ELECTRICITY 20.1-20.3
PHYSICAL SCIENCE
20.1 – ELECTRIC CHARGE & STATIC ELECTRICITY
Electric charge is a property that causes subatomic particles such as
protons and electrons to attract or repel each other.
• Protons have a positive charge.
• Electrons have a negative charge.
• Like charges repel
• Opposite charges attract
• Names of charges are arbitrary
• Positive Charge (+) - defined as the charge on glass after it is rubbed with
silk
• Negative Charge (-) – Defined as the charge on rubber or amber after it is
rubber with fur
• Pith ball demos
In an atom, a cloud of negatively charged electrons surrounds the
positively charged nucleus. The atom is neutral because it has equal
numbers of positive and negative charges.
• If an atom gains electrons, it becomes a negatively charged ion.
• If an atom loses electrons, it becomes a positively charged ion.
• An excess or shortage of electrons produces a charge
CHARGE
• The SI unit of electric charge is the coulomb (C).
• It takes about 6.24 × 1018 electrons to produce a single coulomb.
• A lightning bolt is about 10 to 20 coulombs of charge, and a camera
flash is about 0.025 coulombs.
ELECTRIC FORCES
• Like charges repel
• Opposite charges attract
• The force of attraction/repulsion between electrically charge objects is
electric force
ELECTRIC FIELDS
• The effect an electric charge has on other charges in the space around it is the
charge’s electric field.
• Strength of the field depends on amount of charge producing it and on the
distance from the charge
STATIC ELECTRICITY
• Study of the behavior of electric charges, including how charges are
transferred between objects
• Several ways charge can build up or move
• Friction
• Contact
• Induction
• Law of Conservation of Charge
•
•
Total charge in an isolated system is constant
http://phet.colorado.edu/en/simulation/balloons
CHARGING BY FRICTION
• Rubbing a balloon on your hair causes charging by friction.
• Electrons move from your hair to the balloon because atoms in rubber have a greater
attraction for electrons than atoms in hair.
• The balloon picks up a net negative charge.
• Your hair loses electrons and becomes positively charged.
CHARGING BY CONTACT
• A Van de Graaff generator builds a charge on a metal sphere. Touching the
sphere transfers charge by contact.
• The sphere is still charged, but its net charge is reduced.
• Sphere is supplied a continuous flow of negative charge, which is then given to
you…. And since the negative charges in your hair will repel each other your
hair stands up on end
CHARGING BY INDUCTION
• Walking on a carpet builds a negative charge on your body. The negative
charge in your hand repels electrons in a metal doorknob.
• The doorknob is still neutral, but charge has moved within it
This is
induction, a transfer of charge without contact between materials.
Static Discharge – The shock you
feel. Occurs when a pathway
through which charges can move
forms suddenly
LIGHTNING
• Lightning animation
20.2 – ELECTRIC CURRENT AND OHM’S LAW
ELECTRIC CURRENT
• Continuous flow of electric charge
• Unit – Ampere, or Amp (A) which equal 1 Coulomb per sec
• http://phet.colorado.edu/en/simulation/signal-circuit
•
By convention, Current is the rate at which + charge flows in a circuit, in reality it is the negatively
charged electrons that actually move
CONDUCTORS & INSULATORS
• Electrical conductor
• A material through which charge can flow easily
• Electrons in atoms are not tightly bound to atom
• Copper, Silver
• Electrical insulator
• Material through which charge cannot flow easily
• Do not have free electrons (held more tightly to the nucleus)
• Wood, Plastic, Rubber, and Air
RESISTANCE
• Opposition to flow of charges in a material
• Unit is the Ohm
• A materials thickness, length and temperature affect its resistance
• As temp goes up, R goes up
• As thickness decreases, R goes up
• As length increases, R goes up
VOLTAGE
• In order for charge to flow in a conducting wire, the wire must be
connected in a complete loop that includes a source of electrical energy.
•
•
Charges flow from a higher potential energy to a lower potential energy
PE depends on its position in an electric field.
• Potential Difference is the difference in electrical potential energy between two places in an
electric field.
•
•
Measured in Joules per Coulomb, or Volts. B/c of this potential difference is also called voltage
Voltage sources do work to separate charges, b/c of this they have a potential difference and can
do work
20.2 Electric Current and Ohm’s Law
Voltage
A pump lifts water to
the top of the
fountain, increasing
the gravitational
potential energy of
the water. A voltage
source increases
the electrical
potential energy of
electric charges.
OHM’S LAW
• Voltage = Current x Resistance
•V = IR
• Increasing the voltage increases the current, keeping the same voltage and
increasing the resistance decreases the current
• http://phet.colorado.edu/en/simulation/battery-resistor-circuit
20.3 – ELECTRIC CIRCUITS
• Electric Circuit – A complete path through which charge can flow
• Circuit Diagrams- use symbols to identify different components in a circuit
• Open Circuit -
Circuit is not complete -- Switch is “Off”, so no current can
flow
• The direction of Current is defined as the direction in which positive charges
would flow….. Electrons in a wire flow opposite direction.
SERIES CIRCUIT
• Only one path for current to follow
PARALLEL CIRCUIT
• An electric circuit for two or more paths through which charges can flow
• If one element stops working in a parallel circuit the others can still operate
KEY IDEAS
•
Differences between el conductors & insulators is explained by how freely the electrons flow through the material due
to how firmly the electrons are held to the nucleus
•
Origin, motion and energy of electrons in circuits
•
Role of batteries as energy sources and resistors as energy dissipaters in circuits
•
•
By convention Current is the rate at which + charge flows in a circuit, in reality it is the negatively charged
electrons that actually move
•
•
Current is measured in Amperes (1 Coulomb of charge per sec)
•
Resistors oppose the rate of charge flow in a circuit
Conceptual understanding of potential difference
•
•
•
In a circuit the power source supplies the electrons already in a circuit electric potential energy by doing work to
separate opp charges… this separation is what causes the electrons to flow
Potential difference (voltage) is a measure of PE in Joules supplied to each coulomb of charge.
Potential difference is a property of the energy source and does not depend on devices in the circuit
Current and resistance and the relationship between them
•
Current increases as potential difference increases, or as resistance decreases