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What we are Doing Today ? 1) Question(s) of Day 2) Kirchhoff’s Laws & Magnetism Notes 3) Continue Lab 41 & 42 at the back 4) Make sure you finished the HW p.173 of the textbook questions 10-18 5) Lab is due on Thursday 6) Tutorial after school today. Room 308 1) Question(s) of Day 2) Kirchhoff’s Laws & Magnetism Notes 3) Continue Lab 41 & 42 at the back 4) Make sure you finished the HW p.173 of the textbook questions 10-18 5) Lab is due on Thursday 6) Tutorial after school today. Room 308

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Circuits Series and Parallel Circuits

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Kirchoff’s Laws Kirchoff ’ s Laws show the relationship between current and voltage Kirchoff ’ s First Law…current (junction rule) Kirchoff ’ s Laws show the relationship between current and voltage Kirchoff ’ s First Law…current (junction rule) At any junction, the sum of the currents entering the junction equals the sum of the currents leaving

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Current in a series circuit: Any junction has one path in and one path out…..current is the same everywhere Current in a series circuit: Any junction has one path in and one path out…..current is the same everywhere

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Current in a parallel circuit: Sum of the currents entering equals the sum of the currents leaving Current in a parallel circuit: Sum of the currents entering equals the sum of the currents leaving

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Kirchoff ’ s Second Law…potential difference (loop rule) The sum of the drops in potential (voltages) equals the potential (voltage) at the source

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Potential difference in a series circuit: What are the missing voltages in the above circuits? Voltages add up to the voltage at the source

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Potential difference in a parallel circuit: Voltage drop in each loop is the same as the voltage at the source. What are the voltages in bulb 1 and bulb 2?

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Series Circuits

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Current One path - all current flows through each resistor (remember the junction rule?) Equation:

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Voltage The sum of the drops in potential difference equals the potential difference at the source (Remember the loop rule?) Equation:

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Resistance But V=IR, so….. Since Current cancels in the equation, and…..

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Series Circuit Example: In the circuit below: R 1 =4Ω, R 2 =2Ω, and R 3 =6Ω. If the current in the circuit is 0.5 A, what is the voltage at the source?

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Method 1 Find total resistance in the circuit: R = R 1 +R 2 +R 3 = 4Ω + 2Ω + 6Ω = 12Ω V = IR = 0.5 A x 12 Ω = 6V Then use Ohm’s Law to find total voltage:

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Method 2 Use Ohm’s Law to find voltage drop in each resistor Then add all the voltages: V = V 1 + V 2 + V 3 = 2V + 1V + 3V = 6V

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Parallel Circuits

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Current Sum of currents entering a junction equals the sum of the currents leaving (remember the junction rule?) Equation:

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Example: I 1 =2A I 2 =1A I 4 =6A What is I 3 ? I 1 =2A I 2 =1A I 4 =6A What is I 3 ? 6A = 2A + 1A + ? I 3 = 3A

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Voltage The sum of the drops in potential difference equals the potential difference at the source (Remember the loop rule?) The voltage in each loop is the same as the source of potential: Equation:

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Resistance But I=V/R, so….. Since Voltage cancels in the equation, and…..

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Parallel Circuit Example: R 1 =2Ω, R 2 =2Ω, and R 3 =4Ω. If the voltage at the source is 16V, what is the total current in the circuit? Add a voltmeter to show the voltage drop across R 3.

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Method 1 Find total resistance in the circuit: Need a common denominator

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Then use Ohm’s Law to find total voltage:

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Method 2 Use Ohm’s Law to find current in each resistor Then add all the currents: I = I 1 + I 2 + I 3 = 8A + 8A + 4A = 20A

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What happens to total current in a parallel circuit when you add more appliances? Additional appliances - more resistance More paths…total resistance DECREASES Less resistance, more current More current, more thermal energy (heat) Short circuits, fires……. Solution? Additional appliances - more resistance More paths…total resistance DECREASES Less resistance, more current More current, more thermal energy (heat) Short circuits, fires……. Solution?

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What devices do you have in your home to protect you from too much current? 1. Fuses - short wire melts, breaks circuit (thickness of wire determines current rating) 2. Circuit breakers - switch opens when current reaches a preset value 3. GFI (ground fault interrupter) - found in kitchen and bathroom outlets - detects differences in current caused by extra current paths - works like a circuit breaker 1. Fuses - short wire melts, breaks circuit (thickness of wire determines current rating) 2. Circuit breakers - switch opens when current reaches a preset value 3. GFI (ground fault interrupter) - found in kitchen and bathroom outlets - detects differences in current caused by extra current paths - works like a circuit breaker

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For a Summary Look at table 5.28 on page 163 of the textbook

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What to do now? 1) Homework p. 173-174 questions 19-20 2) Finish lab 41 & 42 at the back 3) Homework check p.173 questions 10-18 1) Homework p. 173-174 questions 19-20 2) Finish lab 41 & 42 at the back 3) Homework check p.173 questions 10-18

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