OHM’S LAW AND CIRCUITS Current Electricity

Electric Current, ( I ), is the flow of electric charge. More specifically, it is defined as the ______ at which charge flows. Units: ____________, which is defined as 1 _______ (____ for short), with the symbol ___ (1 ___ = 1 ____= 6.25x10 18 ___) rate Coulombs/sec Ampere Amp A AC/se - /s

“It’s the _____, not the ______ that produce electric shock”….. Current in AmpsEffect on human body (1 mA)Can be felt (5 mA)Painful (10 mA)Involuntary Muscle Spasms (15 mA)Loss of Muscle Control (70 mA) If through heart, serious disruption; probably fatal if > 1 second ( mA) Uncontrolled “twitching” of heart > 0.2 (> 200 mA) Heart stops, but may be able to be revived easier than 0.1 – 0.2 A AmpsVolts

Voltage, V, is the “electric potential” measured in ______ (_______________) In the same way that heat flows from one end of a conductor to the other when the two ends are at different temperatures, charges flow from one end of a conductor to another when the two ends are at different ________________. We say that there is an electric ________________ between the two ends. For example, if a wire were attached between a Van de Graaff generator at 100,000 Volts and the ground (0 Volts), charge would ______________________. VoltsJoules/Coulomb electric potentials “potential difference” flow through the wire

…But, the flow would be very brief. The surface of the Van de Graaf generator would quickly reach a common potential with the ground. In order to sustain a flow of charge, the difference in potential must somehow be maintained. The situation is analogous to the flow of water… The flow will continue until the water levels are the same and thus there is __ pressure difference. The pressure difference could be maintained by adding a _______. Pictures from Hewitt’s Conceptual Physics pump NO

A “voltage source” performs a similar function as this water pump…. It sustains an electric potential difference as charge flows. A voltage source, then, is like an electrical pump, and voltage is essentially _________________. Two primary voltage sources: (How they work later…) Batteries (_____ cells such as 1.5 V AAA, AA, C, D, 9V, etc. and _____ cells such as 12 V car battery) Generators (both at the power plant and personal gasoline- powered generators) electric pressure Dry wet

Voltage and Current, then, can be thought of as a cause and an effect…. _______ is the cause, & _______ is the effect. So, a 1.5 V battery has a 1.5 V potential difference between the (+) and (-) terminals. When connected to a small light bulb via wires, that difference “pushes” electrons through the circuit, from the (-) terminal to the (+) terminal, supplying 1.5 Joules of energy per coulomb of charge that flows through the circuit. Because of historical precedent, the current direction in textbooks is still drawn in the direction in which positive charge carriers would move. + - e-e- I VoltageCurrent

Examples/Applications: How can birds perch or squirrels run along high voltage (1000’s of volts) wires and not be fried?? To receive a current (shock) there must be a __________ in potential between one foot and the other, but every part of the bird or squirrel is at the _____ potential as the wire. IF they landed with one foot on one wire and the other foot on a neighboring wire at a different voltage, ZAP!!!!! difference same

A “hot wire” is one that is NOT electrically neutral. What would happen if Joe Anatomy, while attempting to fix a light fixture, would touch a 120 V “hot wire” with one hand while touching a neutral or ground wire with the other hand? If you were to grab a “hot wire” with your bare hand, the current could stimulate the muscles in your hand to contract, causing your hand to close and clamp around the wire (BUMMER!) So, if testing whether a wire is “hot”, use the _____ of your hand (or use a meter!) BACK

Resistance (R) While voltage causes current, the amount of current produced also depends on the electrical resistance (electrical “________”) that the conductor offers to the flow of charge. The components of a circuit (such as a light bulb filament, a “resistor”, a motor, etc.), as well as the _____ in the circuit have electrical resistance. Color-coded Resistors The resistance depends fundamentally on the material’s “conductivity” (how good of a conductor the material is.) friction wires

Back to our water analogy… For a given pressure, more water will pass through a ______ pipe than a ______ one. We could say the _____ pipe, then, offers more “resistance”. The resistance of a wire depends on three things: The __________ of the wire The ________ of the wire The _______ of the wire conductivity thickness length thickerthinner

In the same way, for a given voltage, more electric current would pass through a _______ wire than a _______ one. The ________ wire offers more resistance. Logically, a _______ pipe would offer more resistance to the flow of water than a _______ pipe. In the same way, the _______ a wire is, the greater the resistance. thickerthinner longer shorter longer Example: Wire ___ has the most resistance and Wire ___ has the least resistance. b a L

Unit of Resistance: _____; Symbol = ___ We have just seen that the resistance of a wire depends on its thickness, length, and its conductivity. The following formula quantifies the relationship:  = “resistivity” – a measure of how conductive the material is. A constant at a given temperature. (in  m) L = length of wire (in m) A = “cross-sectional” area (in m 2 ) The higher the “gauge” number, the smaller the cross-sectional area. (A=  r 2 for circular wires.) “ohm” 

Metals (good conductors) have an extremely ______ resistivity. Insulators have a very ______ resistivity. As the temperature of a material increases, the resistance __________. The resistance of some materials becomes ZERO at very low temperatures… _______________ (More details later!) small large increases superconductors

Other Voltage Sources Power companies use large generators to provide the 120V delivered to home outlets The alternating potential difference between the two holes in the outlet is 120V When something is plugged in 120J of energy is supplied to each coulomb of charge that is within the circuit In one second,electricity travels 300,000km

Voltage Sources In the U.S. a typical home is set up for 120V In Europe it is 220V Some appliances ( stove, air conditioners etc. ) require 220V Electric service in the U.S. is three wire: one positive wire at 120V, one Negative wire at 120V and a neutral wire at 0 Normally the neutral is connected to either the positive or negative. When more voltage is needed the positive is connected to the negative

Ohm’s Law  Georg Simon Ohm – physicist that tested different wires to see what effect the resistance of the wire had on the current  Discovered Current = Voltage ÷ Resistance  I = V/R or V=IR  ex. What is the resistance of an appliance that draws 12A of current when connected to a 120 V circuit?  Ans: R = 120V ÷12A = 10Ω

Example What would a shock from a 120V battery do to your body if your resistance was at 100,000Ω? Ans. I = V/R I = 120V ÷ 100,000Ω I = A You would feel a little “zap” CurrentEffect 0.001Can be felt 0.005Painful 0.010Muscle Spasms 0.015Loss of muscle control 0.070Possibly Fatal

Power Power is the rate of conversion of electrical energy Converts electrical potential energy into non- electrical forms of energy Unit = Watt Wattage (found on lightbulbs) is used to show how much heat and light will be given off from a bulb

Power Power can also be described as the rate of energy transfer Power companies charge for energy not power Charge in kilowatt-hours (kWh) which is the energy delivered in 1 hour at a constant rate of 1 kW P=IV (Poison Ivy Equation) V = IR P= I (IR) P = I 2 R

Circuits Recall: Direct Current – flows in one direction Alternating Current – Cycles back and forth Diagram: current time DC current time AC

Circuit Circuit: Path along which electrons can flow Series Circuits: Only one pathway for electrons to flow Each device has the same current Turn off one and they all turn off As more resistance is added:  Total resistance increases  Voltage is split between resistors

Series Equations In series I = constant -> (“SIC” series current constant) V total = V 1 + V 2 + V 3 +…….. R total = R 1 + R 2 + R 3 + ……

Parallel Circuit (household) Parallel Circuits: More then one pathway for electrons to flow Each device has the same voltage drop Turn one off the others stay on As you add more resistors:  total resistance decreases  current increases

Parallel Equations V = Constant (“PVC” parallel voltage constant) I total = I 1 + I 2 + I 3 + ….. R total = R 1 R 2 R 3 Remember: In a series circuit current is constant (SIC). In a parallel circuit voltage is constant (PVC).

Series-Parallel Circuit Combination circuit. Find R t for the two parallel circuits. Then add those up because they are in series with each other.

Capacitance A capacitor is a device used to store electric potential energy Stored energy in a capacitor can be reclaimed when needed Parallel Plate Design: 2 metal plates (separated by a small distance) connected to the two terminals of a battery  Charging process stops when potential difference on plate = potential difference of battery After disconnection from voltage source capacitor will discharge (ex: flash on camera)