Transcript Electricity

Electricity & Magnetism
Chapters 17, 19, 20, 21 and 22-2
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.
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.
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.
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
Example problem
 How many electrons are in 0.85 C of charge?
1 electron
18
0.85 C 

5.3

10
electrons
19
1.6 10 C
OR
6.2 10 electrons
18
0.85 C 
 5.3 10 electrons
1C
18
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
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.
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).
Coulomb’s Law
Fe 




k q1 q2
r
2
F = Electric Force (N)
q = charge (C)
r = distance between charges (m)
k = 8.99 X 109 Nm2/C2
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
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=?
kq q
Fe 
1
2
2
r
9
19
19
(8.99 10 )(1.6 10 )(1.6  10 )
Fe 
0.022
Fe  5.75 1025 N
Current, Resistance &
Voltage
Chapter 19
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 = ΔQ/t
 Current = charge / time
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?
ΔQ = 1.6 C I = 0.835 A t= ?
I = ΔQ/t
t = ΔQ/I
t= 1.6C/0.835A
t= 2.00s
Electric Current
 Batteries maintain electric current
by converting chemical energy into
electrical energy.
 Generators convert mechanical
energy into electrical energy.
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.
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.
Resistance
 Resistance- The opposition to the flow of
current in a conductor
 R = V/I
 Resistance = Potential
difference/Current
 SI unit – ohm
Symbol-  (omega)
Resistance
 Resistance depends on length, crosssectional area, material and
temperature.




Length: short = ↓ R; long = ↑ R
Area: skinny = ↑R; wide = ↓R
Material: insulator = ↑R; conductor = ↓R
Temperature: hot = ↑R; cold = ↓R
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.
Example Problem
 The resistance of a steam iron is 19.0 Ω.
What is the current in the iron when it is
connected across a potential difference of
120V?
 R= 19.0 Ω
V= 120V I= ?
 R=V/I
 I=V/R
 I=120V/19.0 Ω
 I= 6.32 A
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).
Potential Difference
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
Electric Power
 Electric power is the rate of
conversion of electrical energy
 Formula for Electric Power:
P = IV
Electric power = current X potential
difference
Electric Power
Because P= IV and V=IR 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
P=V2/R
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 Ω
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
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 kW*24 h = 2.4
kWh
 Cost = 2.4 kWh*$0.080 = $0.19
Circuits
Chapter 20
Schematic Diagrams and
Circuits
 Schematic Diagram or Circuit
Diagram: diagram which depicts the
construction of an electrical circuit.
Symbols
 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.
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.
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.
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.
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.
Resistors in series vs. parallel
Circuit
Series
Parallel
Current
I = I1= I2= I3 …
I = I1 + I2 + I3 …
Current is the same for each
resistor and the same as total
For Total Current:
I = V/Req
Sum of currents = total
current
Current across a resistor:
I1=V/R1 and I2=V/R2 ,etc.
Potential
V = V1 + V2 + V3 …
Difference Sum of potential differences =
total potential difference.
Potential difference across a
resistor:
V = V1 = V2 = V3 …
Same for each
resistor and same as
total
V1 = IR1 and V2 = IR2 ,etc.
Equivalent Req = R1 + R2 + R3 …
resistance Sum for each resistor
1/Req = 1/R1 + 1/R2
+ 1/R3 …
Reciprocal sum of
resistances
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 = 2Ω+4Ω+5Ω+7Ω = 18Ω
 I = V/R
 I = 9V/18Ω = 0.5 A
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/R1+1/R2+1/R3+1/R4
 1/Req = 1/2Ω+1/4Ω+1/5Ω+1/7Ω =
0.92Ω
 I = V/R
 I = 9V/0.92Ω = 9.8 A
Magnetism
Chapter 21
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.
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
Earth’s 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!
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.
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.
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
Solenoid
Electromagnetism