Electricity & MagnetismChapter 8
Student Learning Objectives
• Recall properties of charge
• Characterize static electricity
• Differentiate between series and parallel circuits
• Describe properties of magnetic materials
• Analyze electromagnetic systems
All matter is composed of atoms which contain charged particles.
What is charge?
http://www.pond5.com/stock-footage/68580/atom5.html
electron proton neutronCharge − 1 + 1 0Charge –1.6 x 10−19 C +1.6 x 10−19 C 0Mass 9.1 x 10-31 kg 1.67 x 10–27 kg 1.67 x 10-27 kg
1) Why do the electrons stay in orbit around the nucleus?
2) Does an object that has lost electrons have a positive or negative charge? Does it have more or less mass?
Practice
Law of conservation of charge: The total amount of electric charge in the universe remains constant. Charge is never created or destroyed.
Neutral atoms have the same number of electrons and protons.
2He
4.00260
Atomic Number
Coulomb’s LawObjects become charged when they lose or gain
electrons. (ionized)
F = k(q1)(q2) d2
Opposite charges AttractLike charges Repel
The electric force and the gravitational force are both inverse square laws, and both apply mutual forces between two quantities.
Electric Force Gravitational Force
Attract & Repel AttractStrong Weak
Practice1) If an object has lost 100 electrons, what is the
object’s charge in Coulombs?
2) What is the electric force between the electron and the proton in a hydrogen atom? The average separation between the electron and proton is 5.29 x 10−11 m.
More Practice
3) Rubbing a balloon on your hair results in the transfer of 2000 electrons from your hair to the balloon. What is the force between your hair and the balloon if the balloon is 2 cm from your hair?
How do objects become charged?
Electrons are lost, gained, or redistributed within a charged object.
Friction: electrons are “knocked loose” and transferred
Contact: electrons are transferred
Induction: charge is rearranged
Image Credit: sci-culture.com
Static Electricity
A net electric charge on an object results in static electricity. (Positive or Negative)
Cars & Clothes
Practice1) Is an object charged by induction ionized? 2) Static electricity on clothes and us is less common on
humid days. Why? 3) Why does a plastic slide retain static charge while a
metal slide does not? 4) How do you think fabric softener works?
What is an electric field?
An electric field surrounds charge, modifying the space near the charge.
Lines of Force
Electric Fields Interact
Objects are affected by the field established in that space.
Practice
1) What would the electric field lines look like between two equal positive charges?
2) How would the electric field lines between two equal negative charges compare to the equal positive charges?
What are the properties of electric circuits?
Electric current is the net movement of electrons.
Electrons are present in all electrical systems.
When electrons begin to move, there is current.
Two different electric potentials set-up an electric field, which causes existing electrons to flow from the high potential to the low potential.
EPE → KE as charge is accelerated through the system.
Wall Outlets (ac)
Batteries (dc)
Lightening
No difference in electric potential = no charge will flow.
"Shock War“
Bird on a Wire
Potential Difference E Field Current
Electrical SystemsVoltage is created by a difference in electric
potential.
Electric resistance is a measure how difficult it is for electrons to move within the system.
1. Conductivity 2. Length 3. Diameter 4. Temperature
Voltage & Power
Ohms LawPower is the rate at
which electric energy is used by a system.
V = IR P = IV
Practice1) A flashlight uses a 1.5 volt battery and has a
resistance of 2 ohms. What is the current within the flashlight circuit? How many Watts will this flashlight supply?
2) A 2000 Watt hair dryer, a 100 Watt radio, and five 60 Watt light bulbs are being used simultaneously. Will this cause a 12 A safety switch to open? (V =110 volts)
More Practice3) An Xbox 360 console is attached to a 55 inch plasma
TV. Each of these devices uses about 135 Watts. If you play the Xbox 360 on this TV for 4 hours, what is the cost? The current average utility rate is 22¢/kWh.
Why you should not stick any metal object into the plug!
4) A person with dry skin has a maximum resistance of 500,000 ; moist skin has a resistance of 1000 . If the metal prongs of a plug are touched as the plug is placed into a 110 volt wall outlet, how much current could pass through the person’s body? Is this dangerous? Don't try it!
0.001 A Noticeable0.005 A Painful0.010 A Muscle spasms0.015 A Loss of muscle control0.070 Disrupts heartbeat
How are circuits wired?
Parallel Circuits: multiple parallel loops
Voltage is supplied to each parallel loop. (energy not shared)
Resistance will drop.(decreases)
Series Circuits: single continuous loop
Voltage is divided. (energy shared)
Resistance is additive.(increases)
R = R1 + R2 + … 1 = 1 + 1 + …R R1 R2
Practice1) What do you think
an electrical switch does to the circuit?
2) Why is too much current in a wire dangerous?
What are the properties of the magnetic field?
All magnets have two poles; this is where the magnetic field is strongest.
North Seeking & South Seeking
The poles are not charged!
A magnetic field surrounds a magnet, modifying the space around the magnet.
Opposite magnetic poles AttractLike magnetic poles Repel
Practice
1) How do electric charges and magnetic poles compare and contrast?
2) How do electric fields and magnetic fields compare and contrast?
What produces magnetism?
The motion of electric charge is what makes a material magnetic.
Magnetic MomentsElectric Current
Motion of Ionized Fluid
Magnetic MomentsThe intrinsic magnetic moments of electrons align
creating magnetic regions called domains.
Non-MagneticMaterial
no domains
MagneticMaterial
domains not aligned
Magnetdomains aligned
WiresElectric current forces magnetic moments to align.
I
B-Field
ConvectionEarth is surrounded by a magnetic field.
Ionized material in motion produces a magnetic field.
• Protects us from solar wind• Produces aurora
• And the poles switch!
NASA
http://www.scientificamerican.com/article/earth-s-magnetic-field-flip-could-happen-sooner-than-expected/
How are electric and magnetic effects related?
Electric charge moving induces magnetism.
Magnetic field changing induces current.
E-Fields & B-FieldsInduce Each Other
Faraday’s Law
Faraday’s Law: Induced voltage in a coil is proportional to the number of loops, multiplied by the rate at which the magnetic field changes.
More Loops + Turning Faster = More Voltage
Electric Motor vs Electric Generator
Motors
• Current induces magnetism
• Magnetic fields interact
• Electric E Mechanical E
(Interaction causes Rotation)
Generators
• Magnetic field induces electric current
• Coils turned in B Field
• Mechanical E Electric E
(Rotation causes Interaction)
Practice1) Do electric generators create energy or
convert energy?
2) What are some examples of items you may purchase that utilize electromagntic induction?
Maxwell’s LawMaxwell’s Law: A magnetic field is induced in any
region of space in which an electric field is changing with time.
DCurrent = Magnetism
Electric TransformerThe relative number
of turns determines the change in voltage.
V2 = T2
V1 T1
Practice
1) If a laptop needs only 12 volts and the wall outlet supplies 110 volts, what is the relative number of turns needed in each coil?
2) Is the laptop transformer a step-up or step-down transformer?
