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 ST EST AST

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. ST EST AST

Electrical Forces of Attraction and Repulsion  Like charges repel  Opposite charges attract ST EST AST

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 electrons or protons. ST EST AST

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 ST EST AST

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 ST EST AST

An insulator is a substance that impedes the free flow of electrical charges  The nuclei of insulators hold tight to their valence electrons.  Nonmetals are usually insulators.  E.g. wood, plastic, glass, ceramics, rubber, silk, paper, air ST EST AST

Semiconductors exhibit a variable conductivity, depending on different factors.  Metalloids and carbon are semiconductors  They are widely used in electronics. ST EST AST

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. ST EST AST

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 … ST EST AST

ST EST AST Plastic is rubbed with silk Glass is rubbed with wool What will happen if the glass is then brought close to the plastic?

Charging by Conduction  An object is put in contact with another object that is already charged  The two objects will share the charge between them resulting in both having the same charge, but weaker than that of the original object. ST EST AST

Charging by Induction  The charge is created without direct contact of the two objects.  The charged object will acquire the opposite charge to the original one ST EST AST

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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. ST EST AST

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. ST EST AST

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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”. ST EST AST

Current Intensity  Current 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 ST EST AST 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 X electrons flow through a resistor in 0.50 seconds Determine the current flowing through the resistor. # e - = X Δt = 0.5 s I = ? 1.Find the quantity of charge (q): 2.Find the current (I): ST EST AST

Current intensity is measured using an ammeter. ST EST AST

Potential Difference  The 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 ST EST AST 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 = ? ST EST AST

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. ST EST AST

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

Resistance  Any 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. ST EST AST

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 ST EST AST

Resistance is measured in ohms (Ω) An ohm is equal to a potential difference of one volt per ampere: ST EST AST

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. ST EST AST Where:U is the potential difference, in volts (V) I is the current intensity, in amperes (A) R is the resistance, in ohms (Ω) And …

Sample Problem A 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 = ? ST EST AST

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. ST EST AST Where:P e 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? P e =500 W Δt = 2 min = 120 s W = ? ST EST AST

Another equation for power … Sample problem A 100 W light bulb is plugged into a 120 V outlet. Determine the current passing through the bulb. P e = 100 W V = 120 V I = ? ST EST AST where:Pe is electrical power, in watts (W) V is potential difference, in volts (V) I is current intensity, in coulombs (C)

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 = J  Presently, the residents of Quebec pay about 8 cents per kilowatt-hour. ST EST AST where:E is the electrical energy in joules (J) or kilowatt-hours )kWh) P e 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. P e = 5 W Δt= 0.5 minutes = 30 s E= ? ST EST AST

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. P e = 150 W = kW Δt= 1800 minutes = 30 hours Cost = 8¢/kWh E= ? ST EST AST 1. Find the Energy used: 2. Find the cost:

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) ST EST AST

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

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. ST EST AST

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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. ST EST AST

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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. ST EST AST

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. ST EST AST

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Magnetic Forces of Attraction and Repulsion  All 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. ST EST AST

The following is true of magnets:  Opposite magnetic poles attract each other  Like magnetic poles repel each other ST EST AST

Magnets will always have a north and a south pole. If the magnet breaks, each piece will have a north and a south. ST EST AST

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. ST EST AST

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

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 south 2. Never cross 3. Are closer together near the poles, where the magnetic field is strongest ST EST AST

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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. ST EST AST

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. ST EST AST

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. ST EST AST

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. ST EST AST

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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 the magnetic field ST EST AST