Presentation on theme: "Electricity & Magnetism"— Presentation transcript:
1Electricity & Magnetism Current Electricity:Direct Current Circuits, Ohm’s Law, Resistance, Electric Power, Equivalent Resistance, Kirchoff’s Rules
2Current Current is defined as the flow of positive charge I = Q/t I is current in Amperes (A)Q is charge in Coulombs (C)t is time in seconds (s)In a normal electric circuit, electrons move to carry charge – the current is opposite from the movement of electrons
3Practice #1How many electrons per hour flow past a point in a circuit if it bears 11.4 mA of direct current?If the electrons are moving north, which way is the current?
4Sample problemHow many electrons per hour flow past a point in a circuit if it bears 11.4 mA of direct current?If the electrons are moving north, in which direction is the current?
5Cells Cells convert chemical energy into electrical energy The potential difference (voltage) provided by a cell is called its electromotive force (emf)The emf of a cell is constant until near the end of the cell’s useful lifetimeThe emf is not really a force! It’s one of the biggest misnomers in physics!
7Battery A battery is composed of more than one cell in series The emf of a battery is the sum of the emf’s of the cellsPractice Problem #2:If a typical AA cell has an emf of 1.5 V, how much emf do 4 AA cells provide?Draw the battery composed of these 4 cells.
8Sample problemIf a typical AA cell has an emf of 1.5 V, how much emf do 4 AA cells provide?Draw the battery composed of these 4 cells.
19Conductors & Insulators Conduct electricity easilyHave high “conductivity”Have low “resistivity”Metals are examplesWires are made of conductorsDon’t conduct electricity easilyHave low “conductivity”Have high “resistivity”Rubber is an example
20ResistorsResistors are devices put in circuits to reduce the current flowResistors are built to provide a measured amount of “resistance” to electrical flow, and thus reduce the current
21#7Draw a single loop circuit containing two resistors and a cell. Draw voltmeters across each component.
22Sample problemDraw a single loop circuit containing two resistors and a cell. Draw voltmeters across each component.V
23Resistance, RResistance depends on resistivity and on geometry of the resistorR = ρL/Aρ : resistivity (Ωm) from the materialL: length of resistor (m)A: cross sectional area of resistor (m2)Unit of resistance: Ohms (Ω)
24Practice #8What is the resistivity of a substance which has resistance of 1000 Ω if the length of the material is 4.0 cm and its cross sectional area is 0.20 cm2?
25Sample problemWhat is the resistivity of a substance which has a resistance of 1000 W if the length of the material is 4.0 cm and its cross sectional area is 0.20 cm2?
26#9What is the resistance of a mile of copper wire if the diameter is 10.0 mm?(resistivity of copper is 1.72 x 10-8 Ωm)
27Sample problemWhat is the resistance of a mile of copper wire if the diameter is 10.0 mm?
28Ohm's LawResistance in a component in a circuit causes potential to drop according to the equation:ΔV = IRΔV : potential drop (Volts)I: current (Amperes)R: resistance (Ohms)
29Practice #10Determine the current through a 333 Ω resistor if the voltage across the resistor is observed to be 1.5 V.
30Sample problemDetermine the current through a 333-W resistor if the voltage across the resistor is observed to be 1.5 V.
31Practice #11Draw a circuit with a AA cell attached to a light bulb of resistance 4 Ω.Determine the current through the bulb.
32Sample problemDraw a circuit with a AA cell attached to a light bulb of resistance 4 W.Determine the current through the bulb. (Calculate)
33Ohmmeter Measures Resistance. Placed across resistor when no current is flowing.W
34Ammeter An ammeter measures current It is placed in the circuit in a series connectionAn ammeter has very low resistance, and does not contribute significantly to the total resistance of the circuit
35Power In General In Electrical Circuits P = W/t P = ΔE/Δt Units: WattsJoules/secondP = I Δ VP: Power (W)I: Current (A)Δ V: Potential difference (V)P = I2RP = (Δ V)2/R
36Practice #12How much current flows through a 100-W light bulb connected to a 120 V DC power supply?What is the resistance of the bulb?
37Sample problemHow much current flows through a 100-W light bulb connected to a 120 V DC power supply?What is the resistance of the bulb?
38#13If electrical energy (power x time) is 5.54 cents per kilowatt hour, how much does it cost to run a 100-W light bulb for 24 hours?
39Sample problemIf electrical power is 5.54 cents per kilowatt hour, how much does it cost to run a 100-W light bulb for 24 hours?
40Resistors in circuitsResistors can be placed in circuits in a variety of arrangements in order to control the currentArranging resistors in series increases the resistance and causes the current to be reducedArranging the resistors in parallel reduces the resistance and causes the current to increaseThe overall resistance of a specific grouping of resistors is referred to as the equivalent resistance
42Kirchoff's 1st Rule 3.0 A I4 4.0 A 1.5 A Practice Problem #15 Kirchoff’s 1st Rule is also called the Junction RuleThe sum of the currents entering a junction equals the sum of the currents leaving the junctionThis rule is based upon conservation of chargeFind the current I4 (magnitude and direction)3.0 AI44.0 A1.5 A
43Kirchoff's 2nd RuleKirchoff’s 2nd rule is also referred to as the “loop rule”The net change in electrical potential in going around one complete loop in a circuit is equal to zero.This rule is based upon the conservation of energy
44Practice Problem #16Use the loop rule to determine the potential drop across the light bulb.
49Mini-Lab J Draw and construct the following circuit. Predict all 3 currents. Apply Kirchoff’s 1st Rule to your current measurementsMeasure the voltage across all components. Apply Kirchoff’s 2nd Rule to your voltage measurements.
50Mini-Lab KDraw and construct a circuit containing a cell and one 330-Ω resistor.Measure the potential drop across the resistorMeasure the current through the resistorDoes ΔV = IR?I (A)R(Ω)ΔV(V)calculatedMeasuredDifference (V)
51Ohm's Law GraphMake a table of current and resistance data and graph the data such that voltage is the slope of a best-fit lineWire a circuit with a cell and one or more resistors. Calculate and record the resistance. Measure and record the corresponding current. Do this 8 times without duplicating your resistance values. You will have to use resistor combinations in addition to single resistors.Rearrange the equation V = IR so that V is the slope of a “linear” equation. Construct a graph from your data that corresponds to this rearranged equation. Calculate and clearly report the slope of the line. How does this compare to the emf of a 1.5 V for a D-cell?
52Circuit Mini-Lab ADraw a circuit containing one cell, one bulb, and a switch. Wire this circuit. Measure the voltage across the cell and across the bulb. What do you observe?
53Mini-Lab BDraw a circuit containing two cells in series, one bulb, and a switch. Wire this on your circuit board.What do you observe happens to the bulb:With two cells instead of one?When opening and closing the switch?Measure the voltage across the battery and across the bulb. What do you observe?
54Mini-Lab CDraw a circuit containing two cells in series, two bulbs in series, and a switch. Wire this on your circuit board.What do you observe happens to the bulbs when you unscrew one of them?Measure the voltage across the battery and across each bulb. What do you observe?
55Mini-Lab DDraw a circuit containing two cells in series, two bulbs in parallel, and a switch. Wire this on your circuit board.What do you observe happens to the bulbs when you unscrew one bulb?Measure the voltage across the battery and across each bulb. What do you observe?
56General Rules for Circuits How does the voltage from a cell or battery get dispersed in a circuit:When there is one component?When there are two components in series?When there are two components in parallel?
57Mini-Lab ESet up your digital multi-meter to measure resistance. Measure the resistance of each light bulb on your board. Record the results.Wire three bulbs together in series, and draw this arrangement. Measure the resistance of all three bulbs together in the series circuit. How does this compare to the resistance of the individual bulbs?Wire three bulbs together in parallel, and draw this arrangement. Measure the resistance of all three bulbs together in the parallel arrangement. How does this compare to the resistance of the individual bulbs?
58Mini-Lab FMeasure the resistance of the different resistors you have been given. Make a table and record the color of the first three bands (ignore the silver/gold band) and the resistance associated with the band color. See if you can figure out the code.
59Resistor codes Resistor codes are read as follows: It is helpful to know the code, but you will not be required to memorize it
60Mini-Lab GWhat is the equivalent resistance of a 100-Ω, a 330- Ω, and a 82- Ω resistor when these are in a series arrangement?Draw the circuitBuild the circuitMeasure valuesCalculate and compare measured and calculated values
61Mini-Lab HWhat is the equivalent resistance of a 100-Ω, a 330-Ω, and a 82-Ω resistor when these are in a parallel arrangement?DrawBuild the circuitMeasureCalculate and compare values
62Mini-Lab IDraw and build an arrangement of resistance that uses both parallel and series arrangements for 5 or 6 resistors in your kit. Calculate and then measure the equivalent resistance. Compare the values.
63#14Draw a circuit containing, in order (1) a 1.5 V cell, (2) a 68-Ω resistor, (3) a 330-Ω resistor in parallel with a 100-Ω resistor, (4) an 82-Ω resistor, and (5) a switch.Calculate the equivalent resistanceCalculate the current through the cellCalculate the current through the 330-Ω resistor