Transcript Electric Charge

Electric Charge
Positive and Negative charges
Three subatomic particles:
Protons – positively charged
Electrons – negatively charged
Neutrons – NO charge
In an atom the (+) charges or protons and (-)
charges or electrons are equal, so they atoms
are naturally neutral
Protons and neutrons are tightly bound in the
center of the atom
Atoms cannot lose protons or neutrons!
Atoms can lose electrons though
if an atom loses an electron it becomes (+)
If an atom gains an electron it becomes (-)
Some atoms hold their electrons more tightly
than others
Transferring charges
Some materials hold their electrons very tightly.
Electrons do not move through them very
well.
These things are called insulators.
Plastic, cloth, glass and dry air are good
insulators.
Other materials have some loosely held
electrons, which move through them very
easily.
These are called conductors.
Most metals are good conductors.
Static electricity
The accumulation of excess electric charge on an
object
The imbalance of (+) and (-) charges
Law of Conservation of Charge
Charges cannot be created or destroyed – only
transferred between objects!
Charges Exert Forces
Like charges repel
Unlike charges attract
Forces decrease as the distance between atoms
increases
As the amount of charge increases the electrical
force increases
Electric Fields
Surround any electrically charged particle
Exerts a force that attracts/repels other charged
particles
Represented by arrows drawn in the direction
that a (+) charged particle would move in thee
field
Electrical vs Gravitational Forces
Even though there are gravitational forces
between ALL objects that have mass, even
subatomic particles – this force is weak due to
the extremely small mass of the particles
Electrical forces are stronger and are responsible
for:
~ holding atoms together
~ chemical bonds between atoms
Electrical forces between OBJECTS is less than
the gravitational forces.
Since MOST all objects are electrically neutral,
there is little if any electrical force between
the objects
Conductors and Insulators
When you walk on carpet the electrons transfer
from the carpet to your shoes – your shoes
become (-) charged
Why does it shock you when you touch the door
knob?
Some electrons transfer from your shoes to your
skin – skin is a good conductor!
Conductors
The best electrical conductors are metals
Metals have free moving electrons that can
move between atoms
This allows for electrons to flow easily through
the material
Insulators
A material that holds it’s electrons tightly,
electrons do not move around easily
Charging Objects
Charging by contact – transferring charges
between objects by touching
One object becomes (+) charged and the other
(-)
Charging by Induction- the rearrangement of
electrons in a neutral object caused by a
nearby charged object
Lightning
A large static discharge – a transfer of charge
between 2 objects due to build up of static
electricity
Water molecules in clouds collide and may lose
electrons creating charged particles
Top of the clouds become (+) and the bottom (-)
The negative charge of the cloud causes the
ground below to become (+) charged (attracts
(+) charges to the surface)
When enough (-) charge builds in the bottom of
the cloud, it starts to move toward the (+)
charged ground
When the electrons bet close enough, the (+)
particles move upward
When they meet and complete the connection
The light is due to the extreme heat
The lightning travels up, but the (-) electrons are
moving downward
Thunder
The extreme heat produced by lightning causes
the surrounding air to rapidly expand
producing sound waves
Grounding
Connecting objects to the Earth with a conductor –
Grounding
Since the earth is a large neutral object that is a
good conductor; grounding provides a path for
excess electrical charges before they can build
enough to do damage
Electric Current
Electric Current: the NET movement of electric
charge in a single direction
In any condutor, electrons are in constant
motion – random, and typically not in any one
direction
When a current flows through a conductor,
electrons still move randomly – they just drift
in the direction of the current
Electric Current is measured in AMPERES
1 ampere = 6,250 million electrons flowing past
one point per second
Voltage difference – related to the force at
which electrons flow
Electrons will ALWAYS flow from high pressure
to low pressure
Circuits
Electric current requires a closed path in order
to flow
This path is called a circuit
If any part of the circuit is broken, the current
will stop flowing
Batteries
In order to keep an electric current flowing in a
circuit; a voltage difference must be
maintained
This voltage different can be created by a
battery
As long as there is a closed path between the
two battery terminals, the current will flow
Dry Cell Batteries
Convert chemical energy to electrical energy
An electrolyte paste in the battery reacts with
the zinc in the battery to create an electrical
charge
The carbon rod in the battery becomes (+)
The zinc becomes (-) charged
This voltage difference can cause current to flow
through a closed circuit
Wet Cell Batteries
Contains two connected metal plates (different
metals) that are in a liquid electrolyte solution
A wet cell battery usually contains several cells
connected together
Lead-Acid Batteries
Car batteries
The metal plates are lead, and lead acetate
The electrolyte solution is a high concentration
of sulfuric acid
Each cell provides a voltage difference of 2V, put
6 cells together and you make a 12V battery
Voltage differences can also be created by
electrical outlets, such as a wall outlet
The voltage difference between the two holes in
a wall socket is usually 120V
What makes a light bulb glow?
Part of the closed circuit in a light bulb is a metal
structure called a filament
When the electrons flow through the filament
then collide with the metal particles
The filament heats up and begins to glow
Resistance
The tendency for a material to resist the flow of
electrons, changing electrical energy into
thermal energy or light
All materials have some resistance
Conductors have much less resistance that
insulators
Resistance in measured in Ohms
Electrical resistance increase as Temp increases
Resistance can also depend on the length and
diameter of the object, such as a wire
Resistance increases with wire length
Resistance increased when a wire becomes
thinner
Simple Circuits
A simple circuit contains:
• a source of voltage difference, such as a
battery
• A device, such as a light bulb
• Conductors that connect the battery to the
device
Increasing Current in a circuit
1. Increase the voltage difference
2. Reduce resistance
Ohm’s Law
The current in a circuit equals the voltage
difference divided by the resistance
Electric current (I) = Voltage difference (volts)
Resistance (Ohms)
To measure resistance of a material just
rearrange the equation:
R = V/I
Series Circuits
All circuits must contain:
a source of voltage difference
conductor
device that uses electrical energy
Series Circuit: only has one loop of flow for
electrons
Open circuit
All parts of a circuit are wired one after the
other, the amount of current through each
device is the same
When one part of the series is disconnected the
current flow stops
EX: some Christmas tree lights
Parallel Circuits
Contain 2 or more branches for the current to
move through
Can turn off one device and the current still
flows through the other branches
In a parallel circuit, the more devices that
operate, the great the current in the circuit
Too many devices on the same circuit will cause
the circuit to overheat
Fuses and circuit breakers prevent circuits from
getting too hot
Electric Power
The rate at which electrical energy is converted into
other forms of energy
Devices that convert electrical energy into thermal
energy (heating coils) tend to use more power
Power (watts) = current (amperes) X voltage (volts)
P = IV