1. Current Electricity, Ohm’s Law & Circuits

2. 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

3. 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

4. Resistance (R) Opposition to the flow of charges. “electrical friction” of the moving charges. Measured in Ohms (Ω) Resistance in a wire depends on - material (low R for copper) - temperature (as T increases, R increases) - length (as length increases, R increases) - cross-sectional area ( as A incr., R decr.)

5. 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 (  )

6. Electric Power (Watts) Used for thermal energy

7. 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

8. Voltmeter and Ammeter Ammeter –Measures Current in Amps –Placed in the path with the resistor(s) to be measured –Placed in series with whatever is being measured Voltmeter –Measures Voltage in Volts –Placed around whatever is being measured –Placed in parallel with whatever is being measured

9. 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 total voltage of the battery. The equivalent resistance of a series circuit is the sum of the individual resistances. R1R1 R2R2 R3R3 V I

10. Solving a Series Circuit 6V R 1 =1 Ω R 2 =2 Ω ITIT Step 1: Find the equivalent (total) resistance of the circuit Step 2: Find the total current supplied by the battery Step 3: Find Voltage Drop across each resistor. Remember: The current is the same throughout a series circuit so… I T = I 1 = I 2

11. Parallel Circuits Current splits into “branches” so there is more than one path that current can take Voltage is the same across each branch as the total voltage drop (or voltage across the battery) Currents in each branch add to equal the total current through the battery R1R1 R2R2 R3R3 V

12. Solving a Parallel Circuit R 1 =1 Ω R 2 =2 Ω R 3 =4 Ω 12V Step 1: Finding the total resistance of the circuit. Step 2: Finding the total current from the battery. Step 3: Finding the current through each resistor. Remember, voltage is the same on each branch as the battery voltage. Note: Step 1 & 2 were not necessary to do step 3!! 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.

13. Combo Circuits with Ohm’s Law What’s in series and what is in parallel? 15V 3Ω3Ω 5Ω5Ω 7Ω7Ω 1Ω1Ω 2Ω2Ω 6Ω6Ω 4Ω4Ω 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. AB CD 3Ω3Ω A B 6Ω6Ω 4Ω4Ω 1Ω1Ω C 5Ω5Ω 2Ω2Ω D 7Ω7Ω 15V

14. Combo Circuits with Ohm’s Law Now…again…what’s in series and what’s in parallel? 3Ω3Ω A B 6Ω6Ω 4Ω4Ω 1Ω1Ω C 5Ω5Ω 2Ω2Ω D 7Ω7Ω 15V The 6Ω and the 4Ω 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.

15. Solving Combo Circuits – Find R T first R 2 = 7 Ω R 3 = 2 Ω R 4 = 3 Ω R 1 = 4 Ω V = 15 V Step 1: Find R T for any series within a parallel branch. R 2-3 = 7 + 2 = 9Ω Step 2: Find R T for any parallel parts. Step 3: Find R T for the series circuit. R 2-3 = 9 Ω R 1 = 4 Ω V = 15 V R 4 = 3 Ω R 2-4 = 2.25 Ω R 1 = 4 Ω V = 15 V R 1-4 = 6.25 Ω V = 15 V

16. Solving Combo Circuits Step 4: Find I T using Ohm’s law. Step 5: Find the voltage across any resistors in series with the battery. Step 6: Find the voltage drop across the whole parallel part. (2 methods are shown.) Step 7: The voltage drop across each parallel branch is the same as the total voltage drop across the parallel part (if there are not any resistors in series in that branch).