ELECTRICITY 20.1-20.3PHYSICAL 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

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.

Voltage

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)

• 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

• Resistors oppose the rate of charge flow in a circuit

• Conceptual understanding of potential difference

• 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