Physics I Honors 1 Current and Circuits Current

Physics I Honors 2 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 Current Battery

Physics I Honors 3 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 4 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 5 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 6 CURRENT AND CIRCUITS TYPES OF CURRENT

Physics I Honors 7 Conventional Current vs. Electron Flow Conventional current: Direction a positive charge would flow This is opposite the direction of electron flow in a conductor! -+ electron flowcurrent

Physics I Honors 8 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 9 CURRENT AND CIRCUITS Resistance Current

Physics I Honors 10 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 11 Less Resistance More Resistance

Physics I Honors 12 Resistance (  ) Resistance = Resistivity * Length / Area –R =  L/A –Resistivity depends upon the material. Current L = Length A = Area

Physics I Honors 13 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 VV V

Physics I Honors 14 Resistance (  ) V = 9 V R = 50  R = 100  V=9 V V = IR  I = V/R I = 9 V /50 ohms =.18 A V = IR  I = V/R I = 9 V /100 ohms =.09 A

Physics I Honors 15 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 16 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 17 Current and Circuits Circuits Schematics Use of Meters Safety

Physics I Honors 18 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 19 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 20 Schematics - Symbols wire or conductor resistor or other load bulb voltmeter battery ammeter switch generator V A

Physics I Honors 21 Schematics – Simple Circuit circuit diagram + - A +- A

Physics I Honors 22 Schematics - Complex Circuit Construct the circuit diagram + - A +- A

Physics I Honors 23 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 24 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 25 Fuses and Circuit Breakers If an electric circuit gets “overloaded” (too much current!) fuses or circuit breakers interrupt the flow of current. Fuses Circuit Breaker

Physics I Honors 26 Current and Circuits Series Circuits

Physics I Honors 27 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 28 Series - Computing Current The current reading will be the same anywhere in the circuit. I 1 = I 2 = I 3 …

Physics I Honors 29 Series - Computing Resistance The total resistance is the sum of the resistances of all of the loads. R T = R 1 + R 2 + R 3 …

Physics I Honors 30 Series - Computing Voltage The total electricity (energy) used by the loads is the sum of the usages of all of the loads. V T = V 1 + V 2 + V 3 … All the energy is used before the current returns to the energy source

Physics I Honors 31 Sample Problem Series Circuit Three 20  resistors are connected in series across a 120 V generator. What is the total resistance of the circuit? R T = R 1 + R 2 + R 3 = 20  + 20  + 20  = 60  What is the current in the circuit? I =  V/R T = 120V / 60  = 2 A

Physics I Honors 32 Current and Circuits Parallel Circuits

Physics I Honors 33 Parallel Circuits two or more paths for the electricity. Total current increases and volts remain constant with additional resistors

Physics I Honors 34 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 35 Parallel - Computing Current The sum of the currents through each branch equals the total current I T = I 1 + I 2 + I 3 …

Physics I Honors 36 Parallel - Computing Resistance R eq (total resistance) is smaller than any individual resistor; 1/R eq = 1/R 1 + 1/R 2 + 1/R 3 …

Physics I Honors 37 Parallel - Computing Voltage The voltage across each resistor is the same. V 1 = V 2 = V 3 …

Physics I Honors 38 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/R eq = 1/20  + 1/20  + 1/20  = 3/20  = 1/6.67  R eq = 6.67W 20 

Physics I Honors 39 Sample Problem – Parallel Circuit What is the current through the circuit? I =  V / R eq = 120 V / 6.67  the total current I = 18A 20 

Physics I Honors 40 Sample Problem – Parallel Circuit What is the current through each branch of the circuit? For each branch, I n =  V / R n = 120V/20  = 6A 20 

Physics I Honors 41 Current and Circuits Combination Circuits

Physics I Honors 42 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 43 Combination - Series/Parallel This circuit is composed of two resistance segments: –R 1 alone –R 2 and R 3 in parallel

Physics I Honors 44 Combination - Series/Parallel Resistors R 2 and R 3 can be combined to form a single equivalent resistance: R 2,3 This leave a simple series circuit

Physics I Honors 45 Using the series-parallel schematic What is the equivalent resistance of the circuit? first treat the parallel section –R 1 = 3  –R 2 = 4  –R 3 = 6  –V T = 12 V 1/R eq = 1/R 2 + 1/R 3 = 1/4 + 1/6 = 3/12 + 2/12 = 5/12 = 1/2.4  R eq = 2.4  for the parallel portion

Physics I Honors 46 Using the series-parallel schematic Then, add all the resistances as if they were in series R T = R 1 + R 2,3 = 3   = 5.4 

Physics I Honors 47 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 48 Using the series-parallel schematic What is the current through each resistance? I = V / R I 2 = V 2 / R 2 = 5.33 V / 4  = 1.33 A I 3 = V 3 / R 3 = 5.33 V / 6  = 0.89 A I 2,3 = I 2 + I 3 = 1.33 A A = 2.22 A

Physics I Honors 49 Using the series-parallel schematic How much voltage is lost at each resistor? I = V / R so V 1 = IR 1 = 2.22 A * 3  = 6.67 V V 2,3 = IR 2,3 = 2.22 A * 2.4  = 5.33 V V T = V 1 + V 2,3 = 6.67 V V = 12 V

Physics I Honors 50 Parallel/Series This circuit is composed of two resistance segments: –R 1 and R 2 in series –R 3 in parallel

Physics I Honors 51 Current and Circuits POWER TRANSMISSION OF ELECTRICAL ENERGY

Physics I Honors 52 POWER Power is calculated by multiplying current times voltage P = IV Resistors use energy. Rate = Power = I 2 R V R I

Physics I Honors 53 POWER Energy per unit of time converted into another form of energy. Thermal energy is produced at a rate given by P = I 2 R Power is lost during transmission due to production of heat ( conversion of electrical energy to thermal energy)

Physics I Honors 54 Transmission of Power In transmission of energy, it is important to maintain the power and reduce the loss of energy. Using P = I 2 R, we must reduce either the resistance or the current. Reducing resistance –using wire that is highly conductive and of large diameter.

Physics I Honors 55 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 56 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 10 6 J Light bulbs are often sold in wattage. A 100 Watt bulb uses 100 Watts of electric energy in 1 hour.

Physics I Honors 57 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 = I 2 R R = P/I 2 =  c) How much energy is used in 5 minutes? E = Pt = 840 W * 5 min * 60 s = 2.52 x 10 5 J

Physics I Honors 58 Injuries through Electricity Current Effect Fatal? Current Effect Fatal? 1 mAmild shock no 5 mApainful no 10 mAparalysis of motor muscles no breathing stops minutes 20 mAbreathing stops minutes 100 mAheart stops seconds 100 mAheart stops seconds 1000 mAserious burns instantly