Electricity Electric Current and Circuits

Electricity The energy associated with charged particles as they move from place to place These particles are usually electrons but any charged particles will do such as protons Electric charge is a property of electrons (negative) and protons (positive) When particles with opposite charges come near one another, they attract each other or repel—if they have the same charge Electrical charge is measured in coulombs

Electric Charge Like charges repel Unlike charges attract

Electric Field A region surrounding a charged particle in which forces of attraction or repulsion are noticeable. To measure the intensity of an electric field, place a standard charge & measure the force acting on the region-example: generator

Electricity Terms Electrical potential difference = force required to move electrons; voltage Current = flow of electrons from negative to positive terminals in voltage sources Resistance = resistance of the flow of electrons; users of current electricity; depends on conduction & thickness of wire

POTENTIAL DIFFERENCE Found between 2 points; also called voltage Determined by the amount of work needed to move charges Based on the concentration of charges between the 2 points Formula: Potential difference = field intensity x distance V = E d

Flow of Charge Difference in electric potentials causes charge flow from one end to another until the ends reach the same potential and then there would be no more difference

REMEMBER: VOLTAGE OR POTENTIAL DIFFERENCE HAS TO HAVE “2 POINTS” THAT “CONNECT” FOR THE CURRENT TO BE CARRIED OR TO EXIST. Why do birds sitting on an electrical wire not get “electrified”?

How come a bird on a wire doesn't get shocked? When the bird perches on a live wire, her body becomes charged--for the moment, it's at the same voltage as the wire. But no current flows into her body. A body is a poor conductor compared to copper wire, so there's no reason for electrons to take a detour through the bird. More importantly, current flow [electrons]is from a region of high voltage to one of low voltage. The drifting current, in effect, ignores the bird. But if a bird (or a power line worker) accidentally touches an electrical "ground" while in contact with the high-voltage wire, she completes an electrical circuit. A ground is a region of approximately zero voltage. The earth, and anything touching it that can conduct current, is the ground.

Sources of Electric Energy Charges do not flow unless there is a potential difference—use an “electric pump” or voltage source ie. Battery Types of voltage source include: wet cells(CAR BATTERY), dry cells(FLASHLIGHT BATTERY), generators All supply energy which allows the charges to move due to energy transformations

Voltage Sources Dry Cells-Batteries Wet Cells Generators

Voltage Source Something that provides potential difference is known as a voltage source. To have a sustained current it is required to have a sustained potential difference. Ex. Dry cells, wet cells, and generators (batteries are just 2 or more dry cells)

Voltage source – an “electrical pump”---this is where “electrons” are produced—any type of battery or generator.

Voltage A measure of the energy available to move electrons The electrical potential difference between two points (based on the concentration of electrons): it IS the force needed to move charges Voltage is measured like potential energy Voltage as it is used “ drops”= battery is DEAD

Electric Shock Voltage doesn’t kill, but current does. Wet skin decreases resistance and increases the current. Rubber shoes add resistance.

Ohm’s Law-Current flow Electric current – the flow of electric charge; Measured in AMPERE or Amps (A) To obtain a higher voltage, higher charge is needed; higher voltage = electric field to be more intense = more electrons to move --greater current flow = DIRECT RELATIONSHIP

Resistance Resistance: measure of how easily current flows through a circuit –Higher Resistance: harder for charge to go through –Low Resistance: easier for charge to go through. Resistance increases current goes down Resistance decreases current goes up Resistance is measured in Ohm’s Two types of Resistors –Fixed: always has the same value –Variable: value changes

Electrical Resistance Resistance to the flow of charge. The thicker the wire, lower the gauge, or shorter the wire the less resistance. Increase in resistance- hotter temperatures R=Resistance=measured in Ohms (Ω)

Electrical resistance – the resistance of a material to the flow of an electric current through it –Measured in OHMS (Ω ) Named after Georg Simon Ohm

OHMS’ LAW V = voltage I = current R = resistance V = I R DIRECT RELATIONSHIP : BETWEEN CURRENT & VOLTAGE INVERSE RELATIONS: BETWEEN CURRENT & RESISTANCE

OHM’S LAW Law states: “the current that flows through a given wire varies directly with the applied voltage” Formula: i = v / R

Electrical Currents Direct Current flows in one direction only DC is produced when stored electrical energy is trapped or stored Batteries and Solar Cells have this electrical potential Alternating Current changes direction periodically AC is produced by a generator Most AC in the U.S is 60 cycle.

Circuit Schematics Electric circuits are connected with metal conductors which are represented by straight lines----see diagram provided Electrons will move from negative terminal (short line) to positive terminal (larger line)- see diagrams---terminal is also the voltaic source or potential difference

CIRCUITS: Circuit – any complete path along which a charge can flow

CIRCUIT DIAGRAMS Be sure you know the symbols provided on the hand-out. You will be required to draw circuit diagrams & use Ohm’s Law with the diagrams. Two types: picture or schematics

Schematic Diagrams battery connecting wire open switch

Circuit diagrams A circuit is “complete” when the conducting wires form a complete line to the voltaic source **** see example of complete circuit Note how ammeters are drawn as part of the circuit-----see example Note how voltmeters are drawn “across” the circuit-----see example Resistors (use the current are part of the circuit)

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Calculating Electrical Power Power = Voltage(V) x Current (i) Power is measure in Watts Formula: P = V i P= watts; V= volts; i = amps (A) Power is the rate at which electrical energy is converted to another form such as mechanical, heat or light energy

Electric power = current X voltage ( watt = ampere X volt)

END OF FIRST PART OF NOTES

Electric Circuits Structures that provide paths through which energy (electrons) travels –Two types Parallel Circuit Series Circuit Symbols used with circuits—see hand-out

Electric Circuits

Series Circuit Current can only travel in one path—SEE EXAMPLE IN HEWITT BOOK

Series Circuit Series circuit: break in the circuit, stops the current Resistors connected in “series”[one-after the other], the current travels through EACH resistor exactly the same way Current encounters resistance by each resistor—called “voltage drop”; opposed by the sum of the resistors; total resistance is the sum of the number of resistors in circuit- so, the current is DECREASED WITH EACH RESISTOR

Series Circuits The current through each resistor varies according to the size of the resistor; the voltage depends on the resistance. The sum of the voltage drops across the resistors equals the battery voltage. (19-2)

Series Circuits-math Formula to find “total resistance” –R T = R 1 + R 2 + etc. To find the current flow: use Ohm’s Law The sum of the voltage is equal to the voltage of the entire circuit—or voltage drop of circuit

Resistance in Series Circuits Add the sums of each resistance to get the total resistance. 3 ohms 2 ohms

Series Circuits From this we get the equivalent resistance (that single resistance that gives the same current in the circuit)= TOTAL RESISTANCE. (19-3)

Parallel Circuit Current can follow multiple paths—SEE EXAMPLE IN HEWITT BOOK

Parallel Circuits A parallel connection splits the current; the voltage across each resistor is the same:

Parallel Circuit Placing resistors in parallel always decreases the total resistance of the circuit! Total resistance decreases because each new resistor has a “different” path for the electrons to flow Circuit “divides” the current by the number of branches found in the circuit

PARALLEL CIRCUITS CURRENT: amount of current flow is decreased due to the use of each resistor—divides by the # of “branches”. BUT, the current flow is THE SAME IN EACH BRANCH RESISTANCE: is DIVIDED (1/R t ) through EACH of the branches VOLTAGE is the same for EACH branch

Parallel Circuits This gives the reciprocal of the equivalent resistance: As the number of parallel branches is increased, the overall resistance of the circuit is decreased. Overall resistance is lowered with each added path between any two points of the circuit. This means the overall resistance of the circuit is less than the resistance of any one of the branches.

Resistance in Parallel Circuits The equivalent resistance is less than the sum of the resistances. 6 ohms

Parallel Circuit-math Formula to find total resistance for a parallel circuit: –1/R T = 1/ R 1 + 1/ R 2 + 1/ R 3 + etc. ** Find the “reciprocal “ of the fraction & divide Total current is the “sum” of the currents in the separate branches---also called EQUIVALENT Voltage is equal to the voltage of the generator.

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SERIES / PARALLEL CIRCUITS Find the total resistance in the parallel circuits then, add this resistance to the individual series resistors---use the sum of the “entire” circuit. SEE DIAGRAM

CIRCUIT BREAKERS/FUSES Fuses: automatic switches that prevent circuit over-loads Circuit breakers: automatic switches that prevent circuit over-loads by “breaking” the path of the current

Electric Hazards Fuses-To prevent overloading fuses are connected in a series along the supply line, directing all current through the fuse. Above a certain amount of current the fuse will melt causing it to blow out. Circuit Breakers-Today in more modern homes circuit breakers are used. Such circuit breakers use magnets or bimetallic strips to open the switch when they get hot. These are more convenient because they can just be switched back on rather than having to be replaced.

Electric Hazards Sometimes insulation can wear away. This shortens the path of the circuit and therefore is called a short circuit. A short circuit draws a dangerously large current because it bypasses the normal circuit resistance.

END OF SERIES/PARALLEL CIRCUIT NOTES

Electric Hazards Sometimes insulation can wear away. This shortens the path of the circuit and therefore is called a short circuit. A short circuit draws a dangerously large current because it bypasses the normal circuit resistance.

Magnetism Magnetic means the ability of forces on magnets or other magnetic materials Types of Magnets –Permanent magnets are those magnets that keep their magnetic properties, even when it is not close to other magnetic

Properties of Magnets All magnets have the same properties –Have to opposites called poles one is north and the other is south –All magnets exert magnetic forces on each other –These forces depend on the direction of the poles. Remember opposites attract, likes repel. –Magnetic fields describe how a magnet exerts a magnetic force.

Electromagnets Electromagnets: are magnets that are created when electric current flows through in a wire. Change the current you can increase or decrease the strength of the electromagnet Electromagnetic force exists between the electrical force and the magnetic force If you move a magnet through a coil you create an electrical current, this is called electromagnetic induction

Generators and Transformers Generator: is a combination of mechanical and electrical systems that converts kinetic energy into electrical energy. Transformers: transforms electrical voltage from a higher voltage to a lower voltage and reverse.