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Circuits Chapter 35

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LAB 21 What will happen to bulbs 1 and 2 when you disconnect the wires at various points? Consensus: Current requires a closed loop

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LAB 22 What type of object, when inserted into the loop, will allow the two test bulbs to light? Consensus: Current requires a closed loop made entirely of conductors.

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LAB 23 What parts of a socket and bulb are conductors and which are insulators? What is the conducting path through the bulb? Fig 2. Clip – side view Base Clips Plates Consensus: Trace the Continuous Conducting Path (CCP) through the bulb.

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Schematics Schematics are drawings used to show circuits using symbols instead of actual drawings. Here are some examples:

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Schematics You have a list of several useful symbols in your book. Here are a few more:

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Schematics Draw a schematic of a circuit consisting of a battery made of 2 cells (label + and – ends) and two light bulbs.

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Schematics Draw a schematic of a circuit consisting of a battery made of 3 cells (label + and – ends), a light bulb and a resistor.

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Schematics Draw a schematic of a circuit consisting of a battery (label + and – ends), three light bulbs, an ammeter and a voltmeter on the second bulb.

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Schematics A visual representation of a circuit is shown below. On the right, sketch the corresponding schematic diagram.

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Worksheet 2

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3. bulb 1: LitUnlit bulb 2: LitUnlit

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Worksheet 2 4. bulb 1: LitUnlit bulb 2: LitUnlit

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Worksheet 2 5. bulb 1: LitUnlit bulb 2: LitUnlit

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Worksheet 2 6. bulb 1: LitUnlit bulb 2: LitUnlit

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Worksheet 2 7. bulb 1: LitUnlit bulb 2: LitUnlit

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CIRCUIT LAB 2 How does the number of bulbs in a single loop affect the overall resistance of the circuit? Based on the measurements of the ammeter and observations of bulb brightness, what can you state about the flow rate in each circuit? R R R A A A

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CIRCUIT LAB 2 Consensus: As resistance in a single closed loop goes up, the current goes down. V = IR New Terms: Series circuit – a circuit with all sources of resistance on a single loop with the battery. R R R A A A

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Series Circuits Series circuits – circuits where all sources of resistance are on a single closed loop with the battery.

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Series Circuits Total Resistance = the sum of all resistances. R = R 1 + R 2 + R 3 … This means that two 8- resistors could be replaced with one 16- resistor.

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Activity 25 Consensus: For a series circuit, the voltage across the whole circuit is equal to the sum of the individual voltage drops.

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Activity 25 Consensus: For a series circuit, the total current flows through each resistance on the loop.

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Series Circuits Total Resistance = the sum of all resistances. R = R 1 + R 2 + R 3 … This means that two 8- resistors could be replaced with one 16- resistor.

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Series Circuits Voltage drop = the voltage pulled from the battery by a resistance to produce current. Total voltage = the sum of the voltage drops across each resistance.

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“Rules” for Series Circuits Step 1: Total resistance = sum of all resistances R = R 1 + R 2 + R 3 + … Step 2: Total current is the same throughout I = V/R Step 3: Voltage drop – use Ohm’s Law V 1 = I (R 1 ) Check: Total voltage = sum of voltage drops = battery V = V 1 + V 2 + V 3 + …

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Series Circuits Suppose you have three 3- resistors connected in series to a 9-V battery. What is the total resistance? 9 What is the current in the circuit? 1 A What is the voltage drop across each resistor? 3 V

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Series Circuits There are three 20- resistors connected in series across a 120-V generator. What is the equivalent resistance of the circuit? What is the current of the circuit? What is the voltage drop across each resistor? R = R 1 + R 2 + R 3 … R = 20 + 20 + 20 R = 60 I = V/R I = 120V/60 I = 2 A V = IR V = 2A(20 ) V = 40 V

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Series Circuits A 10- resistor, a 15- resistor and a 5- resistor are connected in series across a 90-V battery. What is the equivalent resistance of the circuit? What is the current in the circuit? What is the voltage drop across each resistor? R = R 1 + R 2 + R 3 … R = 10 + 15 + 5 R = 30 I = V/R I = 90V/30 I = 3 A V = IR V = 3A(10 ) V = 30 V V = IR V = 3A(15 ) V = 45 V V = IR V = 3A(5 ) V = 15 V V = 30V + 45V + 15V = 90V

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CIRCUIT LAB 3 How does the number of bulbs along side affect the overall resistance? Based on the ammeter reading and bulb brightness, what can we state about the flow rate through each bulb?

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CIRCUIT LAB 3 Consensus: Bulbs alongside create their own closed loop increasing the amount of current overall and decreasing total resistance. This is the opposite of bulbs in series. New Terms: Parallel circuit – a circuit with each source of resistance on its own closed loop with the battery.

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Parallel Circuits Parallel circuits – circuits where each source of resistance is on its own closed loop with the battery.

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Parallel Circuits Total Resistance = sum of the inverses of each resistance 1/R = 1/R 1 + 1/R 2 + 1/R 3 … This means that two 8- resistors in parallel could be replaced by one 4- resistor.

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Activity 27 Consensus: The voltage drop across each lamp is equal to the total across the battery.

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Activity 27 b c A A A A f LL d A L e A L a Consensus: The total current in the circuit is equal to the sum of the currents in each lamp.

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Parallel Circuits Total Resistance = sum of the inverses of each resistance 1/R = 1/R 1 + 1/R 2 + 1/R 3 … This means that two 8- resistors in parallel could be replaced by one 4- resistor.

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Parallel Circuits Voltage drop In a parallel circuit, the voltage drop is the same across each branch, equal to the battery.

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Parallel Circuits Current In a parallel circuit, the total current in the circuit is equal to the sum of the currents in each branch.

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“Rules” for Parallel Circuits Step 1: Total Resistance = inverse-sum of inverses 1/R = 1/R 1 + 1/R 2 + 1/R 3 + … Step 2: Voltage drops on each branch = battery Step 3: Current in each branch uses Ohm’s Law I 1 = V/R 1 Step 4: Total current = sum of currents in each branch I = I 1 + I 2 + I 3 + … Check: Calculate Total Current using Ohm’s Law I = V/R

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Parallel Circuits Suppose you have three 3- resistors in parallel connected to a 9-V battery. What is the total resistance of the circuit? 1 What is the current in the circuit? 9 A What is the voltage drop across each resistor? 9 V What is the current in each resistor? 3 A

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Parallel Circuits Three 15- resistors are connected in parallel and placed across a 30-V potential difference. What is the equivalent resistance of the parallel circuit? What is the current through the entire circuit? What is the current through each branch of the parallel circuit? 1/R = 1/R 1 + 1/R 2 + 1/R 3 … 1/R = 1/15 + 1/15 + 1/15 1/R = 3/15 R = 15/3 = 5 I = V/R I = 30V/5 I = 6 A I 1 = V/R 1 I 1 = 30V/15 I 1 = 2 A

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Parallel Circuits A 120- resistor, a 60- resistor and a 40- resistor are connected in parallel and placed across a 120-V potential difference. What is the equivalent resistance of the parallel circuit? What is the current through the entire circuit? What is the current through each branch of the parallel circuit? 1/R = 1/R 1 + 1/R 2 + 1/R 3 … 1/R = 1/120 + 1/60 + 1/40 1/R = 1/120 + 2/120 + 3/120 1/R = 6/120 R = 120/6 = 20 I = V/R I = 120V/20 I = 6 A I 1 = V/R 1 I 1 = 120V/120 I 1 = 1A I 2 = V/R 2 I 2 = 120V/60 I 2 = 2 A I 3 = V/R 3 I 3 = 120V/40 I 3 = 3 A

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Series and Parallel Circuits An Analogy

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Activity 28 What is the effect of adding a bulb in parallel? Consensus: Adding a bulb in parallel to a series circuit causes a split of the current to the two parallel branches and an increase in the total current. New Terms: Combination Circuit – a circuit with both series and parallel parts.

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Combination Circuits Combination circuits – those circuits that contain both parallel and series portions.

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Combination Circuits There are steps to take to find the total resistance: 1) Find the total resistance for any parallel parts. 2) Add that to any resistance in series.

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Combination Circuits Sometimes there are more steps to be taken: 1)Start with the smallest section of parallel. 2)Add any in series. 3)Then add as if two resistances in parallel. 4)If there are still some in series, then add those.

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Combination Circuits You can also do the same thing if each branch of the parallel portion has some in series:

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Worksheet 35-2

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