Electricity

1
Chapter 7

• Electricity

2
Lesson 7.1 Electric Charge

• Topics
• Static electricity
• Opposites attract
• Electric fields
• Conductors and Insulators
• The Electroscope

3
What happens When

• You rub your stocking covered feet across the
  carpet and touch a door knob?

• What happens when you rub a balloon on your hair
  and touch it to the wall?

4
Static Electricity
Static electricity is the net accumulation of
electric charges on an object.
5
The Atom
Neutron
Proton
Electron
6
Opposites Attract, Like Repel

• Positive and Negative Attract
• Positive and Positive Repel
• Negative and Negative Repel

7
Law of conservation of charge

• Charge can be transferred from object to object,
  but it cannot be created or destroyed.

8
Back to examples

• Feet on carpet
• Balloon on wall

9
Conductors

• A conductor is a material that allows electrons
  to easily move through it.
• Examples Earth, skin, metals, electrolytic
  solutions.
• An object connected to the Earth, or ground, is
  said to be grounded.

10
Insulators

• An insulator is a material that doesnt allow
  electrons to flow through it easily.
• Examples Plastic, wood, rubber, cork, and
  glass.

11
Screw Drivers
Why do screw drivers have plastic or wood handles?
Plastic or wood handles help to insulate the
person against electrical shock.
12
Transferring electric charge

• Charging by contact
• Rubbing two materials allows electron transfer
• Charging by induction
• Rearranges electrons in a neutral object without
  contact.

13
Electroscope

• Detects presence of electrical charge
• Does not identify if it is positive or negative
• By touching a charged object to the knob, the
  metal leaves will repel.

14
The Electroscope
15
Lightning

• Large discharge of static electricity.\
• Thunderclouds moving past each other create
  static electricity.
• The build up of electrons is discharged by the
  earth.

16
Lesson 7.2

• Electric Current

17
Potential Difference

• Like temperature flowing from an object with a
  higher temperature to an object with lower
  temperature until the temperatures of the two
  objects are the same, electricity will flow from
  a place of higher potential energy to a place of
  lower potential energy. This is is called
  potential difference.

18
Potential Difference

• Flow of electrons is from a high concentration to
  a lower concentration.
• Measured in volts (v).
• Potential difference is also called voltage.
• Measured by a voltmeter (which should be wired in
  parallel).
• Symbolized by V in Ohms Law.

19
Circuits

• How do you get a light to turn on and to stay on?
• In order to keep a lamp (or some other resistor)
  on there must be a continual flow of electrons,
  therefore you must connect it in a complete
  electric circuit.

20
Electric circuits

• A closed path through which electrons can flow.
• There will be a potential difference across the
  lamp because it is part of the circuit and is
  using some of the voltage. When you turn the
  lamp on electrons flow through it.

21
Current

• The flow of electrons through a wire or some
  other conductor.
• Measured in Amperes (A).
• Current is measured by an ammeter (must be wired
  in series).
• Symbolized by I in Ohms Law.

22
Dry Cells

• Batteries.
• An electron pump due to a potential difference
  across the positive and negative ends.
• The electron pump flows from negative to
  positive.

23
Wet cells

• Car batteries.
• Two connected plates made of different metals or
  metallic compounds in an electrolytic solution.
• The chemical reaction causes the potential
  difference.

24
Resistance

• The tendency for a material to resist the flow of
  electrons turning electrical energy into thermal
  and light.
• All appliances connected into your circuit are
  considered resistors.
• Resistance is measured in Ohms.
• Symbolized by R in Ohms Law.

25
Resistance

• What affects the amount of resistance?
• Three things
• Length of the wire. (shorter, less R)
• Diameter of the wire. (fatter, less R)
• Material of the wire. (better conductor, less R)

26
Ohms Law

• Potential Differencecurrent x resistance.
• V (volts)I (amperes) x R (ohms)
• Relates to a flowing river
• The faster the flow of water the larger the
  current, but the more rocks the more resistance.
  So as you increase voltage, you increase current.
  As you increase resistance, you decrease
  current).

27
Example of Ohms Law
A light bulb with a resistance of 160 Ohms is
plugged into a 120-V outlet. What is the current
flowing through the bulb? V IR 120-V I (160
Ohms) 120-V/160 Ohms I I .75 Amperes
V
I
R
28
Lesson 7.3

• Electrical Circuits

29
The Series Circuit

• The current that has only one path it can travel
  along.
• If one light goes out they all go out.

Show overhead of series circuit
30
The Parallel Circuit

• There are separate branches for current to flow
  through.
• If one light burns out the others stay on.
• Houses are wired to a Parallel circuit so that
  when you turn out your light, for instance,
  everything else still works.

Show overhead of Parallel circuit
31
Fuses

• When there is too much current running through a
  wire, it gets hot. This causes a metal filament
  inside the fuse to melt. This in turn opens the
  circuit shutting off every appliance attached to
  it.
• Not in use much today because of the cost of
  replacement.

32
Circuit Breakers

• In a circuit Breaker, instead of the metal
  melting when it gets too hot, the metal bends
  (bimetallic strip of 2 metals with different heat
  resistances). This also makes a break in the
  circuit which stops the flow of current.

33
Power
The rate at which electrical energy is converted
to some other form of energy.
34
Calculating Power

• Electrical power is expressed in watts (W) or
  kilowatts (kW).
• Most electrical appliances advertise by
  expressing how much power they use.

35
Calculating Power

• power current x voltage
• watts amperes x volts
• P I x V

P
V
I
36
Electrical Energy

• Why is it important not to waste electricity?
• Because most electrical energy is produced from
  unrenewable natural resources.

37
Electrical Energy

• You can calculate the amount of electrical energy
  an appliance uses by using the following formula
• energy power x time
• Ws W x s
• E P x t
• DTE measures in kWh.

E
t
P