2 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 repelOpposite charges attract
3 Names of charges are arbitrary Positive Charge (+) - defined as the charge on glass after it is rubbed with silkNegative Charge (-) – Defined as the charge on rubber or amber after it is rubber with furPith ball demos
5 An excess or shortage of electrons produces a charge 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
6 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 coulombs.
7 Electric Forces Like charges repel Opposite charges attract The force of attraction/repulsion between electrically charge objects is electric force
8 Electric FieldsThe 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
9 Static ElectricityStudy of the behavior of electric charges, including how charges are transferred between objectsSeveral ways charge can build up or moveFrictionContactInductionLaw of Conservation of ChargeTotal charge in an isolated system is constant
10 Charging by FrictionRubbing 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.
11 Charging by ContactA 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
12 Charging by inductionWalking 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
15 Electric Current Continuous flow of electric charge Unit – Ampere, or Amp (A) which equal 1 Coulomb per secBy convention, Current is the rate at which + charge flows in a circuit, in reality it is the negatively charged electrons that actually move
16 Conductors & Insulators Electrical conductorA material through which charge can flow easilyElectrons in atoms are not tightly bound to atomCopper, SilverElectrical insulatorMaterial through which charge cannot flow easilyDo not have free electrons (held more tightly to the nucleus)Wood, Plastic, Rubber, and Air
17 Resistance Opposition to flow of charges in a material Unit is the Ohm A materials thickness, length and temperature affect its resistanceAs temp goes up, R goes upAs thickness decreases, R goes upAs length increases, R goes up
18 VoltageIn 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 energyPE 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 voltageVoltage sources do work to separate charges, b/c of this they have a potential difference and can do work
19 VoltageA 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.
20 V = IR Ohm’s Law Voltage = Current x Resistance Increasing the voltage increases the current, keeping the same voltage and increasing the resistance decreases the current
21 20.3 – electric CircuitsElectric Circuit – A complete path through which charge can flowCircuit Diagrams- use symbols to identify different components in a circuitOpen Circuit - Circuit is not complete -- Switch is “Off”, so no current can flowThe direction of Current is defined as the direction in which positive charges would flow….. Electrons in a wire flow opposite direction.
22 Series CircuitOnly one path for current to follow
23 Parallel CircuitAn electric circuit for two or more paths through which charges can flowIf one element stops working in a parallel circuit the others can still operate
24 Key IDeasDifferences 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 nucleusOrigin, motion and energy of electrons in circuitsRole of batteries as energy sources and resistors as energy dissipaters in circuitsBy convention Current is the rate at which + charge flows in a circuit, in reality it is the negatively charged electrons that actually moveCurrent 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 flowResistors oppose the rate of charge flow in a circuitConceptual understanding of potential differencePotential 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 circuitCurrent and resistance and the relationship between themCurrent increases as potential difference increases, or as resistance decreases