Current Electricity, Ohm’s Law & Circuits

Current Electricity,Ohm’s Law & Circuits

Current (I) • The rate of flow of charges through a conductor • Needs a complete closed conducting path to flow • Must have a potential difference (voltage) • Measured with an “ammeter” in amps (A) named for Ampere – French scientist • I = current, A Q = charge, C t = time, s So: 1 Amp = 1

Voltage (V) • Electric potential difference between 2 points on a conductor. Equal to the electric potential energy per charge. • Sometimes described as “electric pressure” that makes current flow • Supplies the energy of the circuit • Measured in Volts (V) using a voltmeter • 1 Volt = 1 Joule / Coulomb

Resistance (R) • The “electrical friction” encountered by the charges moving through a material. • Depends on material, length, and cross-sectional area of conductor • Measured in Ohms (Ω) Where: R = resistance, = length of conductor, A = cross-sectional area of conductor, ρ = resistivity of conducting material

Resistivity (ρ) • Property of material that resists the flow of charges (resistivity, ρ, in Ωm) • The inverse property of conductivity • Resistivity is temperature dependent…as temperature increases, then resistivity increases, and so resistance increases.

Ohm’s Law • A relationship between voltage, current, and resistance in an electric circuit • used to make calculations in all circuit problems • V = potential difference (voltage) in volts • I = electric current in amperes (amps , A) • R = resistance in ohms (  )

Electric Power (Watts) • Used for thermal energy

Electric Energy • Electric energy can be measured in Joules (J) or Kilowatt hours ( kWh ) • for Joules use Power in watts and time in seconds • for kWh use Power in kilowatts and time in hours

R1 R2 R3 Series Circuits • Current can only travel through one path • Current is the same through all parts of the circuit. • The sum of the voltages of each component of the circuit must equal the battery. • The equivalent resistance of a series circuit is the sum of the individual resistances. V I

Solving a Series Circuit Step 1: Find the equivalent (total) resistance of the circuit R1=1 Ω IT 6V R2=1 Ω Step 2: Find the total current supplied by the battery Step 3: Find Voltage Drop across each resistor. Note: Since both resistors are the same, they use the same voltage. Voltage adds in series and voltage drops should add to the battery voltage, 3V+3V=6V

R1 R2 V R3 Parallel Circuits • Current splits into “branches” so there is more than one path that current can take • Voltage is the same across each branch • Currents in each branch add to equal the total current through the battery

R2=2Ω 12V R3=3Ω R1=1Ω Solving a Parallel Circuit Step 1: Find the total resistance of the circuit. Step 3: Find the current through each resistor. Remember, voltage is the same on each branch. Step 2: Find the total current from the battery. Step 4: Check currents to see if the answers follow the pattern for current. The total of the branches should be equal to the sum of the individual branches.

Combo Circuits with Ohm’s LawWhat’s in series and what is in parallel? A B It is often easier to answer this question if we redraw the circuit. Let’s label the junctions (where current splits or comes together) as reference points. 5Ω 1Ω 3Ω 6Ω 15V D C 4Ω 7Ω 2Ω 6Ω 4Ω C 2Ω B 1Ω D 7Ω A 3Ω 5Ω 15V

Combo Circuits with Ohm’s LawNow…again…what’s in series and what’s in parallel? 6Ω 4Ω C 2Ω B The 6Ω and the4Ω resistors are in series with each other, the branch they are on is parallel to the 1Ω resistor. The parallel branches between B & C are in series with the 2Ω resistor. The 5Ω resistor is on a branch that is parallel with the BC parallel group and its series 2Ω buddy. The total resistance between A & D is in series with the 3Ω and the 7Ω resistors. 1Ω D A 7Ω 3Ω 5Ω 15V

Combo Circuits with Ohm’s LawFinding total (equivalent) resistance 6Ω 4Ω 2Ω C B To find RT work from the inside out. Start with the 6+4 = 10Ω series branch. So, 10Ω is in parallel with 1Ω between B&C… Then, RBC + 2Ω=2.91Ω and this value is in parallel with the 5Ω branch, so… 1Ω D A 7Ω 3Ω 5Ω Finally RT = RAD +3 + 7 = 1.84 + 3 + 7 RT = 11.84Ω 15V

Combo Circuits with Ohm’s LawSolving for current and voltage drops in each resistor RT = 11.84Ω IT=1.27A IT=1.27A 6Ω 4Ω 2Ω C Then… B The total current IT goes through the 3Ω and the 7Ω and since those are in series, they must get their chunk of the 15V input before we can know how much is left for the parallel. So… 1Ω D A 7Ω 3Ω So… Since parallel branches have the same current, that means the voltage across the 5Ω resistor V5Ω=4.84V and the voltage across the parallel section between B&C plus the 2Ω is also 4.84V 5Ω 15V

Combo Circuits with Ohm’s LawSolving for current and voltage drops in each resistor (continued) I2Ω=0.81A I5Ω=0.46A IT=1.27A IT=1.27A Known values from previous slide. 6Ω 4Ω To calculate the top branch of the parallel circuit between points A & D we need to find the current and voltage for the series 2 Ω resistor. Since the current through the resistor plus the 0.92A for the bottom branch must equal 1.3A. 2Ω C To calculate the current through the 5Ω resistor… B 1Ω D A 7Ω 3Ω 5Ω 15V So…

Combo Circuits with Ohm’s LawSolving for current and voltage drops in each resistor (continued) I6Ω=I4Ω =0.068A Known values from previous slide. I1Ω=0.68A I2Ω=0.81A I5Ω=0.46A IT=1.27A IT=1.27A 2Ω Next we need to calculate quantities for the parallel bunch between points B&C. The voltage that is left to operate this parallel bunch is the voltage for the 5Ω minus what is used by the series 2Ω resistor. The 1Ω resistor gets all of this voltage. Finally we need to calculate the current through the 6Ω and 4Ω resistors and the voltage used by each. C B 6Ω 4Ω 1Ω D A 7Ω 3Ω All we need now is the voltage drop across the 6Ω and 4Ω resistors. So… 5Ω 15V THE END!

Voltmeter and Ammeter • Ammeter • Measures current in amps • Placed in series where current is to be measured • Voltmeter • Measures voltage in Volts • Placed in parallel across whatever is being measured

Current Electricity, Ohm’s Law & Circuits