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Transcript
Current and Circuits Current Physics I Honors 1 Electric Current • Voltage (which indicates the presence of an electric field) causes charges to move in conductors provided there is a complete path in which the charges can move. Resistor Battery Current Physics I Honors Current 2 Electric Current • The rate at which electric charges move through a wire from higher potential to lower potential. • This works just like gravity. – For water flowing down a hill, the greater the elevation change (greater change in gravitational potential) the swifter the water flows. – For electrical current, the greater the electrical potential difference (or voltage) the greater the current. Physics I Honors 3 Electric Current • Electric potential (V), or voltage, is a measurement of the energy available at a location – higher voltage means more energy per electron Physics I Honors 4 Electric Current • I = Q/t • 1 Ampere = 1 Coulomb/sec – In wires, moving electrons create the current. (Electrons moving in one direction are mathematically equivalent to protons moving in the other direction.) – Conventional current says positive charges move. • In some materials (liquids, gases, semi-conductors) positive or negative changes can move. Physics I Honors 5 CURRENT AND CIRCUITS TYPES OF CURRENT Physics I Honors 6 Conventional Current vs. Electron Flow Conventional current: Direction a positive charge would flow This is opposite the direction of electron flow in a conductor! current electron flow + Physics I Honors 7 Alternating Current vs Direct Current direct current (DC) – electrons flow in one direction only i.e. batteries alternating current (AC) – the direction of flow changes (60 cycle in United States) i.e. power station Although net electron flow over one cycle is zero, can still do useful work! Physics I Honors 8 CURRENT AND CIRCUITS Resistance Current Physics I Honors 9 Resistance () Opposition to the flow of electrons Resistance depends on: – – – – Length (L) Cross-sectional area (A) {diameter} Conductivity of the material Temperature (T) • Ohm’s Law is not always valid!! • Increased temperature increases resistance Physics I Honors 10 Less Resistance Physics I Honors More Resistance 11 Resistance () • Resistance = Resistivity * Length / Area – R = L/A – Resistivity depends upon the material. Current A = Area Physics I Honors L = Length 12 Resistance () • R = V/I (Resistance = Voltage/Current) – or V = I * R It takes one volt to push one amp through one ohm 1 = 1 V/A V Physics I Honors V V 13 Resistance () R = 50 V = 9 V R = 100 V=9 V V = IR I = V/R V = IR I = V/R I = 9 V /50 ohms = .18 A I = 9 V /100 ohms = .09 A Physics I Honors 14 Current (I) • Current is the flow of charge. • Charges flow when there is a potential difference (voltage) and there is a complete path for the charges to flow. • To maintain a constant flow of charge (current), a continuous potential difference must be maintained. Physics I Honors 15 Teen dementia test • 4 electrons pass a point in a circuit every second. What is the current? • A resistor experiences a current of 2 amps and a voltage of 4 volts. What is the size of the resistor? Physics I Honors 16 Current and Circuits Circuits Schematics Use of Meters Safety Physics I Honors 17 Circuit • A path through which electrons flow. • The path is from a power source’s negative terminal, through the various components and on to the positive terminal Physics I Honors 18 Circuit • Open circuit – – a break prevents the flow of electricity • Closed circuit – – electricity can flow • Short circuit – – the electricity completes a circuit without going through the load • Load – – item using electricity Physics I Honors 19 Schematics - Symbols wire or conductor resistor or other load bulb V voltmeter battery A ammeter switch Physics I Honors generator 20 Schematics – Simple Circuit + circuit diagram A + - A Physics I Honors 21 Schematics - Complex Circuit + Construct the circuit diagram A + Physics I Honors - A 22 Meters - Ammeter • Measures current. • It is a Galvanometer wired in parallel to a resister. • Ammeters are connected in series to the circuit. • Have low resistance Physics I Honors 23 Meters - Voltmeter • Measures electric potential…voltage. • It is a Galvanometer wired in series with a resistor. • Voltmeters are connected in parallel to the load • Have high resistance Physics I Honors 24 Fuses and Circuit Breakers If an electric circuit gets “overloaded” (too much current!) fuses or circuit breakers interrupt the flow of current. Circuit Breaker Fuses Physics I Honors 25 Current and Circuits Series Circuits Physics I Honors 26 Series Circuit • Only ONE path for the electricity. – Christmas lights – Batteries in a series (more current storage) • Total current is constant and voltage decreases with additional resistors (i.e dimmer lights) Physics I Honors 27 Series - Computing Current • The current reading will be the same anywhere in the circuit. I1 = I2 = I3 … Physics I Honors 28 Series - Computing Resistance • The total resistance is the sum of the resistances of all of the loads. RT = R1 + R2 + R3 … Physics I Honors 29 Series - Computing Voltage • The total electricity (energy) used by the loads is the sum of the usages of all of the loads. VT = V1 + V2 + V3 … • All the energy is used before the current returns to the energy source Physics I Honors 30 Sample Problem Series Circuit Three 20 resistors are connected in series across a 120 V generator. What is the total resistance of the circuit? RT = R1 + R2 + R3 = 20 + 20 + 20 = 60 What is the current in the circuit? I = V/RT = 120V / 60 = 2 A Physics I Honors 31 Current and Circuits Parallel Circuits Physics I Honors 32 Parallel Circuits • two or more paths for the electricity. • Total current increases and volts remain constant with additional resistors Physics I Honors 33 Parallel Bulbs Voltage dropped is the same at each resistance Current is different But: total current out of battery = sum of the currents through each bulb Physics I Honors 34 Parallel - Computing Current • The sum of the currents through each branch equals the total current IT = I1 + I2 + I3 … Physics I Honors 35 Parallel - Computing Resistance • Req (total resistance) is smaller than any individual resistor; 1/Req = 1/R1 + 1/R2 + 1/R3… Physics I Honors 36 Parallel - Computing Voltage • The voltage across each resistor is the same. V1 = V 2 = V 3 … Physics I Honors 37 Sample Problem – Parallel Circuit Three 20 resistors are connected in parallel and placed across a 120 V potential difference. What is the equivalent resistance? 1/Req = 1/20 + 1/20 + 1/20 = 3/20 = 1/6.67 Req = 6.67W 20 20 20 Physics I Honors 38 Sample Problem – Parallel Circuit What is the current through the circuit? I = V / Req = 120 V / 6.67 20 the total current I = 18A 20 Physics I Honors 20 39 Sample Problem – Parallel Circuit What is the current through each branch of the circuit? For each branch, In = V / Rn = 120V/20 = 6A 20 20 20 Physics I Honors 40 Current and Circuits Combination Circuits Physics I Honors 41 Combination • circuit with both series and parallel paths for the electricity • The parallel section of the circuit is like a sub-circuit and actually is part of an over-all series circuit. Physics I Honors 42 Combination - Series/Parallel • This circuit is composed of two resistance segments: – R1 alone – R2 and R3 in parallel Physics I Honors 43 Combination - Series/Parallel • Resistors R2 and R3 can be combined to form a single equivalent resistance: R2,3 • This leave a simple series circuit Physics I Honors 44 Using the series-parallel schematic • What is the equivalent resistance of the circuit? • first treat the parallel section – R1 = 3 – R2 = 4 – R3 = 6 – VT = 12 V • 1/Req = 1/R2 + 1/R3 = 1/4 + 1/6 = 3/12 + 2/12 = 5/12 = 1/2.4 • Req = 2.4 for the parallel portion Physics I Honors 45 Using the series-parallel schematic • Then, add all the resistances as if they were in series RT = R1 + R2,3 = 3 + 2.4 = 5.4 Physics I Honors 46 Using the series-parallel schematic • What is the current in the circuit? I = V/R = 12 V / 5.4 = 2.22 A Physics I Honors 47 Using the series-parallel schematic • What is the current through each resistance? • I=V/R • I2 = V2 / R2 = 5.33 V / 4 = 1.33 A • I3 = V3 / R3 = 5.33 V / 6 = 0.89 A • I2,3 = I2 + I3 = 1.33 A + 0.89 A = 2.22 A Physics I Honors 48 Using the series-parallel schematic • How much voltage is lost at each resistor? • I = V / R so • V1 = IR1 = 2.22 A * 3 = 6.67 V • V2,3 = IR2,3 = 2.22 A * 2.4 = 5.33 V • VT = V1 + V2,3 = 6.67 V + 5.33 V = 12 V Physics I Honors 49 Parallel/Series • This circuit is composed of two resistance segments: – R1 and R2 in series – R3 in parallel Physics I Honors 50 Current and Circuits POWER TRANSMISSION OF ELECTRICAL ENERGY Physics I Honors 51 POWER Power is calculated by multiplying current times voltage P = IV R Resistors use energy. Rate = Power = I2R V I Physics I Honors 52 POWER • Energy per unit of time converted into another form of energy. • Thermal energy is produced at a rate given by P = I2R • Power is lost during transmission due to production of heat ( conversion of electrical energy to thermal energy) Physics I Honors 53 Transmission of Power • In transmission of energy, it is important to maintain the power and reduce the loss of energy. • Using P = I2R, we must reduce either the resistance or the current. • Reducing resistance – using wire that is highly conductive and of large diameter. Physics I Honors 54 Transmission of Power • Reducing the current. – Looking at P = IV, – to reduce current, increase the voltage to maintain the same power delivered. – Long distance lines may carry voltages of more than 500,000 V (that’s half a million volts) Physics I Honors 55 Paying for Electricity • You pay for electric energy, not power. • Electric energy is paid for by the kilowatt hour (kWh). • 1 kWh = (1000 J/s)(3600 s) = 3.6 x 106 J • Light bulbs are often sold in wattage. A 100 Watt bulb uses 100 Watts of electric energy in 1 hour. Physics I Honors 56 Electric Power A microwave oven is rated at 120 V and 840 W. a) What is the current drawn? P = IV I = P/V = 7 A b) What is the resistance of microwave oven? P = I2R R = P/I2 = 17.14 c) How much energy is used in 5 minutes? E = Pt = 840 W * 5 min * 60 s = 2.52 x 105 J Physics I Honors 57 Injuries through Electricity Current 1 mA 5 mA 10 mA 20 mA 100 mA 1000 mA Physics I Honors Effect mild shock painful paralysis of motor muscles breathing stops heart stops serious burns Fatal? no no no minutes seconds instantly 58