Electricity

Chapter 17, Section 1 - Electric Charge and Force                                                                                                                    

Electric Charge - An electrical property of matter that creates electric and magnetic forces and interactions.
Objects can have...
  • positive charge
  • negative charge
  • no charge (neutral)
An object with equal amounts of positive and negative charge has no net charge.
Law of Conservation of Charge - Electric charge cannot be created or destroyed.  (But opposite charges can balance.)

Like charges repel, opposite charges attract.

Electric charge depends on an imbalance of protons and electrons.
Protons (+) positive charge
Electrons (-) negative charge
Neutrons (o) no charge (neutral)
  • If an object has...
    • more electrons than protons => negative charge.
    • fewer electrons than protons => positive charge.
      • The amount of protons remains fairly constant, but objects can gain/lose electrons.
Electric charge is measured in coulombs (C).
  • 1 proton has a charge of +1.6x10-19 C.
  • 1 electron has a charge of -1.6x10-19 C.
    • To make a charge of -1 C, an object would need 6.25x1018 excess electrons.
Transfer of Electric Charge
When different materials are rubbed together, electrons can be transferred.
The direction of transfer depends on the materials.

Electrical conductors let electric move freely.  (ex - metals)
Electrical Insulators do not.  (ex - plastic, glass, cardboard, silk)

Charges can move inside neutral objects.
Induced Charge - The separation of charges in a neutral object caused by bringing a charged object near it.  (One side becomes negatively charged, the other becomes positively charged.)

Objects can be charged by...
  • Contact - When a charged object touches a neutral object, electrons are transferred.
  • Friction - Electrons are transferred when different materials rub against each other.  (ex - static)
Surface charges can be induced on insulators...
Bringing a charged object near it forces the electrons in the molecules to move a bit => makes polar molecules => makes an induced surface charge.

Electric Force - The force of attraction or repulsion on a charged particle due to an electric field.
Electric force (at atomic/molecular level) is responsible for most of the forces around you.
  • Electrical repulsion of electrons in your hand and your desk make the desk feel solid.
  • Electric force holds atoms and molecules together.
Electric force depends on charge and distance.
  • If charge is doubled, force is doubled.  (IF distance is constant.)
    • 2 x charge = 2 x electric force
  • If distance is doubled, force is quartered.  (IF charge is constant.)
    • 2 x distance = 1/4 x electric force
Electric Field - The space around a charged particle in which another charged particle experiences an electric force.
Electric Field Lines are drawn to represent electric fields.  The arrows show the force on a positive particle.
Electric field lines never cross.

(l) You can see the two positive charges repelling each other.

(r) If the positive charge was twice a big as the negative charge, only half of the field lines would connect them.





Homework:
    Pg. 592, #1-4, 6.

Chapter 17, Section 2 - Current                                                                                                                                               

Electrical Potential Energy - The ability to move an electric charge from one point to another.
  • Depends on the charge's position in an electric field.
    • A negative charge will move away from another negative charge.  (Like a ball rolling downhill.)
      • The charge is moved => work is done => electrical PE is decreased.
    • If a charge is pushed against the electric force, electrical PE is increased.  (Like rolling the ball uphill.)
  • Potential Difference - Change in electrical potential energy of a particle divided by its charge. OR the voltage difference in potential between two points in a circuit.
    • Measured in volts (V).
      • 1 volt = 1 Joule / coulomb          1 V = 1 J/C
    • Potential difference = voltage.
Batteries
  • There is voltage across the terminals (+ and - ends) of a battery.
  • Cell - A device that makes an electric current by converting chemical or radiant energy into electrical energy.
    • Electrochemical cells contain a solution that conducts electricity and two electrodes (of different conducting material).
      • Dry Cells (ex - flashlight batteries) use a paste-like electrolyte.
      • Wet Cells (ex - car battery) use a liquid electrolyte.
  • Voltage causes charge to move.
    • When a flashlight is turned on, the terminals of the battery are connected through the flashlight.
    • Charge flows from an area of high electrical PE to an area of low electrical PE.  
      • (Like a ball rolling downhill!)
  • Current - The rate at which charges pass through a certain point.  (The rate that charges flow.)
    • Measured in amperes (A).
      • 1 ampere = 1 coulomb of charge moving past in 1 second             1 A = 1 C/s
    • Direct Current - Charges always move from one terminal to another in the same direction (as in a battery).
    • Current can be made up of...
      • Negative charges
      • Positive charges
      • A combination of both.
    • Negative charge moving in one direction has the same effect as a positive charge moving in the opposite direction.
      • Conventional Current - The model of current in which current flows out of the positive terminal, through a circuit and into a negative terminal.
        • In reality, electrons (negative) flow out of the negative terminal and into the positive terminal.
        • Conventional current is opposite to electron flow!
Electrical Resistance
  • Resistance - The opposition to current flow by a material or device.
    • Caused by internal friction.
      • When moving electrons collide with atoms, they transfer some KE => generates heat.
    • Slows the charge moving through a conductor.
      • Resistance in a light bulb determines how bright the light will be.  More resistance = dimmer bulb.
    • Resistance = voltage / current
    • Measured in ohms (Ω).
      • Ω = 1V/A    1 ohm = 1 volt per ampere
        • If a voltage of 1 V produced a current of 1 A, then the resistance is 1 Ω.
      • Resistor - A special conductor used to control current.
        • Resistors have specific resistances.
    • Conductors have low resistance.
    • Insulators have high resistance.
    • Semiconductors conduct electricity under certain conditions.
    • Superconductors - materials with zero resistance when below a critical temperature.
      • Once a current is established, it continues even if the voltage is removed.
      • Can create super strong magnets - used to lift trains above tracks and eliminate friction.

Homework:
    Pg. 599, #2-8.

Chapter 17, Section 3 - Circuits                                                                                                                                               

Electric Circuit - A set of electrical components connected so that they make a complete path (or paths) for the movement of charges.
(A complete path that electricity flows through.)
Circuits are connected across a source of voltage (like a battery).
Closed Circuit - A complete path that current can flow through.
Open Circuit - A path that is not complete, so current cannot flow through it.

Switch - A device that can open or close a circuit.  (ex - a light switch)

Schematic Diagram - A representation of a circuit that uses lines for wires and different symbols for components.
  • Schematic diagrams use standard symbols.
Types of Circuits
Series Circuit - A circuit in which the parts are joined one after another.
  • One path for current.
  • The current in each device is the same.
  • The voltage is divided among the devices.
    • If the different devices have different amounts of resistance, the voltage will be different.
  • Any break in the circuit will cause the whole circuit to fail.
    • Imagine a string of Christmas lights - if one goes out, they all go out!  Now try finding the burnt-out bulb!
Parallel Circuit - A circuit in which the parts are joined in branches.
  • Multiple paths for current.
  • The current is divided among the paths: the sum of the current in each device = the total current.
  • The voltage is the same across each device.
    • If the resistance is less in one device, more charge will flow through that device.
  • A break in one branch of the circuit does not stop the flow of current to other branches.
    • Now, imagine the Christmas lights with one bulb burnt out, but the rest are still glowing.  Ah, the beauty of parallel circuits!

Electrical Energy & Electric Power
  • Electric Power - The rate at which electrical energy is converted into other forms of energy.
    • As current goes through a circuit, some energy is transformed into...
      • useful work.
      • heat.
    • Power = Current x Voltage         P = IR
    • This can be combined with the formula for resistance to calculate power lost by a resistor.
    • Measured in watts (W).
      • 1 W = 1 J/s
      • 1 W = 1 A x 1 V
      • 1,000 W = 1 kW (kilowatt)
    • Electric companies measure energy in kilowatt-hours.
      • 1 kW*h is the energy delivered in 1 h at a rate of 1 kW.

Fuses and Circuit Breakers
  • As more devices are connected in a circuit, resistance is lowered and current increases.
    • If the current rises above a safe level, the circuit is overloaded.
      • This could cause a fire!
    • Short Circuit - An alternate pathway for current, with less resistance.  (Like a short cut for electricity!)
      • Can happen when wire insulation wears out and two wires touch.
      • Can cause electrical fires.
  • Fuse - An electrical device that contains a metal strip that melts when current becomes too great.  (This breaks the circuit.)
  • Circuit Breaker - A switch that opens a circuit automatically when the current exceeds a certain value.
    • Ground Fault Circuit Interrupter (GFCI) - A special outlet that acts as a circuit breaker.
      • Usually found in kitchens and bathrooms (where water is near the electrical outlet).

Homework:
    Pg. 607, #1-5