Electricity & Magnetism

Electrostatics Ch 32

Basic Concept In Mechanics, the basic property of matter is Mass. In Electricity, the basic property is Charge.

Static Electricty Electric charges that are at rest

Basic Units Compare Smallest Possible Unit Practical Unit US Currency Penny $0.01 = 1/100th of a dollar Dollar $1 = 100 Pennies Electric Charge Elementary Charge (1e) electron or proton 1e = 1.6 x 10-19C Coulomb 1C = 6.25 x 1018e

Subatomic Particles Particle Proton Neutron Electron Location Relative Charge Actual Charge (C) Relative mass (u) Actual mass (kg) Easily detected ? Easily removed?

Subatomic Particles Particle Proton Neutron Electron Location Nucleus Outside Nucleus Relative Charge Actual Charge (C) Relative mass (u) Actual mass (kg) Easily detected ? Easily removed?

Subatomic Particles Particle Proton Neutron Electron Location Nucleus Outside Nucleus Relative Charge +1 -1 Actual Charge (C) Relative mass (u) Actual mass (kg) Easily detected ? Easily removed?

Subatomic Particles Particle Proton Neutron Electron Location Nucleus Outside Nucleus Relative Charge +1 -1 Actual Charge (C) +1.6 x 10-19 -1.6 x 10-19 Relative mass (u) Actual mass (kg) Easily detected ? Easily removed?

Subatomic Particles Particle Proton Neutron Electron Location Nucleus Outside Nucleus Relative Charge +1 -1 Actual Charge (C) +1.6 x 10-19 -1.6 x 10-19 Relative mass (u) 1 Actual mass (kg) Easily detected ? Easily removed?

Subatomic Particles Particle Proton Neutron Electron Location Nucleus Outside Nucleus Relative Charge +1 -1 Actual Charge (C) +1.6 x 10-19 -1.6 x 10-19 Relative mass (u) 1 Actual mass (kg) 1.67 x 10-27 9.11 x 10-31 Easily detected ? Easily removed?

Subatomic Particles Particle Proton Neutron Electron Location Nucleus Outside Nucleus Relative Charge +1 -1 Actual Charge (C) +1.6 x 10-19 -1.6 x 10-19 Relative mass (u) 1 Actual mass (kg) 1.67 x 10-27 9.11 x 10-31 Easily detected ? yes no Yes Easily removed?

Subatomic Particles Particle Proton Neutron Electron Location Nucleus Outside Nucleus Relative Charge +1 -1 Actual Charge (C) +1.6 x 10-19 -1.6 x 10-19 Relative mass (u) 1 Actual mass (kg) 1.67 x 10-27 9.11 x 10-31 Easily detected ? yes no Yes Easily removed? NO!!!!

Charged objects Positively charged objects have more protons than electrons Negatively charged objects have more electrons than protons Neutral objects have equal amounts of protons and electrons

Law of Charges Opposite charges attract Like charges repel Neutral objects are attracted to positive or negative objects because of polarization Polarization Separation of charges without charging object

Examples How many electrons are on a conductor if it has a charge of -4 x 10-17 C? 250 electrons What charge will 120 protons have? +1.92 x 10-17 C

Example Why is it not possible for any object to have a charge of 8 x 10-20 C? Electrons (or protons) cannot be broken down into smaller pieces.

Review Gravitational Force What factors affect gravitational force? Size of each mass Distance of separation

Electrostatic Force What factors will affect the amount of attraction or repulsion? Size of each charge Distance of separation

Coulomb’s Law k = Electrostatic constant, 8.99 x 109 Nm2/C2 q = charge (C) r = distance separating the center of each charge (m)

Remember Opposite charges attract Like charges repel -F means attractive force +F means repulsive force

Movement of Charge Conductors Insulators Electrons are free to move Most metals Insulators Electrons are not free to move

Movement of Charge Grounding Examples Excess charges on an object will try to move away from like charges or towards opposite charges Examples Lightning, Static electricity

Charging by Friction Objects become charged when they are rubbed together One becomes (+) while the other becomes (–) Gaining electrons becomes negative Losing electrons becomes positive Both will have equal charge, opposite sign

Electroscope Object used to detect and measure electric charge on an object + - - + + -

Electroscope – Charge separation + + + + + + + + + + + - - - - - - - - - - - - - - - - + + - - - - + +

Charging by Conduction When 2 conductors of unequal charges touch, the electrons will move to balance out the charge, leaving both conductors with the same (equal) charge -6 -10 +4 -3

Charging by conduction using - rod - - - - - - - - - - - e- rod to scope - Scope charged negative + - e- scope to ground + -

Charging by conduction using + rod e- scope to rod + + + + + + + + + + Scope charged positive + - e- ground to scope + -

Charging by Induction If a charged object is brought near a conducting surface, even without physical contact, electrons will move in the conducting surface. Read 32.6

Charging by Induction

Charging by induction using - rod + - e- scope to ground - - - - - - - - - - - - - - - - - - + - - - - - - - - - - + Scope charged positive + + -

Charging by induction using + rod - + + + + + + + - + e- ground to scope + + + + + + + + - + + + + + + + Scope charged negative - + -

Electric Fields Ch 33

Electric Fields Electric Field is a region around a charged object through which a force is exerted on any other charged particle. Direction of the electric field is the direction a positive charge would move if placed in the field

Electric Field Lines Lines are not real Positive  Negative Can not cross Closer lines mean stronger field

Electric Field Lines

Electric Fields Parallel Plates Electric Field is uniform between plates + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - -

Electric Field Strength, E Force per charge Amount of force felt by a charge Vector E (N/C) Fe (N) q (C)

Electric Field Strength

Work Which way will each particle be pushed by the electric field? Which way does work have to be exerted in order to move each particle against the electric field? + -

Electric Potential Difference Amount of work done per unit charge as a charged particle is moved between points A.K.A. Electric Potential Potential Difference Voltage

Electric Potential Difference V (V) W (J) q (C) 1 V = 1 J/C

Electric Potential Difference Rearranged (1C)*(1V) = 1J

Electron Volts (eV) Amount of work(energy) done by 1 volt on 1 electron Unit of Energy (1C)(1V)=1J (1e)(1V)=1eV 1eV = 1.6 x 10-19J

Why something moves Gravitational Electrical

Electric Current Chapter 34

Electrodynamics The study of charges and their motion Often use an analogy of water moving to illustrate

Flow A measure of water flow is called ____? Current How much charge flows per unit time

Current q (C) t (s) 1 Ampere (A) = 1 C/s

Current Which charge flows? Negatives (Reality) Conventional Current Positive charges (rest of the physics world) Electrons actually move

Motion What causes water to move? Change in height (Potential Difference) Water, flows from high potential to low potential

Voltage Electrical Potential difference causes charges to move. Batteries provide a Potential Difference Batteries act like a pump to raise charges from a low to high potential

Resistance Opposition to flow Any device that opposes the flow of current can be called a resistor Unit is Ohm, Ω

Ohm’s Law Potential Difference encourages charges to flow Resistance discourages charges to flow Ratio of Potential Difference to Resistance equals Current

Ohm’s Law V (V) I (A) R (Ω) 1 V = 1 A* Ω

Ohm’s Law

Circuit Continuous loop that allows for the flow of charges Must follow Ohm’s Law Overall and individual components

Electric Power Rate at which electrical energy is converted into another form such as mechanical, heat, and light. Measured in Watts(W)

Power

Electrical Power P (W) I (A) V (V) 1 W = 1 A*V

Electrical Power

Electrical Energy (Work) W (J) P (W) t (s)

Electrical Energy (Work)

Kilowatt-Hour (kW-hr) Amount of energy consumed in 1 hour at a rate of 1 kilowatt

Resistance What factors affect the resistance of a material? Size Type of material Temperature

Resistance The size of a resistor affects the resistance Length Cross-sectional Area

Length Would water flow faster through a short pipe or a long pipe?

Length More Length increases Resistance L R

Cross-Sectional Area Would more water flow through a wide pipe or narrow pipe?

Cross-Sectional Area Larger Area decreases Resistance A R

Type of Material How does the type of material affect resistance? Electrons need to flow through the resistor. The more material that gets in the way, the slower the electrons

Resistivity, ρ Measure of how resistive a material is. As Resistivity increases Resistance Increases ρ R

All together now R (Ω) L (m) A (m2) ρ (Ω *m) Selected materials in reference tables

Temperature How would temperature affect resistivity? What does increasing the temperature do to the molecules in a resistor? Increasing Temp increases molecular movement

Temperature Imagine walking down the hall with sophomores standing every where. Imagine walking down the hall with sophomores running every where. Which is easier?

Temperature Increasing Temperature increases Resistivity Increasing Temperature increases Resistance

Resistors

Resistor Color Code 4 bands First band is first number Second band is second number Third band is multiplier (x10, …) Last band indicates tolerance (±%)

Circuits Chapter 35

Simple Circuit Continuous loop for electrons to move (current)

Simple Circuit Simple Circuit includes: Power source (battery) Connecting wires (no resistance) Electrical device (resistors, lights, etc) Switch (optional)

Circuit Symbols Power source Connecting Wires Resistor Switch

Basic Circuit

Ohm’s Law Reminder: Circuits as a whole and individual resistors must obey Ohm’s Law V = IR

Measuring Current Ammeter Measures current as it flows through meter Must be connected IN the flow Very low resistance in meter

Circuit with Ammeter A

Measuring Potential Difference Voltmeter Measures the difference between two points Must be connected to TWO points Outside loop Very high resistance in voltmeter No current flows through voltmeter

Circuit with Voltmeter

Series Circuits Multiple electrical devices, resistors, in one continuous loop

Series Circuits Current must flow equally through all resistors. IT = I1 = I2 = …

Series Circuits For a complete loop, potential gains must equal potential drops.(Kirchhoff’s Loop Rule) VT = V1 + V2 + V3 + …

Equivalent Resistance Equivalent resistance is the idea that all the resistors in a circuit can be removed and a single resistor can be placed in the circuit and have the same resistance as if the original resistors were still in place.

Series Circuits VT = V1 + V2 + V3 + V4 IRT = IR1 + IR2 + IR3 + IR4 IRT = I(R1 + R2 + R3 + R4) RT = R1 + R2 + R3 + R4

Series Circuits For series circuits, the equivalent resistance is equal to the sum of each resistor RS = R1 + R2 + R3 + R4 + …

Parallel Circuits Multiple electrical devices, or resistors, in multiple loops.

Parallel Circuits Potential Difference, voltage, across each resistor is the same (Kirchhoff’s Loop Rule) VT = V1 = V2 = …

Parallel Circuits Total Current entering junction must equal total current leaving junction (Kirchhoff’s Junction Rule) IT = I1 + I2 + I3 + … IT I1 I2

Parallel Circuits Equivalent Resistance for Parallel Circuits follows the following equation:

Combination Circuits Circuits that have both series and parallel parts.

Combination Circuits Combination circuits must follow Ohm’s Law V = IR

Combination Circuits Use idea of equivalent resistance to simplify circuit

Combination Circuits 120V 30Ω 15Ω 15Ω Req = ? 30Ω

Combination Circuits 120V 30Ω 15Ω 15Ω 30Ω

Combination Circuits 30Ω 120V

Combination Circuits 10Ω 120V Req =10Ω

Combination Circuits 4Ω 30Ω 120V 10Ω 15Ω 20Ω 40Ω 20Ω Req = ?

Combination Circuits 4Ω 30Ω 120V 10Ω 15Ω 60Ω 20Ω

Combination Circuits 15Ω 120V 4Ω 15Ω 10Ω 30Ω

Combination Circuits 120V 4Ω 15Ω 5Ω

Combination Circuits 24Ω 120V Req = 24Ω

Magnetism Chapter 36

What is a Magnet? Material or object that produces a magnetic field. Two types: Permanent Electromagnet

What causes Magnetism? In order to create a magnetic field, a charged particle must be moving. Moving and spinning electrons cause magnetic fields in every object.

Domains A small region of space where the magnetic fields produced by moving electrons are aligned together. Often, the directions of the domains cancel each other out. Ferromagnetic material Cancellations do not occur, resulting in a net magnetic field

Domains

Magnetism Opposites poles attract Like poles Repel Magnetic Poles can not be separated Every object that has a north pole has a south pole

Magnetism

Magnetic Field Region around a moving charged particle through which a force is exerted on another moving charged particle Similar to Electric Fields

Magnetic Field Lines Lines are not real North  South (outside magnet) Can not cross Closer lines mean stronger field

Magnetic Field Lines

Magnetic Field Lines

Magnetic Field Magnetic Field is a region around a moving charged object through which a force is exerted on another moving charged particle Motion of particle must be perpendicular to the magnetic field

Magnetic Fields Magnetic Fields are often illustrated using arrows

Magnetic Fields What about into the page or out of the page? Into Page Out of Page

Magnetic Hand Rules To determine the direction of the force, we use hand rules. Different hands for different charges Right hand for Positive charges Left hand for Negative charges

Conventional Current Conventional Current follows the old “convention” that positive charges are the charges that are moving in current Use Right Hand Rule

Electron Current Electron Current is the reality that negative charges are the charges that are moving in current Use Left Hand Rule

Magnetic Hand Rules Point index finger in direction of motion Point palm or other fingers in direction of magnetic field Point thumb in direction of Magnetic Force

Example An electron is moving through a magnetic field as shown below. In what direction will the magnetic force be? Out of page e

Another Example An electron is moving through a magnetic field as shown below. In what direction will the magnetic force be? Down e

Force on Wire Still use hand rule to determine the direction of the magnetic force Index finger is the direction of the current

Magnetic Fields produced by Currents A current carrying wire also produces a magnetic field Direction follows second Hand Rule

Magnetic Fields produced by Currents Second Hand Rule Thumb in direction of current Curl fingers around wire Curled fingers show direction of magnetic field

Example What is the direction of the magnetic force exerted on wire 2 by the magnetic field produced by wire 1? I- 1 2 I- Down

Example What is the direction of the magnetic force exerted on wire 1 by the magnetic field produced by wire 2? I- 1 2 I- Down

Magnetic Fields produced by current carrying loops Imagine current flowing through the loop below In what direction will the magnetic field be produced inside the loop? Into Page I-

Electromagnets Temporary magnet caused by an induced magnetic field from current carrying wires.

Electromagnets Current carrying wire produces a magnetic field Coiling the wire bunches up the magnetic field inside the coil

Electromagnets Increasing the strength of the electromagnet: Increase Current in wire Increase number of coils Add an iron core

Electromagnetic Induction If charged particle moving through a magnetic field feels a force, shouldn’t a moving magnetic field exert a force on a charged particle?

Electromagnetic Induction A voltage can be “induced” in a wire by moving a magnet near the wire. often a coil of wire is used Faraday’s Law Induced voltage is directly proportional to the number of coils, cross-sectional area of the coils, and rate of change of magnetic field

Electromagnetic Induction

Electromagnetic Induction

Electromagnetic Induction Inducing a current in a coil of wire creates its own magnetic field

Electromagnetic Induction Changing direction of magnetic field changes direction of induced voltage Creating an alternating current (AC)

Alternating Current Direct Current Current alternates direction at a regular rate Electrical Outlets Direct Current Current flows in one direction only Batteries Sim

Generators & Motors Device to convert between Electrical and Mechanical Energy Generator Converts Mechanical Energy to Electrical Energy Motor Converts Electrical energy to Mechanical Energy

Generator

Motor

Electromagnetic Induction Current flowing through a coil wires produces a magnetic field A changing magnetic field induces a current in an adjacent coil

Electromagnetic Induction Using AC produces a consistent changing magnetic field Adding an iron core strengthens the magnetic field

Transformer Device used to increase or decrease voltage using electromagnetic induction

Transformer Complete loop is more efficient

Transformers V = voltage N = number of coils

Transformers Step Up Transformer Step Down Transformer Secondary has more coils than primary Resulting Voltage is larger Step Down Transformer Secondary has less coils than primary Resulting voltage is lower

Conservation of Energy Energy transferred must be equal Power is equal

Electric Field An electric field is produced by a charged particle A changing electric field can be produced by a moving charged particle A changing electric field produces a magnetic field

Electromagnetic Induction A changing magnetic field also induces an electric field When magnetic fields and electric fields are produced they are at right angles to each other

Electromagnetic Wave Oscillating electric and magnetic fields that regenerate each other (light) No medium is required

Electromagnetic Radiation Only one speed can preserve this regeneration Speed of Light is 300,000,000 m/s 3 x 108 m/s Discovered by James Clerk Maxwell

Maxwell’s Equations