1. Find the quantity of charge (q):

2. Find the current (I):

A 7s 0.5

C 3.5Δt

qI

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Current intensity is measured using an ammeter.

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Potential DifferenceThe potential difference is the amount of

energy transferred between two points in an electrical circuit.

The formula for potential difference is:

A voltmeter is used to measure the potential difference in various parts of the circuit

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q

EV

Where: V is the potential difference in volts )V)E is the energy in joules (J)q is the quantity of charge in coulombs (C)

Practice problem 25 coulombs of electricity carry 12.5 joules of

energy. What is the potential difference in the circuit?

q = 25 C

E = 12.5 J

V = ?

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V 2C 12.5

J 25

q

EV

In an electrical circuit, the energy comes from a battery, a power supply, or from a generator. A battery converts chemical energy into electrical energy. The power supply is a transformer converting 120 V to a lower voltage. A generator converts mechanical energy into electrical energy.

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A voltmeter is used to measure the potential difference in a circuit.

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ResistanceAny part of an electrical circuit that uses

energy is called a resistor. Resistors transfer electrical energy into another form.

In a circuit, the resistance can be described as a force that hinders the flow of the current. The higher the resistance, the more energy it takes for the current to flow.

Electrical resistance is the ability of a material to hinder the flow of electric current.

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There are four factors that determine the resistance of a substance:

Conductance is the ability of a material to allow the flow of electric current. Good conductors are poor insulators.

The best conductors are good conducting material and … Short Fat Cold

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Resistance is measured in ohms (Ω) An ohm is equal to a potential difference of one

volt per ampere:

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1A

1V1Ω

Ohm’s Law Ohm’s Law states that, for a given resistance, the

potential difference in an electrical circuit is directly proportional to the current intensity.

Or …

Ohm’s Law applies only to conductors, and not to insulators or semiconductors.

IRV

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Where: U is the potential difference, in volts (V)I is the current intensity, in amperes (A)R is the resistance, in ohms (Ω)

I

VR And …

R

VI

Sample ProblemA toaster has 3 A of current flowing through it while plugged in to a 120 V outlet.Calculate the resistance of the toaster.

I = 3 A

V = 120 V

R = ?

Ω 40A 3

V 120I

VR

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3.2 Electrical Power Electrical power is a measure of the rate of

transformation of electrical energy. The more powerful a device, the faster it works.

Electrical power is the amount of work an electrical device can perform per second.

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t

WPe

Where: Pe is electrical power, in watts (W)

W is work, in joules (J)Δt is time, in seconds (s)

Sample Question

A 500 W electric motor runs for 2 minutes. How much work does it use? How much energy does it use?

Pe=500 WΔt = 2 min

= 120 sW = ? kJ 60or J 60000

s 120 W500

ΔtPWΔt

WP

e

e

kJ 60

WE

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Another equation for power …

Sample problemA 100 W light bulb is plugged into a 120 V outlet. Determine the current passing through the bulb.

Pe = 100 W

V = 120 V

I = ?

VIPe

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where: Pe is electrical power, in watts (W)V is potential difference, in volts (V)I is current intensity, in coulombs (C)

A 0.83V 120

W100V

PI

VIP

e

e

The Relationship Between Power and Electrical Energy Electrical energy can be measured in joules or

kilowatt-hours The kilowatt-hour is the unit used to calculate

electrical consumption for electricity bills. 1kWh = 1000 W X 3600 s = 3 600 000 J Presently, the residents of Quebec pay about 8 cents

per kilowatt-hour.

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tPE e where: E is the electrical energy in joules (J)or kilowatt-hours )kWh)Pe is the electrical power in watts (W) or kilowatts (kW)Δt is time in seconds (s) or hours (h)

Sample Problem 1 A 5 W toy motor runs for 0.5 minutes. Calculate the energy used.

Pe = 5 W

Δt = 0.5 minutes

= 30 s

E = ?

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J 150

s 30 W5

ΔtPE e

Sample Problem 2 A 150 W light bulb is on for 1800 minutes. Hydro Québec charges 8 cents per kWh. Calculate the cost of using this bulb.

Pe = 150 W

= 0.150 kW

Δt= 1800 minutes

= 30 hours

Cost = 8¢/kWh

E = ?

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1. Find the Energy used:

2. Find the cost:

¢ 36

kWh 4.5 ¢/kWh 8

kWh of # cost/kWh Cost

kWh 4.5

h 30kW 0.150

ΔtPE e

3.3 Electrical Circuits An Electrical Circuit is a network in which

electrical charges can flow continuously in a loop.

Diagrams and symbols are often used to represent electrical circuits. See the “Toolbox” for information on how to draw a circuit diagram.

The current direction shown in a diagram usually corresponds to conventional current (+’ve to –’ve)

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There are two ways in which components in an electrical circuit can be connected: In Series In Parallel

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Series Circuits The components are connected one after the other.

There is only one way for the current to flow. There are no branches in the circuit.

A series circuit is a circuit in which the elements are connected end to end.

If one of the components is defective, the entire circuit stops working.

The amount of energy used by the resistors adds up so that with each new resistor, the amount of energy available for each resistor decreases. If you add more light bulbs in series, the brightness of each bulb decreases.

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Parallel Circuits Parallel circuits have branching. There is more than

one way for the current to go. A Parallel circuit is a circuit that has at least one

branch. The points where the circuits branch into two or more

paths or combine into one are called nodes. If one of the components is defective, the other

components will continue to operate. The amount of energy available to each component

does not change as new resistors are added. Adding new resistors does not decrease the energy used by the others.

As resistors are added, the total resistance decreases.

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4. What is Magnetism? A magnet is an object that can attract

other objects containing iron, cobalt or nickel.

Magnetism describes all the phenomena caused by magnets.

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4.1 Magnets Some substances never acquire magnetic

properties. Others, like iron, can become magnetic under certain circumstances.

Iron is made up of domains that are like tiny magnets. When the iron is not magnetized, the domains are not aligned. Their magnetic effects cancel out. When the iron is magnetized, the domains line up. The greater the number of aligned domains, the stronger the magnet.

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Magnetic Forces of Attraction and RepulsionAll magnets have a north-seeking pole and a

south seeking pole. The north pole of a magnet, by convention, is

the pole that turns towards the earth’s magnetic pole located near the geographical North Pole. The other end of the magnet is its south pole.

So, what we call the “North Pole” is really the south pole of the earth’s magnetic field.

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The following is true of magnets:Opposite magnetic poles attract each otherLike magnetic poles repel each other

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Magnets will always have a north and a south pole.

If the magnet breaks, each piece will have a north and a south.

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4.2 Magnetic Fields The force of attraction or repulsion

between two magnets is called the magnetic force. This force can act over a distance through a magnetic field.

A magnetic field is the area of space in which the magnetic force of a magnet can act on another magnet.

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The effect of the field can be observed by sprinkling iron filings around a magnet …

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By convention, the magnetic field is said to have the direction indicated by the north pole of a compass if it was placed in the field.

The magnetic field can be represented by magnetic field lines.

Magnetic field lines:1. Go from north to south2. Never cross3. Are closer together near the poles, where the

magnetic field is strongest

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4.3 Magnetizing Objects A nonmagnetic substance cannot be magnetized

and is not attracted to a magnet. A ferromagnetic substance has the ability to

acquire magnetic properties. (Iron, nickel or cobalt) A magnet is a substance with magnetic properties.

It can attract ferromagnetic objects. Although it is usually made of iron, nickel or cobalt, some rare earth element can make extremely strong magnets.

Magnetic remanence describes the ability of a material to acquire and conserve magnetic properties.

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5. Electromagnetism Electricity and magnetism are connected. Electric currents will always generate a

magnetic field. Magnetic fields can be used to generate

an electric current. Electromagnetism describes all the

phenomena resulting from the interaction between electricity and magnetism.

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5.1 Magnetization by Electricity An electric current produces a magnetic

field. The field is created by moving electrons.

Static electricity does not involve moving electrons, therefore, a statically charged object will not possess a magnetic field.

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The Magnetic Field of a Live Wire In 1819 Hans Christian Oersted observed that

a compass was deflected when it came close to a live wire. He was the first of observe the relationship between electrical and magnetic phenomena.

The direction of the magnetic field around a live wire depends on the direction of the current.

The “Right-Hand Rule” can be used to determine the direction of the magnetic field.

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Steps for the Right Hand Rule

1. Find the positive and negative ends of the wire

2. The conventional current flows from positive to negative

3. Point thumb of your right hand in the direction of the conventional current

4. Wrap your fingers around the wire

5. Your fingers will point in the direction of the magnetic field

-

+Direction of the Conventional Current

Direction of themagnetic field

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