Electric Current and Circuits Electrons will flow if there is a difference in electric pressure. Electric pressure is called Potential, and is measured in Volts. If there is no difference in pressure from one location to another, the electrons will not flow. In other words, if there is no Potential Difference, V from one location to another, there will be no electric current. HEAT will flow if there is a difference in temperature 2 nd Law of Thermodynamics: Heat flows spontaneously from an object of higher temperature to an object of lower temperature. Current CUENT: a flow of charged particles (usually electrons) through a conductor Current, I, is measured in amperes, A, or amps. I = charge time Andre Ampere Fluids will flow if there is a difference in fluid pressure. The greater the difference in pressure, the greater the flow! Example:, =,, What is the current, I, if 8 C of charge passes through a wire in 2 seconds? I = q / t I = 4 amps How long would it take for 3.2 x 0 8 electrons to pass through the wire? t = q / I But what is the charge, q? q = (3.2 x 0 8 )(.6 x 0-9 C) = 0.52 C t = 0.52 C / 4 A = 0.28 seconds
The Damage Caused by Electric Shock ma = 0.00 A Mild shock can be felt 5 ma = 0.005 A Shock is painful 5 ma = 0.05 A Muscle control is lost 00 ma = 0. A Death can occur 60W light bulb - 0.5 A Starter motor 20 A Clothes dryer 8 A Iron- 3 A Circuits This potential difference (Voltage) is sometimes called the emf, ε (electromotive force) Examples of charge pumps: batteries, solar cells, generators, power supplies ε Circuits ELECTIC CICUIT: Charges moving in a closed loop A circuit requires a both a conductor, usually metal wires, and a charge pump. CHAGE PUMP: a device that provides a potential difference so that charges keep moving. Alessandro Volta The source of the electrons moving in the circuit is NOT the battery or the wall outlet! The free electrons are contained within the wire itself. An individual electron does not actually travel all the way around a circuit. One electron bumps into the next that bumps into the next that bumps into the next.. It is the ENEGY that gets transferred all the way around the circuit. You are not buying electrons from your electric companyyou already have them! You are buying energy! The Potential Difference, V, provided by the charge pump is called its VOLTAGE. V = 9 V If the voltage of a battery is 9 V, this means there is a difference of 9 V of potential (pressure) between the positive terminal and the negative terminal. V =.5 V If the voltage of a battery is.5 V, this means there is a difference of.5 V of potential (pressure) between the positive terminal and the negative terminal. esistance all conductors offer some resistance to the flow of charges, even metal wires. Generally, the thicker the wire, the less the resistance. The pressure DIFFEENCE, the VOLTAGE, is required for charges to flow! So, charge pumps such as batteries, are called voltage sources. 2
esistance ESISTANCE = voltage V = = current I The unit for resistance is the OHM, Ω. This equation is often called OHM S LAW George Ohm- first determined the math relationship now called Ohm s Law WATE ANALOGY Water Electricity flow of water current flow of charge water pump keeps flow going charge pump p.s.i. pressure volts type of pipes resistance type of conductor PUMPS CUENT PESSUE ESISTANCE Example = What is the resistance of an appliance if 2 amps of current run through it when supplied with 20 V? = V / I = 20 V / 2 A = 60 Ω Certain metals offer less resistance to the flow of charges than others. Example: Copper is a better conductor than iron The resistance of a wire of length L and cross sectional area A is given by ESISTANCE, = L ρ A where ρ is the resistivity of that specific metal. Small electrical components called resistors are inserted into circuits to control the amount of current flowing through that part of the circuit. The resistivity also increases as temperature increases. So, a hot wire resists the flow of charges more than a cooler wire. 3
SEIES CICUITS Series and Parallel Wiring If one bulb was disconnected or if one bulb was burned out, what would happen to the other bulbs that are in series with that bulb? www.physicsclassroom.com Schematic Symbols for circuits SEIES CICUITS EQUIVALENT ESISTANCE: What is the net resistance? What one resistor could replace a group of resistors? For resistors wired in series, the equivalent resistance is given by: eq = + 2 + 3 + 4
Using Ohm s Law in a Series Circuit: = V = I # #2 #3 #4 #5 #6 #7 #8 #9 What is V + V 2 + V 3? PAALLEL CICUIT The current in a series circuit is everywhere the same. Charge does NOT pile up and begin to accumulate at any given location such that the current at one location is more than at other locations. Current is the same at the first resistor as it is at the last resistor as it is in the battery. I battery = I = I 2 = I 3 =... where I, I 2, and I 3 are the current values at the individual resistor locations. If one bulb is disconnected or is burnt out, what would happen to the other bulbs that are wired in parallel with that bulb? Potential Difference in a Series Circuit PAALLEL CICUIT For resistors wired in parallel, the equivalent resistance is given by: V = I In series circuits, the resistor with the greatest resistance has the greatest voltage drop. eq = + 2 + 3 +... 5
Easy Effects of series and parallel resistors on current More difficult eq = Use the X - + 2 +... button on the calculator! 5 + 7 X - + 2 X - = Answer + X - X - 3 Circuits and Ohm s Law: = V total = 2 V V total = 2 V #2 What is the total current in the series circuit? # 3 What is the total current in the parallel circuit? Example: What is the equivalent resistance of a 20 Ω, 30 Ω, and 40 Ω resistor wired in series? In parallel? # 0 Series: eq =? # Parallel: eq =? (two decimal places) Current in parallel branches There s much less resistance if resistors are wired in parallel than if they re wired in series. With less resistance, the charge pump will able to push much more current around the circuit. 6
Voltage drop in parallel branches Where is the voltage exactly the same? V A = V B = V C = V E = V G and V D = V F = V H = V I = V J SO = = = Ohm s Law V = I A 6 Ω and a 4 Ω resistor are wired in parallel across a 9 V battery. Determine the current in the circuit. - First find the equivalent resistance, then the current. = 2.4 Ω I = V / I = 3.75 A How much current goes through the 6 Ω resistor? - esistors in parallel have the same potential difference (voltage) across them, even if they have different resistance. I = V / I = 9 V / 6 Ω =.5 A How much current goes through the 4 Ω resistor? I = 9 V / 4 Ω = 2.25 A Calculating current in parallel branches: = Combination Circuits # 4 I =? #5 I 2 =? # 6 I 3 =? # 7 What is the total current in the entire circuit? Characteristics of Series and Parallel Wiring Series: If one component goes out, They all go out! As more resistors are added, the equivalent resistance Increases! which means that the current in that part of the circuit Decreases! Parallel: If one component goes out, The rest still work! As more resistors are added, the equivalent resistance Decreases! which means that the current in that part of the circuit Increases! 7
For maximum resistance- use series wiring. For minimum resistance- use parallel wiring. Analyzing Combination Circuits. 2. 3. For resistors wired in series the same current flows through each one, however the potential difference, voltage, is additive Voltage gain through battery = Sum of Voltage drop through resistors For resistors wired in parallel, the potential difference, voltage, is the same for each of them, however the current is additive. Total Current pushed by battery = Sum of Current going through all the resistors Ohm s Law: V = I 4. What is the total resistance of the circuit? 5. What is the total current in the circuit? I total = V total / total 8
5 Ω 4 A 4 A 20 V 2 A 6 V 2 A 6 V 4 A 24 V total = 5 Ω + 4 Ω + 6 Ω total = 5 Ω I total = V total / total = (60 V) / (5 Ω ) I total = 4 Amp V = I Schematic diagrams - symbols to represent circuit components. wires: Charge pumps: esistors: ALL of the current goes through esistors and 4, so I = I 4 = 4 A But, the current splits at 2 and 3. Since they have equal resistance, I 2 = I 3 = 2 Amp V = I = (4 A) (5 Ω ) V 2 = I 2 2 = (2 A) (8 Ω ) V 3 = I 3 3 = (2 A) (8 Ω) V 4 = I 4 4 = (4 A) (6 Ω) V = 20 V V 2 = 6 V V 3 = 6 V V 4 = 24 V Switches: Ground: All devices connected to a circuit (light bulbs, TV s, toasters, etc.) resist the flow of charges and are sometimes drawn as a resistor in the circuit (if you re considering the unit as a whole). V = I 6. What is the equivalent resistance of 2 and 3? 7. What is the total resistance of the circuit? 8. What is the total current in the circuit? 9. What is the potential difference across? 0.What is the potential difference across 4?.What is the potential difference across the combination of 2 and 3? 2.What is the current through 2? 3.What is the current through 3? 4.Which resistor will have a greater current, 3 or 4? Ohm s Law: Voltage, esistance, Current = = = 9
Electric Power Power is the rate that work is done or energy is transferred, that is Power = Energy(Joules) Time(sec onds) Power is measured in Watts, W Examples How much energy does a 75 W light bulb give off in five minutes? Power = Energy / time Energy = Power x time Energy = 75 W x (5 x 60) seconds Energy = 22500 J What is the power output of a 3 A motor running on regular house voltage? P = IV P = 3 A x 20 V P = 360 W Electric power delivered to a circuit by a power supply is given by Power = Current x Voltage P = IV Power lines The higher the current, the more the wires in the circuit heat up, thereby wasting energy. This is a big problem when electric companies must provide electricity at great distances away from the power plants. The solution: Electric lines that carry current great distances are at very high voltage, so the current can be relatively small. P = IV P = IV Electrons do NOT leave the circuit- Energy leaves the circuit through the different resistors in the form of light, heat, and any kind of work done by the appliance the current is running through. The rate that the energy leaves the circuit is the power output. High Voltage / Low Voltage TANSFOMES: devices that stepup the voltage at the power plant and then step-down the voltage at the customers location. 0
Even with very high voltage, there is still some current running through those wires. Therefore, some electrical power (dissipated through heat) is lost. If the entire length of wire has a total resistance, the power lost along the way is given by Power lost (dissipated) = I 2 Voltmeter- device to measure voltage The voltmeter is placed in parallel with the component whose voltage is being measured. Ammeter- device to measure amps (current) The current must flow THOUGH the meter, therefore an ammeter is placed in series with the component whose current is being measured. Circuit applet Ohmmeter- device to measure resistance esistance is measured with the power OFF! 2.3 Ω
Multimeter- can measure current, resistance, and voltage! Open circuit Closed circuit Parallel Circuit Series Circuit Electrical Safety eq = + + +... 2 3 Each bulb has the same voltage across it! That voltage is the voltage of the battery. However, the current splits up so that each bulb only receives a portion of the total current pushed by the battery. If one of the bulbs goes out, the others remain lit, and will have EXACTLY the same brightness (WATTS) as they did before the bulb went out. P = V 2 / eq = + 2 + 3 + Each bulb has the same current running through it! That current is the TOTAL current pushed by the battery, I T = V battery / eq If one of the bulbs goes out, they ALL go out! Fuse: a short piece of metal that melts if current exceeds a set value (to protect device) Circuit breaker: an automatic switch that opens the circuit when current exceeds a set value. (uses a bimetallic strip) Electrical Safety Ground-fault circuit interrupter: opens a circuit if the current going into a device is not the same as the current coming out of the device. gfci 2
Short Circuit- a circuit that is formed when wires touch. This effectively shortens the path of the circuit because the current no longer passes through the resistor. The extremely low resistance in the circuit produces very high current, which could cause melt down and fire. Ground wire: provides a safety valve an additional pathway for charges in case of high current And some miscellaneous topics NOT a good idea!! Light bulbs Power = I 2 The brightness (Watts) of a light bulb depends on both the current going through the bulb and the resistance of the bulb filament. Which one has a greater influence on the brightness- the current or the resistance? Thicker wires can carry more current. Thinner wires have more resistance. So, which bulb has a thicker filament- a 40 W or a 00 W? 3
Diode- allows only one direction of current to pass through it. Light emitting diode, LED As electrons pass through these semiconductors, light is emitted Transistors are devices that are often used to amplify small signals into a larger one. Alternating current means that the voltage pushes back and forth, not just one direction. The electrons just wiggle back and forth. In the US, they wiggle back and forth 60 times every second. In other words, the frequency is 60 Hz. Power lines The higher the current, the more the wires in the circuit heat up, thereby wasting energy. This is a big problem when electric companies must provide electricity at great distances away from the power plants. The solution: Electric lines that carry current great distances are at very high voltage, so the current can be relatively small. P = IV P = IV The first use of alternating current in the US was at the Chicago World s Fair, in 893, where Nicholas Tesla s alternating current circuits were chosen instead of direct current, which was backed by Thomas Edison and his General Electric Company. Alternating Current, AC The electricity in your houses is alternating current, AC, not direct current, DC as in batteries. The main reason for this is that transformers can only step up or step down the voltage if it is alternating current. Without transformers, there would be great loss of electrical power between the power stations and your houses. For electricity to be very efficient, there would have to be a power generating plant in EVEY community! Tesla Coil Alternating current Systems for wireless communication (radios, remote controls) robotics electrotherapy wireless transfer of electricity x-ray tubes arc lights concepts for electric vehicles devices for lightning protection concepts for vertical takeoff aircraft 4
Ferris designed and built the first 264 foot (80 meter) wheel for the World's Columbian Exposition in Chicago, Illinois in 893. This first wheel was 26 stories tall and could carry 2,60 persons. There were 36 cars accommodating 60 people each (40 seated, 20 standing). It took 20 minutes for the wheel to make two revolutions - the first to make six stops to allow passengers to exit and enter; the 2nd a single non-stop revolution - and for that, the ticket holder paid 50 cents. The wheel was moved twice after the 893 Fair and was eventually disassembled in 906. 5