A Unit 4 Review

When an object is charged, it has an imbalance of electrons. Static electricity is another term used to describe this. You can charge different materials by rubbing them together. Friction! Remember, only electrons (-) move! Ben Franklin created the Law of Charges: Like charges repel Unlike charges attract Charged objects attract uncharged objects

Conductors allow charges to move freely. (eg. Copper wire) Insulators hold charges stopping their flow. (eg. plastic) Semi conductors allow some charges to flow but put up a bit of resistance. (eg. Silicon or Tungsten) Superconductors offer no resistance. They can only be made at extremely low temperatures and are usually made of metal alloys or ceramics

 Typically all circuits have 4 parts:  Source like a battery or cell.  Conductor like a copper wire  Load something that transforms electricity into another useful form like light (bulb!), sound (buzzer!), or mechanical (motor!) energy.  Control like a switch

 Current is the flow of electrons.  It is measured in amperes (A) using a galvanometer (weak current) or an ammeter (larger current).  Voltage is the potential difference in energy between two points in a circuit.  It is measured in volts (V) using a voltmeter.  Circuits can be compare to rivers (see p. 278)

 There are 2 types of circuits:  Parallel circuits (all loads are attached to the power source in branches)  Series circuits (loads share the power source)  In a parallel circuit, all the loads will have the same voltage passing through them. This means the battery may burn out faster.  In a series circuit, the loads share the electrical energy. What will happen if we add too many bulbs to a series circuit?

 All materials off resistance to the flow of electrons inside of them. For example, a good conductor offers little resistance so electrons can flow smoothly and quickly!  Common resistors in circuits would be Nichrome wire or tungsten.  Resistance is measured in ohms (Ω) using an ohmmeter.  Variable resistors can change how much resistance they offer. They can be found in dimmer switches, volume controls, etc…

 Ohm’s Law: Resistance equals voltage over current:

 Resistance increases if:  Length increases  Area decreases  Temperature increases  The material has a high resistance

TransformationDevices Thermal  Electrical Thermocouple Thermopile Thermo-electric generator Electrical  Motion Piezoelectric crystal Motion  Electrical Piezoelectric crystal Electrical  Light LED (light emitting diode) Light  Electricity Photovoltaic cell

 An electrochemical cell (aka voltaic cell) has the following parts:  2 metal electrodes (ex. Copper and Zinc)  An electrolyte (contains dissolved ions/charged particles).  A conducting wire connecting the two electrodes.  Ions from the negative electrode (anode) to the positive electrode (cathode) through the conducting wire.  Eventually, the negative electrode gives up all its electrons (you can see it shrink visibly!) and the cell dies.

 Primary cells cannot be recharged.  Secondary cells can be regcharged.  Wet cells have a liquid electrode.  Dry cells have a solid electrode.  When cells are connected in series, you get a battery

An electromagnet   Electromagnets are created when current is passed through an iron core.  The more batteries or the more coils, the stronger the magnet.  No current = no magnet  Generators transform mechanical energy  electrical energy.  In a generator, a coil of wire rotates within a field magnet and produces electricity.  Normally current is alternating (AC) but if we add a split ring commutator, then you create a DC generator.

 The current entering our homes is AC current. This current can be “stepped up.”  Motors convert electrical energy  mechanical energy.  An electromagnet rotates within a field magnet. When magnets repel & attract, this created a rotating motion. This motion allows for work to be done!  Please see diagrams in Topic 6.

 Power Generating Station   Step up transformer   High Voltage Power lines   Low Voltage Power lines   Step down transformer  Your Home  Meter   Service Panel & Main Circuit Breaker   Branch Circuit Breakers   Branch Circuits in each room

 Power = E/t = I x V  Power is measured in Watts.  Energy is measured in Joules  Time is measured in seconds  Current is measured in Amperes  Voltage is measured in Volts  Efficiency = (E output /E input ) x 100%  Try practice problems on p. 325 & 325 & 329

 Electrical energy can come from non-renewable sources:  Fossil fuels (coal, natural gas, etc.)  Nuclear  Electrical energy can come from renewable sources:  Biomass-- Wind  Solar-- Hydroelectric  Geothermal

 All methods of energy production have their pros and cons. Read about it in topic 8!  For example: The burning of fossil fuels creates are pollution.  Technology like scrubbers can help reduce the amount of harmful gases released into the air.