Use the following symmetry properties of circles:



Lesson Plan

Subject: Physics

Topic: D.C. Circuits

Subtopic: Kirchhoff’s Laws

Class: Secondary 4 (GE)

Date: Thursday, 23 March 2006

Time: 10.25am – 11.25am

Location: Physics Laboratory

Prerequisite Knowledge

Before the lesson, students should be able to:

1. Recall and utilise the equation R = V/I to solve problems.

2. Recall the definition of potential and potential difference, and that the potential along any point of a connected wire is the same.

3. Draw circuits using standard symbols

4. Solve problems using the formula for the combined resistance of two or more resistors in series

5. Solve problems using the formula for the combined resistance of two or more resistors in parallel

Specific Instructional Objectives

At the end of the lesson, the students should be able to:

1. Recall Kirchhoff’s First Law: that the sum of currents into any junction is the same as the sum of currents flowing out

2. Recall Kirchhoff’s Second Law: that the sum of potential differences around a circuit must be zero.

3. State and apply the principle that current at every point in a series circuit is the same.

4. Apply Kirchhoff’s Laws to solve simple circuit problems.

Key Concepts

Kirchhoff’s Laws, combined resistances

Resources

Activity Worksheet

Lab Worksheet 20.1

Slides

Prepared circuits – 2 ( 2 sets

Detailed Lesson Plan

Key questions are in italics.

|Time |Teaching / Learning Activities |Remarks |

|5 min |Introduction – real-life application | |

| | | |

| |Show picture of “electric animals” – electric eel and Pikachu – and solicit for |Relate the topic to real-life examples|

| |responses. |before moving on to potentially dull |

| |How much current do you think these animals put out? How much current does it |drawings of lines and circuitry. |

| |take to kill a person? Why doesn’t the animal get electrocuted itself when | |

| |sending a current through something else? | |

| |Distribute activity worksheet. | |

|10 min |Review of past concepts, leading into Kirchhoff’s Current Law | |

| | | |

| |Before answering the above questions, ask students to recall what they remember | |

| |about combined resistance. |Due to brevity of previous lesson on |

| |What is the combined resistance of two or more resistors in series? What is the |resistors, students might not have |

| |combined resistance of two or more resistors in parallel? |been completely clear on the concept. |

| |What is the combined e.m.f. of two or more cells in parallel? |This will serve as reinforcement. |

| |Using these formulae, introduce relevant numbers for the electric eel: an eel | |

| |has, on average, 140 “rows” of 5000 electroplaques – physiological e.m.f. |Students should be able to recall |

| |devices – each with e.m.f. 0.15V and internal resistance 0.25(. |that, in series, combined resistance R|

| |Show students a diagram of the eel’s circuit – 140 parallel circuits of 5000 |= R1 + R2 + … , whereas in parallel, |

| |electroplaques in series. |1/R = 1/R1 + 1/R2 + … |

| |Individual work: Students are to complete the first part of the worksheet on | |

| |finding the total equivalent e.m.f. of the eel, as well as the total equivalent |For cells, students should be able to |

| |resistance. This would require the synthesis of some skills and concepts learned|recall from the previous unit’s |

| |previously about redrawing circuits and equipotential points; however, if |experiment that cells in parallel |

| |students are stuck, they can ask questions or discuss with their neighbours. |provide the same e.m.f. across their |

| |How do you simplify each row into one cell and one resistor?How do you simplify |ends. |

| |the entire parallel network into one cell and one resistor? | |

| |How much current can the eel produce through the water? | |

| |To provide a hook for the day’s topic, and to pique the students’ curiosity, | |

| |tell the students that not all this current flows through the eel at once. Ask: | |

| |How much current travels through each “row” of the eel? Can you guess | |

| |intuitively? | |

| | | |

| | | |

| | |This question will be left unanswered |

| | |while the students move on to inquiry |

| | |learning. |

|15 min |Inquiry-based learning on Kirchhoff’s Laws | |

| | | |

| |Group work: Students will be divided into four groups. Each group will be given |This will let students see actual |

| |one of two sets of prepared circuitry to experiment with, according to the |voltmeter and ammeter readings |

| |laboratory handout (attached – Lab 20.1.2, Lab 20.1.3). One set will demonstrate|themselves, instead of just |

| |Kirchhoff’s Current Law, the other Kirchhoff’s Voltage Law. |calculating them. |

| |These simple experiments will demonstrate to the students the behaviour of | |

| |current and voltage in various configurations, and allow them to draw their own | |

| |conclusions about Kirchhoff’s Laws. | |

| |Each group is to have 5 minutes to complete the worksheet on that specific | |

| |circuit. The groups will then swap and try the other circuit. | |

|15 min |Kirchhoff’s Current Law | |

| | | |

| |Students will be asked to volunteer the readings they obtained from Lab 20.1.2 |Students should be able to conclude |

| |and the conclusions they have drawn. |that the current flowing into a |

| |Introduce Kirchhoff briefly, to provide some historical perspective |junction is equal to the current |

| |Introduce the first law by asking: Can we create charge out of nothing? |flowing out of it. |

| |Ask students to consider the circuit given in Lab 20.1.2. What is the voltage | |

| |across each resistor? What is the current across each resistor? What can you |Trick question – yes, we can create |

| |conclude about the current across each resistor and the total current in the |charge out of nothing but energy |

| |circuit? |(e.g., gamma rays becoming positrons |

| |Introduce Kirchhoff’s First Law / Current Law / Junction Theorem: “The current |and electrons), but a single charge |

| |flowing into any junction equals the sum of the current flowing out.” |cannot be created nor destroyed in a |

| |What does this imply for current in a series circuit? Are there any junctions? |circuit. Hence, charge is conserved in|

| |Ask students to extrapolate using this law to convince themselves that current |a circuit. |

| |along a series circuit is always the same. | |

| |Return to initial eel question. What does this imply about the amount of current| |

| |that flows through each row on the eel? | |

| |Give students time to go through a worked example of Kirchhoff’s Current Law on | |

| |the board. Students are to copy down the diagram and do the necessary | |

| |calculations in their activity worksheet. |Answering this question here will |

| | |reinforce the idea of a “current |

| | |split” in the students’ minds. |

|10 min |Kirchhoff’s Voltage Law | |

| | | |

| |Students will be asked to volunteer the readings they obtained from Lab 20.1.3 |Students will conclude that the |

| |and the conclusions they have drawn. |voltages sum to the total |

| |Ask students to now consider a simple series circuit with two resistors. If the |electromotive force. |

| |e.m.f. across the battery is 1.5 V, and the potential difference across one | |

| |resistor is 0.5 V, what is the p.d. across the other resistor? | |

| |Introduce Kirchhoff’s Second Law / Voltage Law / Loop Rule: “In any closed loop | |

| |of a circuit, the sum of the voltage drops must equal the sum of the e.m.f.s in | |

| |the loop.” | |

| |What is this an example of the conservation of? Leading questions to follow up | |

| |if necessary: What is the equation for voltage? (Energy per unit charge.) | |

| |Clarify possible misconceptions by showing a parallel circuit: How do you | |

| |account for this law in this “closed loop” – the “loop” containing just two | |

| |resistors? Wouldn’t there be two potential drops around the circuit? | |

| |Using the same diagram, visualise for students some “loops” in the circuit – one| |

| |loop through each resistor and the e.m.f. source. What does this imply about the| |

| |potential difference across each resistor? |Without a source of e.m.f. in that |

| | |“closed loop”, there is no potential |

| | |in the circuit and hence no potential |

| | |drop. |

| | | |

| | | |

| | | |

| | | |

|5 min |Conclusion and homework | |

| | | |

| |Conclude and summarise the two laws learnt. | |

| |Assign homework: Students are to complete the worksheet, where they will be |Appeal to the students’ sense of |

| |challenged to derive the combined resistance solutions using what they have |intellectual competition in letting |

| |learnt today, as well as to solve the given combined resistance problems, and |them set hard problems for one |

| |come up with the hardest possible resistor problem with six equal resistors. The|another. |

| |best problems from each class will then be used to “test” other classes, and the| |

| |student responsible for the winning entry will be rewarded. The entire worksheet| |

| |is to be handed in on the next lesson. | |

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