Electricity

1
Electricity Magnetism

• Chapters 17, 19, 20, 21 and 22-2

2
Chapter 17 - Charge

• The two different kinds of Electric charges are
  positive and negative
• Like charges repel unlike charges attract
• Protons and neutrons are relatively fixed in the
  nucleus of the atom but electrons are easily
  transferred from one atom to another.

3
What causes charge?

• All charge is a result of the movement of
  electrons.
• All atoms begin as neutral- with no charge.
• If you take away negative electrons then the atom
  has a positive charge.
• If you add negative electrons then the atom
  becomes negatively charged.
• All atoms with a charge are called ions.

4
How do we charge objects?

• What causes the electrons to move?
• Friction! When objects rub together electrons are
  moved from one object to the other.
• This causes one object to be positively charged
  and the other to be negatively charged and the
  process is called charge by contact.

5
(No Transcript)
6
Calculating charge

• 1 electron contains 1.6 X 10-19 coulombs of
  charge
• C (coulomb) is the SI unit of electric charge
• 1.0 C contains 6.2 X 1018 electrons

7
Example problem

• How many electrons are in 0.85 C of charge?

8
Types of Materials

• Materials in which electric charges move freely
  are called conductors.
• Ex Copper, Aluminum, most metals
• Materials in which electric charges do not move
  freely are called insulators.
• Ex Wood, glass, styrofoam
• Semiconductors are materials between conductors
  and insulators.
• Ex silicon, germanium

9
More Terms to Know

• Grounding is when a conductor is connected to the
  Earth by another conducting object such as copper
  wire. Many times it is a safety precaution in
  electrical devices.
• Induction is the process of charging a conductor
  by bringing it near another charged object and
  grounding the conductor.

10
More Terms to Know

• Electric Force two or more charged objects near
  one another may experience motion either toward
  or away from each other because each object
  exerts a force on the other objects.
• Electric force is an example of a field force (a
  force which does not require physical contact to
  act).

11
Coulombs Law

• F Electric Force (N)
• q charge (C)
• r distance between charges (m)
• k 8.99 X 109 Nm2/C2

12
Electric Field

• Electric field a region in space around a
  charged object in which a stationary charged
  object experiences an electric force because of
  its charge.
• No contact needs to take place for this to occur

13
What is the electric force between a proton and
an electron if they are separated by 2 cm?

• q (proton) q (electron) 1.6 x 10-19 C
• r 2 cm 0.02 m
• k 8.99 x 109
• F ?

14
Current, Resistance Voltage

• Chapter 19

15
Electric Current

• Current is the rate at which electric charges
  move through a given area.
• SI unit is the Ampere or Amp.
• 1 A 1 C/s
• I ?Q/t
• Current charge / time

16
Example problem

• The current in a light bulb is 0.835 A. How long
  does it take for a total charge of 1.67 C to pass
  a point in the wire?
• ?Q 1.6 C I 0.835 A t ?
• I ?Q/t
• t ?Q/I
• t 1.6C/0.835A
• t 2.00s

17
Electric Current

• Batteries maintain electric current by converting
  chemical energy into electrical energy.
• Generators convert mechanical energy into
  electrical energy.

18
AC/DC

• There are two kinds of current
• Direct current is where charges are always moving
  in the same direction.
• Batteries produce direct current because the
  positive and negative terminals always stay the
  same.

19
AC/DC

• Alternating current is where the charges change
  the direction of flow constantly.
• Power plants supply alternating current to homes
  and businesses by using giant electromagnets to
  change positive and negative terminals.
• In the US current alternates (changes direction)
  60 times every second while in Europe, current
  alternates 50 times every second.

20
Resistance

• Resistance- The opposition to the flow of current
  in a conductor
• R V/I
• Resistance Potential difference/Current
• SI unit ohm Symbol- ? (omega)

21
Resistance

• Resistance depends on length, cross-sectional
  area, material and temperature.
• Length short ? R long ? R
• Area skinny ?R wide ?R
• Material insulator ?R conductor ?R
• Temperature hot ?R cold ?R

22
Resistance

• Resistance is important in controlling the amount
  of current in a circuit.
• If the voltage is constant, resistance is the
  only way to adjust the current.
• Change the material of the wires, or add
  resistors to the circuit.

23
Example Problem

• The resistance of a steam iron is 19.0 O. What is
  the current in the iron when it is connected
  across a potential difference of 120V?
• R 19.0 O V 120V I ?
• RV/I
• IV/R
• I120V/19.0 O
• I 6.32 A

24
Potential Difference

• The electric potential is the amount of energy
  contained in each unit of charge.
• Only differences in electric potential from one
  point to another are measured and used in
  calculations.
• Potential Difference is the change in energy per
  unit of charge.
• Potential Difference is also known as VOLTAGE,
  and is measured in volts (V).

25
Potential Difference
26
Potential Difference

• The potential difference between the positive and
  negative ends of batteries
• All AA, AAA, C, D Cell Batteries 1.5 V
• The only difference is how long they produce the
  1.5 V.
• Car battery 12 V
• Positive and Negative
• slots of an electrical outlet 120 V

27
Electric Power

• Electric power is the rate of conversion of
  electrical energy
• Formula for Electric Power
• P IV
• Electric power current X potential difference

28
Electric Power

• Because P IV and VIR we can also say
• P IV I(IR) I2R
• P I2R
• Or, because I V/R, we can also say
• P IV (V/R)V V2/R
• PV2/R

29
Electric Power

• An electric space heater is connected across a
  120 V outlet. The heater dissipates 1320 W of
  power in the form of electromagnetic radiation
  and heat. Calculate the resistance of the heater.
• P V2/R R V2/P
• R 1202/1320
• R 10.9 O

30
Electric Power

• Power companies measure energy not power, using
  the kilowatt-hour as the unit
• One kilowatt-hour the energy delivered in 1
  hour at the constant rate of 1 kW.
• To convert between kWh and the SI unit of Joule
• 1 kWh 3.6 X 106 J

31
Example Problem

• How much does it cost to operate a 100.0 W light
  bulb for 24 h if electrical energy costs 0.080
  per kWh?
• P 100W 0.100 kW t 24 h
• Energy Pt 0.100 kW24 h 2.4 kWh
• Cost 2.4 kWh0.080 0.19

32
Circuits

• Chapter 20

33
Schematic Diagrams and Circuits

• Schematic Diagram or Circuit Diagram diagram
  which depicts the construction of an electrical
  circuit.

34
Symbols
35

• Since bulbs have internal resistance, sometimes
  bulbs are drawn as resistors in circuit diagrams
  and treated as resistors in calculations.
• Electric circuit- a set of electrical components
  connected so that they provide one or more
  complete paths for the movement of charges.
• Load- energy user of a circuit
• All complete circuits must contain a source of
  potential difference and a load.

36
Closed vs. Open

• Closed circuit- there is a closed-loop path for
  the electrons to follow
• Open circuit- no complete path, no charge flow,
  no current.

37
Resistors in series

• Series- describes a circuit or portion of a
  circuit that provides a single conduction path
  without junctions.
• If any one bulb burns out, all of the bulbs go
  out because the broken filament becomes a break
  in the circuit.

38
Resistors in series

• When connected in series, the current is the same
  in all bulbs (or resistors).
• The equivalent resistance (Req) in a series
  circuit is the sum of all resistances.
• V I/R can be used to find current and potential
  difference in a series circuit.

39
Resistors in parallel

• Parallel- describes two or more components in a
  circuit that are connected across common points
  or junctions, providing separate conduction paths
  for the current
• Because of this, a bulb can burn out and will not
  effect any other bulbs.

40
Resistors in series vs. parallel
Circuit Series Parallel
Current I I1 I2 I3 Current is the same for each resistor and the same as total For Total Current I V/Req I I1 I2 I3 Sum of currents total current Current across a resistor I1V/R1 and I2V/R2 ,etc.
Potential Difference V V1 V2 V3 Sum of potential differences total potential difference. Potential difference across a resistor V1 IR1 and V2 IR2 ,etc. V V1 V2 V3 Same for each resistor and same as total
Equivalent resistance Req R1 R2 R3 Sum for each resistor 1/Req 1/R1 1/R2 1/R3 Reciprocal sum of resistances
41
A 9V battery is connected to four light bulbs.
Find the equivalent resistance for the circuit
and the current in the circuit.

• Req R1 R2 R3 R4
• Req 2O4O5O7O 18O
• I V/R
• I 9V/18O 0.5 A

42
A 9V battery is connected to four resistors. Find
the equivalent resistance for the circuit and the
total current in the circuit.

• 1/Req 1/R11/R21/R31/R4
• 1/Req 1/2O1/4O1/5O1/7O 0.92O
• I V/R
• I 9V/0.92O 9.8 A

43
Magnetism

• Chapter 21

44
Magnets

• Every magnet has poles which contain opposite
  charges.
• Like poles repel each other, and unlike poles
  attract each other due to their magnetic fields.

45
Magnetic Fields

• Magnetic Field (B) region around a magnet with
  magnetic force
• Magnetic Fields are measured in Teslas (T)
• The direction of the magnetic field at any
  location is the direction in which the north pole
  of a compass needle points at that location

46
Earths Poles

• A compass is a magnet
• Its north pole points north with regard to the
  Earth
• That means the magnetic South pole of the Earth
  is near the geographic North pole and the
  magnetic North pole of the Earth is near the
  geographic South pole!

47
Electromagnetism

• When a wire is carrying a current it creates a
  magnetic field of concentric circles around the
  wire.
• We use the right hand rule to describe the
  direction of the field around the wire. If the
  current changes direction the magnetic field
  changes direction.

48
Electromagnetism

• Right hand rule Pretend the wire is grasped in
  your right hand with your thumb pointing in the
  direction of the current. Your fingers curl
  around the wire in the direction of the magnetic
  field.

49
Solenoids

• When wires are looped, the magnetic field works
  the same way.
• Several closely spaced loops create a device
  called a solenoid.
• Solenoids generate a strong magnetic field
• The more loops, the stronger the magnetic field
• The magnetic field can also be increased by
  inserting an iron rod through the center of the
  loops

50
Solenoid
51
Electromagnetism