Transcript Electricity & Magnetism - West Johnston High School

 When you rub a balloon on cloth, it becomes charged & will
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“stick” to the wall.
If you rub your hair with a balloon, your hair will stand up and
point in all directions.
You are wearing socks and you walk across a carpet dragging
your feet. When you touch the metal doorknob, you will feel a
shock.
During the winter, you turn on the heat in your car. When you
step out of the car and close the door, you get a shock.
During a thunderstorm, lightning can be seen going from the
cloud to the ground.
If you were to touch (DO NOT DO THIS) an electrical appliance
with a wet hand, you would receive a large shock.
What is your hypothesis as to why these things occur?
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Static Electricity: build-up of stationary
charges

“Static” means: not moving or stationary
Subatomic Particles
What does an atom look like?
Charge
Proton
Neutron
Electron
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Opposite charges __________. Example: _____________________
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Like charges _____________. Example: ______________________
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A neutral object has the same number of _______ and ________.
1.
2.
3.
Friction
Conduction
Induction
1. Friction – charges transferred by rubbing
a. Example –rubbing a balloon, dragging your
feet across the carpet, sliding along car seat
2. Conduction – charges transferred between objects in
contact.
a. Example – touching metal and feeling a shock,
lightning rod
b. Conductors – materials that allow charges to move
through (low resistance)
▪ Examples – metals, water
c. Insulators – materials that do not transfer charges
easily
▪ Examples – rubber, plastic, glass, cardboard
d. How are conductors different from insulators in
terms of their electrons?
- Conductors allow their electrons (charges) to
move freely.
e. Why are electrical wires surrounded by a rubber
coating?
- To prevent charges from escaping.
3. Induction – charging by bringing a charged object
close to a neutral object.
 Example – balloon with hole punches (see below)
 Is direct contact necessary? No.
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What do you think is happening?
_________________________________________________________________________________
______________________________________________________________________________
Charge Balloon
Before
After
Long Time After
Type of Charging:
_______________
_________________________
____________
1.
Which objects were picked up by the balloon?
2.
Explain how do you think the objects were
picked up? What happened with the
charges?
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Electric Discharge is the loss of static electricity as
electric charges move off an object. Often extra
electrons escape into water molecules in the air. On dry
days, do you think you would be more or less likely to
see static electricity?
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Demo:
Van der Graaf Generator
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Lightning is a form of electric discharge. How does lightning
occur?
The term, “grounding,” comes from the fact that the Earth
(the ground) is an extremely good conductor of electric
charge.
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Lightning rods work according to the principle of grounding.
How do lightning rods protect houses from lightning?
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Label with the type of charging:
Friction (F), Conduction (C), Induction (I)
 When you rub a balloon on cloth, it becomes charged & will





“stick” to the wall.
If you rub your hair with a balloon, your hair will stand up and
point in all directions.
You are wearing socks and you walk across a carpet dragging
your feet. When you touch the metal doorknob, you will feel a
shock.
During the winter, you turn on the heat in your car. When you
step out of the car and close the door, you get a shock.
During a thunderstorm, lightning can be seen going from the
cloud to the ground.
If you were to touch (DO NOT DO THIS) an electrical appliance
with a wet hand, you would receive a large shock.
Only Electrons Can Move!
Electroscope: Used to detect a charge
Neutral
Charged by Conduction
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Touch with negative rod
Touch with positive rod
Only Electrons Can Move!

Charged by Induction
Bring positive rod close.
Bring negative rod close.

What happened when you brought a charged
object close to the electroscope?
_____________________________________
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Current Electricity: moving charges
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Electrical Circuits: Complete (closed) paths
through which charges can flow.
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Parts of a Circuit
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Wire
Load – Lightbulb (resistor)/electrical appliance
Switch – Open/Closed
Source of Energy - Battery (Voltage Source)
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Current (I) – the rate at which charges move
through a conductor
 Unit – ampere (Amp, A)
 Ammeters & Galvanometers
are used to measure current.
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Voltage (V) – or Electric Potential Difference – a
difference in the quantity of negative charge between
two locations.
 Unit – volts (V)
 A voltmeter is used to measure potential difference
(voltage).
 Electrons move from areas having more
electrons to areas having few electrons .
 How is a potential difference created in a
battery?
A chemical reaction causes a build-up of
electrons.
 The greater the potential difference, the
greater the current produced in the wire.
COPPER WIRE
electron
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Resistance (R) – internal friction which slows
the movement of charges through the
conducting material.
 Unit – ohms (Ω)

How do lightbulbs work?
 A thin filament has high
resistance.
Some electrical energy
is changed into
heat/light because of
internal friction.
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Resistance depends somewhat on Temperature:
▪ At high temps, the resistance of a metal increases. Why?
▪ Copper atoms are moving faster. It is harder for e- to get through
▪ At very low temps, the resistance of certain materials
becomes essentially zero  superconductors
V = IR
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Voltage (V) = volts (V)
Current (I) = amps (amps, A)
Resistance (R) = ohms (Ω)
1.
A current of 0.5 A flows in a lightbulb when the
potential difference between the ends of the
filament is 120 V. What is the resistance of the
filament?
2.
Calculate the current in a circuit with a
resistance of 25 Ω if the potential difference in
the circuit is 12.5 volts.
“Series is simple”
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Series Circuit – Current only has one path to travel.
 Used in flashlights & in some holiday lights.
 Draw a series circuit:
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What would happen if your home were wired
in a series circuit and you turned off one light?
 All electrical appliances would turn off.
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Current in a Series Circuit:
 Is the same throughout.
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Voltage drop in a Series Circuit:
 Electrical appliances all have to share the voltage
from the battery.
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Parallel Circuit – Current has multiple paths to
flow through.
 Used in houses.
 Draw a parallel circuit:
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What would happen if your home were wired
in a parallel circuit & you turned off one light?
 The other electrical appliances would be unaffected.
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Current in a Parallel Circuit:
 Current splits to go through separate paths &
recombines before returning to the battery.

Voltage in a Parallel Circuit:
 Each loop gets the entire battery voltage
(appliances do not have to share).
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Do electrons have to move all the way from the
power plant to your house to turn on your light
when you flip the switch?
 No. They are “waiting in line.”
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How is current affected by an increase in
voltage?
 Current increases.
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How is current affected by an increase in
resistance?
 Slows the current.
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Electric Power: The rate at which electrical
energy is converted into another form of energy.
 Unit: watts (W)
P = VI
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Power (P) – watts (W)
Voltage (V) – volts (V)
Current (I) – amperes (amps, A)
1.
A toaster oven is plugged into an outlet that
provides a voltage difference of 120 V. What
power does the oven use if the current is 10 A?
2.
A flashlight bulb uses 2.4 W of power when the
current in the bulb is 0.8 A. What is the voltage
difference?
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Why are fuses and circuit breakers used in
circuits?
 To prevent circuit from overheating and causing a fire.
Demo – Bimetallic Strip
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What is the difference between a circuit
breaker and a fuse?
 Fuse must be replaced. To reset a circuit breaker, you
just have to flip the switch.
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What is a magnet made of?
 Soft Magnets: iron (easy to make, loses magnetism easily)
 Hard Magnets: cobalt, nickel
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Magnetic poles – North and South poles
 => Opposite poles attract. Like poles repel.
 Can you isolate a north pole from a south pole? – NO!
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Magnetic fields
 Region where magnetic force can be detected.
Field is
strongest
at poles.
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Magnetic domains
 Inside the magnet, each atom acts like a mini-
magnet (domain).
 If the domains are aligned so that all the north
poles face in one direction, the material is
magnetized.
What would happen if you cut this
magnet in half?
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How can you destroy a magnet?
 1) drop it
 2) hammer it
 3) heat it
Mess up the
alignment of the
domains!
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Earth’s Magnetic Field
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Electromagnets – the relationship between
electricity and magnetism
 Moving electric charges (current) through wire creates a
magnetic field around the wire.
 To increase the strength of an electromagnet:
1. Increase the coils in the wire.
2. Increase the current in the wire.
Solenoid = Coil of Wire
1. If you increase the number of coils of an electromagnet, how does
that affect the strength of the electromagnet?
_______________________________________________________
2. How would increasing the current affect the strength of the
electromagnet? _________________
3. How would changing the direction of the current affect the
electromagnet?
_______________________________________________________
4. How would changing the core material (the iron nail) affect the
electromagnet?
_______________________________________________________
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Generator:
 changes
mechanical energy
into electrical energy
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Electric Motor:
 changes
electrical energy into
mechanical energy
1.
You rub a rubber rod with wool. The rod
becomes negatively charged. Explain what
happened in terms of electrons & protons.
2.
You rub a glass rod with silk. The rod
becomes positively charged. Explain what
happened in terms of electrons & protons.