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Chapters 17-18

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Title: Chapters 17-18


1
Electricity and Magnetism
  • Chapters 17-18

2
Electrical Energy
  • Electricity is a form of energy related to the
    flow of electrons (small, negatively charged
    parts of an atom) through a conductor.
  • It is associated with the movement of an electric
    charge.

3
Electrical Charge
  • Electrical charge is an electrical property of
    matter that creates force between objects.
  • The SI unit for charge is a coulomb, C.

4
Electrical Charge
  • Differences in electric charge come from natural
    charges that exist within the atom.
  • The nucleus is made of positively charged protons
    and neutrally charged neutrons.
  • Electricity is associated with the negatively
    charged electrons moving outside the nucleus.

5
Electrical Charge
  • An atom is typically neutrally charged because it
    has the same number of protons as it does
    electrons.
  • gain of electrons negative charge
  • loss of electrons positive charge
  • Objects that have like charges repel each other.
  • Objects with opposite charges attract each other.
  • Charge can created in 3 ways

6
Electrical Charge
  • Charging by Friction
  • Also called static electricity or static charge.
  • Occurs when two objects are rubbed together
    forcing electrons from one object to be
    transferred to another.
  • This causes the positive charge of one object and
    the negative charge of another.

7
Electrical Charge
Hair and balloon are neutrally charged.
The balloon and hair become charged when
electrons transfer due to friction.
Since the hair is now positively charged and the
balloon is now has a net negative charge, the
balloon and hair attract.
8
Electrical Charge
  • Van de Graaf generators create electric charge
    through friction.

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9
Electrical Charge
  • Charging by Induction
  • Caused by the movement of charges within an
    object.
  • The overall charge of an object is zero, but the
    opposite sides will be charged.

Stays neutral even though charges are separated.
10
Electrical Charge
  • Charging by Conduction
  • Method of charging occurs when electrons flow
    through one object into another by direct
    contact.

11
Electrical Charge
  • Conductors are materials that transfer charge
    easily.
  • Examples Materials such as silver, copper,
    aluminum, and magnesium.
  • Insulators are a material that does not transfer
    charge easily.
  • Examples Materials like rubber, glass, silk,
    and plastic slow or stop the movement of
    electrons.

12
Electrical Discharge
  • Electrical discharge relieves a build-up of
    charges by creating a spark if the object comes
    in contact with a conductor.
  • Once an object has had an electrical discharge it
    becomes electrically neutral.

13
Electric Discharge
  • Lightning is an electrical discharge from a
    buildup of static electricity in the clouds.

14
Electric Force
  • Electric force is the attraction or repulsion
    between objects dues to charge.
  • Create strong ionic bonds in atoms.
  • Affected by
  • amount of charge
  • distance between charges.
  • Electrical force causes electrons to move.

15
Electric Force
  • Electric fields are the region around a charged
    object in which other charged objects experience
    and electric force.

16
Ohms Law
  • Electric current (I) measures the rate that
    electric charges move through a conductor.
  • Measured in amperes (A).
  • increased current increased speed of
    electricity flow

17
Ohms Law
  • Voltage or electrical potential is the ability to
    move an electric charge from one point to
    another.
  • The voltage difference between two points in a
    circuit is called the potential difference.
  • Measured in volts (V).

18
Ohms Law
  • Voltage tells us how much work a battery can do.
  • More voltage is like a stronger pump, giving more
    force and more current.
  • To increase voltage you could use a stronger
    battery OR add batteries.

19
Ohms Law
  • Electrical Resistance is a measure of how much an
    object opposes the passage of electrons.
  • Measured in Ohms (?).
  • Caused by internal friction in a circuit.
  • Resistance slows down electrical current.
  • Adding devices in a circuit increases resistance.

20
Ohms Law
  • A resistor is a material with a high resistance
    that is inserted into a circuit to increase its
    resistance.
  • Example Filament in a light bulb.

21
Ohms Law
  • Differences in resistances
  • Conductors have low resistances while insulators
    have high resistances.
  • Below a certain temperature some substances can
    actually exhibit no resistance (superconductors).
  • Semiconductors have an intermediate resistance
    between conductors and insulators.

22
Ohms Law
  • Resistance can be calculated using Ohms Law
  • Resistance voltage/current
  • RV/I
  • (R Ohms O, V Volts V, I Amperes A)

23
Ohms Law
  • Find the resistance of a portable lantern that
    uses a 24 V power supply and draws a current 0f
    0.80 A.

V 24 V R ? O I 0.8 A
R V / I
R 24 V / 0.8 A
R 30 O
24
Ohms Law
  • The current in a video game is 0.50 A. If the
    resistance of the games circuitry is 12 O, what
    is the voltage of the battery?

V ? V R 12 O I 0.5 A
V I R
V 0.5 A 12 O
V 6 V
25
Ohms Law
  • A 1.5 V battery is connected to a small light
    bulb that has a resistance of 3.5 O. What is the
    current in the bulb?

V 1.5 V R 3.5 O I ? A
I V / R
I 1.5 V / 3.5 O
I 0.43 A
26
Circuits
  • A circuit is one or more complete, closed paths
    for electron flow.
  • Electricity flows through circuits made of
    conductors that are connected in a complete loop.
    (closed circuit).
  • Any break in the circuit will cause the circuit
    to fail (open circuit).

Open
Closed
27
Circuits
  • A schematic diagram is graphic representation of
    an electric circuit with standard symbols for the
    electrical devices.

28
Circuits
  • When the electric charges in a circuit have only
    one path in which to flow, the circuit is called
    a series circuit.

Flow Interrupted
29
Circuits
  • If the circuit has different branches in which
    the electric charges can flow, the circuit is
    called a parallel circuit.
  • Parallel circuits also reduce the total
    resistance making the lights brighter.

No Interuption
30
Circuits
31
Circuits
  • A short circuit can occur if a wires insulation
    breaks down and two wires touch and create an
    alternate pathway.
  • Circuit breakers and fuses help prevent this.

32
Circuits
  • Electric Power is the rate at which electrons are
    moved across a circuit.
  • Electric companies use electric power to charge
    you for your electricity used (kW / h).

33
Ohms Laws in Series Circuits
  • The total voltage (VT) is calculated by adding
    all of the voltages in the circuit.
  • VT V1 V2
  • VT 6 V 6 V
  • VT 12 V

34
Ohms Laws in Series Circuits
  • The total resistance (RT) in a is calculated by
    adding all of the resistances in the circuit.
  • RT R1 R2
  • RT 4 O 1 O 1 O
  • RT 6 O

35
Ohms Laws in Series Circuits
  • The total current (IT) is calculated by using
    Ohms Law.
  • IT VT / RT
  • IT 12 V / 6 O
  • IT 2 A

36
Ohms Law in Parallel Circuits
  • The total voltage (VT) is calculated by adding
    all of the voltages in the circuit.
  • VT V1 V2
  • VT 1.5V 1.5 V
  • VT 3V

37
Ohms Law in Parallel Circuits
  • VBranches VT
  • VT 3 V
  • VB1 3 V
  • VB2 3 V

3 V
3 V
38
Ohms Law in Parallel Circuits
  • Use Ohms Law to find the I in each branch
  • Branch 1 I1 3 V / 1 O
  • I1 3 A
  • Branch 2 I2 3 V / 1 O
  • I2 3 A

3 V
3 V
3 A
3 A
39
Ohms Law in Parallel Circuits
  • IT I1 I2
  • IT 3 A 3 A
  • IT 6 A

3 V
3 V
3 A
3 A
6 A
40
Ohms Law in Parallel Circuits
  • Use Ohms Law to find the total resistance (RT)
  • RT VT / IT
  • RT 3 V / 6 A
  • RT 0.5 ?

3 V
3 V
3 A
3 A
6 A
41
Ohms Law in Parallel Circuits
  • The total resistance (RT) in a parallel circuit
    is can also be calculated by adding the inverses
    of the resistances in the circuit.
  • 1/RT 1/R1 1/R2 1/R3
  • 1/RT 1/1O 1/1O
  • 1/RT 2/1 (Take the inverse to find the answer!)
  • RT 1/2 or 0.5 O

3 V
3 V
3 A
3 A
6 A
42
Magnets
  • A magnet is a device made of a ferromagnetic
    metal (typically iron or nickel) that gives off a
    magnetic field.
  • Magnets are created due to the alignment of
    several small magnetic fields in a magnetic
    material called domains.

Non Magnet
Magnet
43
Magnets
  • Permanent magnets never lose their magnetism.
  • Temporary magnets lose their magnetism over
    time
  • Magnets can be man-made, but the first magnets
    used were natural (ex. lodestone, magnetite)

44
Magnetic Fields
  • A magnetic pole is an area of a magnet where the
    magnetic force appears to be strongest.
  • There are two types
  • North Pole
  • South Pole

45
Magnetic Fields
  • Unlike magnetic poles (south pole-north pole)
    will attract each other while like magnetic poles
    (north pole-north pole) repel each other.

46
Magnetic Fields
  • A magnetic field is a region around a magnet or
    current-carrying wire where magnetic forces can
    be measured.
  • Magnetic fields come out of the north pole and
    into the south pole.

47
Magnetic Fields
  • Compasses are a magnet suspended on top of a
    pivot so that the magnet can rotate freely.
  • Compasses can track magnetic fields.

48
Magnetic Fields
  • The Earths geographic North and South poles
    are different from Earths magnetic poles.
  • The geographic poles are straight up and down
    versus the magnetic poles that are slightly
    tilted (magnetic north is in Canada).

49
Magnetism and Electric Current
  • A solenoid is a coil of wire with an electric
    current in it.
  • The electric current creates a magnetic field
    around each loop of the wire.
  • more loops more magnetic force
  • more current more magnetic force

50
Magnetism and Electric Current
  • The magnetic force of a solenoid can also be
    strengthened by adding a soft iron core to its
    center.
  • This is called an electromagnet.

51
Creating Electric Current
  • Faradays Law of Induction describes the
    production of a current in a conducting circuit
    by a change in the strength, position, or
    orientation of an external magnet.

52
Creating Electric Current
  • Electrical generators change mechanical energy to
    electrical energy using electromagnetic
    induction.
  • In a commercial generator, an electric current is
    produced when a large coil of wire is rotated
    through a strong magnetic field.

53
Creating Electric Current
  • Electrochemical cells change chemical energy into
    electrical energy.
  • Electrochemical cells batteries contain an
    electrolyte (a solution that conducts
    electricity) and two electrodes, each a different
    conductor.
  • There are two types
  • dry (contain electrolyte paste)
  • wet (contains electrolyte liquid)

54
Creating Electric Current
  • Electricity from a generator changes direction
    moving back and forth in cycles. This is called
    alternating current (AC).
  • Electrochemical cells have direct current (DC)
    which means that the charges always move in the
    same direction through a circuit.

55
Using Electric Current
  • Electric Motors convert electrical energy into
    mechanical energy by using magnets to create
    motion.
  • Inside an electric motor, the attracting and
    repelling forces of two magnets create a
    rotational motion.
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