Transcript Electric charge

Unit 10
Chapter 13
Consumer warning:
 You will have carpal tunnel syndrome by the end of
this unit if you don’t learn how to summarize the
information contained in this powerpoint.
Think of all the ways electricity
can be found in your daily life.
Could you survive without
electricity?
Electric charge and force
 Electric charge is an electrical property of matter
that creates a force between objects.
 All matter is made of atoms
 Atoms are composed from 3 essential particles
 Neutrons – in the nucleus with a neutral charge
 Proton – in the nucleus with a (+) charge
 Electron – outside the nucleus with a (-) charge
Electric charge cont.
 SI unit of electric charge is the coulomb, C
 Electrons and protons have exactly the same
amount of charge… 1.6 x 10-19 C
 Because they are oppositely charged.
 Protons is a +1.6 x 10-19 C
 Electrons is a -1.6 x 10-19 C
 If an object has excess electrons then it has an
electric charge.
 The net electric charge is always a multiple of
1.6 x 10-19 C.
 Atoms become charged by gaining or
losing electrons
 Static electricity is the accumulation
of excess electric charges on an object
What are the rules?
Law of Conservation of Charge
Charge may be transferred from
object to object but it cannot be
created or destroyed
More rules
 Opposites attract and like charges
repel
 Charges can act at great distances
 Any charge placed in an electric field
will be pushed or pulled by the field
Still more rules
 Electrons move more easily through
conductors like metals
 Electrons do no move easily through
insulators such as plastic, wood, rubber,
and glass
Really dry air
With a big enough build up of charge,
electricity WILL flow
Like say half a megavolt trying to pass through a wire
Conductors and Insulators
 A conductor is a material that transfers charge
easily
 It allows the charges to flow
 Example: metal
 An insulator is a material that does not transfer
charge easily
 It will not allow the charges to flow
 Examples: cardboard, glass, silk, and plastic
Transferring a Charge
 Three main methods
 Charging by contact
 Charging by Induction
 Static Discharge
Charging by contact
 Done by touching or rubbing
 Example: feet rubbing the carpet
then touching a metal door knob
Charging by Induction
 Electrons move because of a nearby
charged object
 Example: negatively charged balloon
near your sleeve causes your sleeve to be
positively charged
Static Discharge
 A transfer of charge through the air
between two objects because of the
buildup of a static electricity – a
spark!
 Example: lightning
Last bits
 Grounding – using a conductor to
direct the charge into the ground
 It an important part of electrical
safety!
 The presence of a charge can be
determined by an electroscope
Electric force
 Electric force is the force of attraction or
repulsion between objects due to charge
 It is the reason why some clothes cling
together when you take them out of the
dryer
 It is part of what holds an atom together
 It also plays a part in chemical bonding
 Without electric force, life itself would be
impossible
Electric force cont.
 The electric force between 2 charged objects
varies depending on the amount of charge on
each object and the distance between them
 Acts through a field
 Electric field is the region around a charged
object in which other charged objects experience
an electric force
Electric force cont.
 Regardless of the charge, electric field lines
never cross one another
Current
Electrical potential energy
 Electrical potential energy is the
potential energy of a charged object due
to its position in an electric field
 Potential difference is the change in the
electrical potential energy per unit
charge
 SI unit for potential difference is the volt (V)
which is equivalent to 1 joule per coulomb
 Often called voltage
Electric current
 The flow of charges through a wire or
conductor is called electric current
 Current is the rate that electric charges
move through a conductor
 SI unit of current is Amperes (A or amp)
Voltage
 Charges flow from high voltage to low
voltage
 A voltage difference is the push that
causes charges to move
 Voltage difference is measured in volts
(V)
 For charges to flow, the wire must always
be connected in a closed path or circuit
Sources of electricity
 Cell is a device that is a source of electric
current because of a potential difference,
or voltage between the terminals
Dry cell – standard battery – produces a
voltage difference by a chemical reaction
Wet cell – car battery – produces a voltage
difference by chemical reaction
 Wall sockets – AC – produced at a
generator
Types of electric cells
 Electrochemical – Electrons transferred
between different metals immersed in an
electrolyte
 Uses – common batteries and automobile
batteries
 Photoelectric and photovoltaic – Electrons are
released from a metal when struck by light of
sufficient energy
 Uses –satellites, calculators, and streetlights
Types of electric cells
 Thermoelectric – Two different metals are
joined together, and the junctions are held at
different temperatures, causing electrons to
flow.
 Uses – Thermostats for furnaces and ovens
 Piezoelectric – Opposite surfaces of certain
crystals become electrically charged when
under pressure.
 Uses – crystal microphones and headsets,
computer keypads
Conventional current was the
original idea……
 Conventional current is the
movement of positive charge
 It is current made of positive charge
that would have the same effect as
the actual motion of charge in the
material
Electrical Resistance
 Resistance is the ratio of the voltage
across a conductor to the current it
carries
 The tendency for a material to
oppose the flow of electrons,
changing electrical energy into
thermal energy and light
 All materials have some resistance
Still resisting
 SI unit of resistance is ohms (Ω)
 It is equal to volts per ampere
 Resistor is a special type of conductor
used to control current
 Making wires thinner, longer, or
hotter increases resistance
Light bulbs
 light bulbs make use of resistance
Ohm’s Law
I=V
R
I – current (A)
V – voltage (V)
R – resistance (Ω)
Practice problem
What is the resistance of a wire that has a current
of 1.4 A in it when it is connected to a 6.0 V
battery?
R=V
I
Practice problem
What is the resistance of a wire that has a current
of 1.4 A in it when it is connected to a 6.0 V
battery?
R=V
I
= 6.0 V = 4.3 Ω
1.4 A
Practice problem
An electric space heater is plugged into a 120 V
outlet. A current of 12.0 A is in the coils in the
space heater. What is the resistance of the
coils?
Practice problem
An electric space heater is plugged into a 120 V
outlet. A current of 12.0 A is in the coils in the
space heater. What is the resistance of the
coils?
R=V
I
= 120 V = 10 Ω
12.0 A
Practice problem
A steam ion has a current of 9.17 A when plugged
into a 120 V outlet. What is the resistance of
the steam iron?
Practice problem
A steam ion has a current of 9.17 A when plugged
into a 120 V outlet. What is the resistance of
the steam iron?
R=V
I
= 120 V = 13 Ω
9.17 A
 Practice problems……………………..
1
120.12V
2

4.8V
3

116.5V
4

119.56V
5

4.55V
1.5V per battery
6

2.9A
7

15A
8

4.79A
9

5.2A
10

230.77Ω
11

179.2 Ω
Anybody do #14?

5.74A
More on resistance….
 Conductors have low resistances
 They allow the electrons to flow
easily and therefore have a low
resistance
Still resisting
 Some materials become superconductors
below a certain temperature
 Certain metals and compounds have zero
resistance when their temperature falls
below the critical temperature
 The critical temperature varies between 272oC and -123oC
 Examples – tin, mercury, and some
metallic compounds containing barium,
copper, and oxygen
Still resisting
 Insulators have high resistance to charge
movement
 They are used to prevent electric current
from leaking
 Examples - plastic coating around copper
wire of an electric cord keeps the current
from escaping into the floor or your body
Circuits
What are circuits?
 Electric circuit is an electrical device
connected so that it provides one or more
complete paths for the movement of
charges
 Closed circuit is one in which there is a
closed-loop path for electrons to follow
 Open circuit is one without a complete
path, there is no charge flow and therefore
no current
Circuits
 Schematic diagram is a graphic
representation of an electric circuit
or apparatus, with standard symbols
for the electrical devices
Electrical Circuits
 Circuits rely on generators at power
plants to produce a voltage difference
across the outlet, causing the charge
to move when the circuit is complete
Series and Parallel circuits
 Series describes a circuit or portion of a
circuit that provides a single conducting
path
 Parallel describes components in a circuit
that are connected across common
points, providing two or more separate
conducting paths
Series circuit
 The current has only one loop to flow
through
 The parts of the series circuit are
wired one after another so the
amount of current is the same
through every part
Series circuits cont.
 Open circuit – If any part of a series
circuit is disconnected, no current
flows through the circuit
 Example – Christmas lights
Parallel Circuits
 Contain two or more branches for
current to move through
 Individual parts can be turned off
without affecting the entire circuit
 Example – circuits in a house
Electric Power and Electrical Energy
 Electrical energy is the energy associated
with electrical charges, whether moving
or at rest
 The energy may come from a battery or a
power plant
 Electric Power
Power = current x voltage
P = IV
 SI unit for power is the watt (W)
Practice problem
A flashlight has a potential difference of 3.0
V. The bulb has a current of 0.50 A. What
is the electric power used by the
flashlight?
P = IV
= 3.0 V x 0.50 A
= 1.5 W
Practice problem
What is the current in a 60 W light bulb
when it plugged into a 120 V outlet?
Practice problem
What is the current in a 60 W light bulb
when it plugged into a 120 V outlet?
I=P
V
= 60 W = 0.5 A
120 V
Household circuits
 Use parallel circuits
 Each branch receives the standard
voltage difference
 Electricity enters your home at the
circuit breaker or fuse box and
branches out to the rest of your
house
House guards
 Electric fuse – contains a small piece of
metal that melts if current becomes too
high and breaks the circuit
 Circuit breaker – uses a magnet or
bimetallic strip (a strip with two different
metals welded together) that responds to
current overload by opening the circuit.
 It protects a circuit from current overloads
 Used in many homes and businesses