BC Science Connections 9 Series and Parallel Circuits

BC Science Connections 9: Series and Parallel Circuits Unit 3. 4 -2 and Unit 3. 4 -3 Pages 238 -241

Topic 3. 4: How are circuits used in practical applications? 3. 4 -1 Voltage, current, and resistance in a circuit are related by Ohm’s law. 3. 4 -2 Loads can be connected in series or in parallel in a circuit. 3. 4 -3 Parallel loads are practical for circuits in the home.

Remember from last class: Ohm’s Law The electrical potential difference between two points in a circuit is equal to the current times the resistance between those two points. V = IR V I R V: electrical potential difference (voltage) in volts (V) I: current in amperes (A) R: resistance in ohms (Ω)

Concept 2: Loads can be connected in series or in parallel in a circuit. There are two main types of circuits: • Series circuit: A circuit in which current can only flow along one path • Parallel circuit: A circuit that has at least one branch point where the current splits into two or more pathways

Series Circuit: One Pathway Series circuit: A circuit in which current can only flow along one path Example: Figure 3. 24 • All of the circuit components are connected in series • Three light bulbs (loads) are connected in series • There is only one path in which the current can flow through the battery, switch, and loads Figure 3. 24

Voltage (V) in a series circuit Voltage: the total voltage a battery supplies must equal the sum of the loss from each load. Example #1: In Diagram A if the total voltage supplied by the battery is 120 V and one lightbulb uses 80 V, what is the voltage across the second light bulb? Answer: 120 V – 80 V = 40 V Example #2: Calculate the voltage of the missing lightbulub in Diagram B Answer: 12 V- 3 V – 4 V = 5 V VTotal = V 1 +V 2

Current (A) in a Series Circuit Current in a series circuit: The electrons repel each other at the same force and therefore remain fairly evenly spaced apart. Since there is only one path for the electrons to travel in a series circuit, the current in each part of the series circuit is equal. ITotal = I 1 = I 2 = I 3 Example: I total = 2 A I 1 = 2 A I 2 = 2 A I 3= 2 A

Resistance (Ω) in a series circuit In a series circuit the more resistors that you place in the series, the greater the total resistance. • Therefore if you increase the total resistance (according to Ohm’s Law), you decrease the current. RTotal = R 1 +R 2 +… Example: RTotal =10 Ω +20Ω + 30 Ω = 60Ω

Parallel Circuit: Multiple Pathways Parallel circuit: A circuit that has at least one branch point where the current splits into two or more pathways Example: Figure 3. 25 • The light bulbs (loads) are connected in parallel • The battery and switch are connected in series • At the branch point, the current splits into two pathways (the sum of the currents in the branches is the same as the current in the single conductor before the branch point) Figure 3. 25

Voltage (V) in a parallel circuit The voltage across each load or resistor in a parallel circuit is the same. A battery that has a 12 V electric potential difference will have the same voltage across each light bulb VTotal = V 1 = V 2 = V 3 Example: What is the voltage at R 2? V = 9 V at every point in this parallel circuit

Current (A) in a Parallel Circuit In a parallel circuit the current branches into different paths and eventually rejoins. To calculate the current of the total circuit, you must add up the current for each path or branch. The total current entering a junction point must equal the sum of the current leaving the juncton point. ITotal = I 1 +I 2 + I 3 Example: I 1 = 4. 0 A I 2 = 4. 0 A I 3 = 10. 0 A ITotal = 4. 0 A + 10. 0 A = 18. 0 A

Resistance (Ω) in a parallel circuit In a parallel circuit the more resistors that you place in the parallel, the less the total resistance. • Therefore if you decrease the total resistance of a circuit, you increase the current (according to Ohm’s Law). • Analogy… they open up another check out stand at the grocery store so the customers have 2 paths to check out their groceries and therefore wait less time in line. 1/RTotal = 1/R 1 + 1/R 2 + 1/R 3… Example: 1/Rtotal = 1/20 Ω + 1/30 Ω = Answer: 1/Rtotal = 3/60 Ω + 2/60 Ω = 5/60 Rtotal =60/5 = 12 Ω

Comparison: Series Circuit and Parallel Circuit Figure 3. 26 A) Series circuit: One pathway for current to flow; current is equal in all parts of the circuit B) Parallel circuit: Current splits into three paths; current is reduced in each path

Concept 3: Parallel loads are practical for circuits in the home. Series circuits are impractical for homes. Example: Loads connected in series in a kitchen If one load (ceiling lamp) burns out: • The circuit will be open • Charges stop moving • No loads (microwave, toaster) in the circuit will work Example: Christmas lights on a series circuit. If one light burns out then the whole string turns off !

Parallel Loads: Household Circuits Parallel circuits are practical because each appliance is controlled by its own switch without shutting off others. Example: Figure 3. 27 A) All of the appliances are running. A large amount of current is passing through the conductor wire (arrow). When large amounts of current flow through a wire, it can overheat and start a fire. B) The ceiling lamp and microwave are turned off, but the toaster and radio are still running Figure 3. 27

Multiple Circuits Within a Building Figure 3. 28: Buildings have multiple parallel circuits. The yellow disks represent ceiling lights. Many separate parallel circuits are installed in buildings • A large electrical cable carrying electrical energy branches out and is connected to each parallel circuit in a circuit panel