Wednesday 7 October · Series and parallel
One battery, two lamps
prediction
Recall the switch idea
Use Tuesday’s switch rule before moving to two lamps.
1 complete path. Lamp A is on. Source current 1 amperes.
- Path
- 1 complete path
- Source current
- 1 A
- Equivalent resistance
- 12 Ω
- Lamp A
- ON · 1 A · 12 W
- Lamp B
- · ·
Hint 1: Trace the circuit’s one current path through the switch.
Hint 2: An open switch separates its contacts.
prediction
One path through two lamps
Both lamps are in one series circuit. Predict before changing it.
1 complete path. Lamp A is on; Lamp B is on. Source current 0.5 amperes.
- Path
- 1 complete path
- Source current
- 0.5 A
- Equivalent resistance
- 24 Ω
- Lamp A
- ON · 0.5 A · 3 W
- Lamp B
- ON · 0.5 A · 3 W
Hint 1: Count the complete paths from one source terminal to the other.
Hint 2: Both lamps share the same single path in series.
circuit
Test the series prediction
Open the path between the lamps, then check both lamp states.
Use these buttons with touch, a mouse or the keyboard. No dragging is needed.
1 complete path. Lamp A is on; Lamp B is on. Source current 0.5 amperes.
- Path
- 1 complete path
- Source current
- 0.5 A
- Equivalent resistance
- 24 Ω
- Lamp A
- ON · 0.5 A · 3 W
- Lamp B
- ON · 0.5 A · 3 W
Hint 1: Choose the control that opens the shared path between the lamps.
Hint 2: After opening it, compare Lamp A and Lamp B in the readings.
explanation
Compare series and parallel
Use the two selectors to compare the same source and two identical model lamps.
Series has one current path through both lamps. The steady current is the same through each lamp; it is not gradually used up. With two identical 12 Ω model lamps on this 12 V ideal source, the model shows 0.5 A and 3 W at each lamp.
Parallel gives each lamp its own complete branch across the source. Each connected branch has 1 A and 12 W in this model, while the source supplies the two branch currents together. These fixed-resistance lamps are a teaching approximation.
1 complete path. Lamp A is on; Lamp B is on. Source current 0.5 amperes.
- Path
- 1 complete path
- Source current
- 0.5 A
- Equivalent resistance
- 24 Ω
- Lamp A
- ON · 0.5 A · 3 W
- Lamp B
- ON · 0.5 A · 3 W
One current path passes through both lamps. The model shows 0.5 A through each lamp.
2 complete paths. Lamp A is on; Lamp B is on. Source current 2 amperes.
- Path
- 2 complete paths
- Source current
- 2 A
- Equivalent resistance
- 6 Ω
- Lamp A
- ON · 1 A · 12 W
- Lamp B
- ON · 1 A · 12 W
Each branch is a complete path across the source. The model shows 1 A in each branch and 2 A from the source.
- Series
- One shared current path through both loads.
- Parallel
- Separate complete branch paths across the source.
- Common feed
- The shared connection before the parallel paths divide.
movement break
Two-minute movement break
Pause the screen task for a moment.
Stand up, stretch or get a drink. Return when you are ready; there is no countdown.
prediction
Two separate branch paths
Both parallel branches are complete. Predict before opening one branch.
2 complete paths. Lamp A is on; Lamp B is on. Source current 2 amperes.
- Path
- 2 complete paths
- Source current
- 2 A
- Equivalent resistance
- 6 Ω
- Lamp A
- ON · 1 A · 12 W
- Lamp B
- ON · 1 A · 12 W
Hint 1: Trace branch B from the top connection to the bottom connection.
Hint 2: Branch B still connects across both source terminals when only branch A opens.
circuit
Test the branch prediction
Open only branch A, then compare both lamp states and the source current.
Use these buttons with touch, a mouse or the keyboard. No dragging is needed.
2 complete paths. Lamp A is on; Lamp B is on. Source current 2 amperes.
- Path
- 2 complete paths
- Source current
- 2 A
- Equivalent resistance
- 6 Ω
- Lamp A
- ON · 1 A · 12 W
- Lamp B
- ON · 1 A · 12 W
Hint 1: Choose “Open branch A”, not the common feed.
Hint 2: After the change, look for one complete path and 1 A source current.
prediction
The common-feed trap
Parallel branches work independently only while their shared feed is complete.
1 complete path. Lamp A is off; Lamp B is on. Source current 1 amperes.
- Path
- 1 complete path
- Source current
- 1 A
- Equivalent resistance
- 12 Ω
- Lamp A
- OFF · 0 A · 0 W
- Lamp B
- ON · 1 A · 12 W
Hint 1: The common feed is shared before the circuit divides into branches.
Hint 2: If that shared connection opens, trace whether either branch still reaches both source terminals.
circuit
Test the common feed
Open the common feed and check what happens to both parallel lamps.
Use these buttons with touch, a mouse or the keyboard. No dragging is needed.
1 complete path. Lamp A is off; Lamp B is on. Source current 1 amperes.
- Path
- 1 complete path
- Source current
- 1 A
- Equivalent resistance
- 12 Ω
- Lamp A
- OFF · 0 A · 0 W
- Lamp B
- ON · 1 A · 12 W
Hint 1: Choose the shared connection before the branches.
Hint 2: Use “Open the common feed”, then check both lamp readings.
check
Choose for the requirement
Use the required behaviour rather than deciding that one arrangement is always better.
prediction
Predict in a changed layout
The branches now run across the screen. Use connections rather than remembered positions.
2 complete paths. Lamp A is on; Lamp B is on. Source current 2 amperes.
- Path
- 2 complete paths
- Source current
- 2 A
- Equivalent resistance
- 6 Ω
- Lamp A
- ON · 1 A · 12 W
- Lamp B
- ON · 1 A · 12 W
Hint 1: Follow the lower branch through Lamp B.
Hint 2: Opening the upper branch does not break the lower branch’s connections.
check
Show the remaining path
Select the three pieces that trace Lamp B’s complete path. Leave out the broken branch.
1 complete path. Lamp A is off; Lamp B is on. Source current 1 amperes.
- Path
- 1 complete path
- Source current
- 1 A
- Equivalent resistance
- 12 Ω
- Lamp A
- OFF · 0 A · 0 W
- Lamp B
- ON · 1 A · 12 W
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