6Electromotive force and internal resistance of current source.

Long-term plan unit: Electrostatics

School:

Date:

Teacher name:

Grade: 10

Number present:

absent:

Theme of the lesson

Electromotive force and source (terminal) voltage.

Learning objectives that are achieved at this lesson (Subject Programme reference)

Ø  To investigate the relationship between electromotive force and source voltage

Lesson objectives

By the end of this section, students will be able to:

·         Describe energy changes in an electric circuit.

·         Define the emf of a cell.

·         Demonstrate how the load on a cell reduces its terminal voltage.

·         Describe what happens to the terminal voltage, current, and power delivered to a load as internal resistance of the voltage source increases (due to aging of batteries, for example).

·         Explain why it is beneficial to use more than one voltage source connected in parallel.

Assessment criteria

Analysis

Express the relationship between the electromotive force and terminal voltage in a form of equation;

Describe what happens to the terminal voltage, current, and power delivered to a load as internal resistance of the voltage source increases (due to aging of batteries, for example);

Application

Investigate experimentally the relationship between electromotive force and source (terminal) voltage;

Language objectives

Subject-specific vocabulary & terminology

Electromotive force (EMF) (The voltage generated by a battery or by the magnetic force according to Faraday's Law. It is measured in units of volts, not newtons, and thus, is not actually a force)

Terminal voltage (The voltage output of a device measured across its terminals)

Lost voltage

Internal resistance

Load resistance

Useful set(s) of phrases for dialogue/writing

All voltage sources have two fundamental parts: a source of electrical energy that has a characteristic electromotive force (emf), and an internal resistance r.

The emf is the potential difference of a source when no current is flowing.

The numerical value of the emf depends on the source of potential difference.

The internal resistance r of a voltage source affects the output voltage when a current flows.

The voltage output of a device is called its terminal voltage V and is given by V = emf − Ir, where I is the electric current and is positive when flowing away from the positive terminal of the voltage source.

When multiple voltage sources are in series, their internal resistances add and their emfs add algebraically.

Values instilled at the lesson

v  Appreciation for the unique abilities of each learner.

v  Respect for other’s opinion while working in a group.

v  Lifelong learning that theories learned from our discussion could be applied also to practical life.

v  Internationalism – the students must recognize that the discovery of capacitors is made by physicists who came from different countries.

Cross-curricular links

      Visual Literacy Instruction – the students must understand, appreciate and comprehend what they have seen in the presentation and other video materials.

      *Utilization of IT in teaching and learning process.

      Mathematics-knowledge on algebra, fundamentals and higher mathematics are pre-requisites to this lesson

ICT skills

The following links are good websites and additional resources for this lesson

https://tap.iop.org/electricity

https://opentextbc.ca/physicstestbook2/

Previous learning

Grade 8: d.c. circuits; V = IR; Ohm’s Law; electrical characteristics; circuit symbols; circuit diagrams; electrical energy and power;

Course of the lesson

Planned stages of the lesson

Planned activities at the lesson

Resources

Beginning

10 min

Teacher:

-Introduces the topic of day and spelling out the learning outcome they will possess after the study.

1. Organizational moment to acquaint students with the

• The theme of the lesson
• The objectives of the lesson
• The criteria of success for the lesson
• The plan of events for the lesson

2. Demonstration.  

Demonstrate some simple circuits in which a power supply delivers electrical energy to at first one then another one lamp connected in series. As the voltage of lamps is increased, the voltage across each falls and the brightness falls. Why?

Middle

11-20 min

21-34 min

35-37 min

Voltage is a measure of the ENERGY of the electricity

Each coulomb of electricity that flows from a battery has some energy. The voltage of the battery tells us just how much energy. So if one coulomb of electric charge flows from a 240 V source, it will have more energy than one from a 12 V source.

We can compare voltage with gravitational (potential) energy in the pictures below.

(T) Teacher explanation.

Introduce learners to a concept of e.m.f as: a work done by external applied forces on displacement of a unit positive charge:

E=Wext./ q

Recall the definition of voltage from the Unit ‘Electrostatics’: potential different or voltage V is: the work done by an electric field on displacement of a unit charge in subcircuit

V= W_e/ q

(G) Group work. EMF (electromotive force) VS Potential difference

Ask learners to fill in the table about the difference between Electromotive Force and Potential Difference and similarities as they watch a video.

Ø  Terminal voltage and internal resistance

Any voltage source (in this case, a carbon-zinc dry cell) has an emf related to its source of potential difference, and an internal resistance r related to its construction. Also shown are the output terminals across which the terminal voltage V is measured. Since V = emf − Ir, terminal voltage equals emf only if there is no current flowing.

Ø  Measuring e.m.f. and Internal Resistance - Open Circuit/Close Circuit

Open Circuit

In open circuit (when the switch is off), the voltmeter shows the reading of the e.m.f.

ε= Vterminal

Close Circuit

In closed circuit (when the switch is on), the voltmeter shows the reading of the potential difference across the cell. With the presence of internal resistance, the potential difference across the cell is always less than the e.m.f.

V_lost =  V_terminal = Ir

V_terminal + V_lost

Activity. True or False:

"The terminal voltage is usually greater than the emf."

Appendix 1

Appendix 2

End

  38-40 min

At the end of the lesson, learners reflect on their learning:

-           What has been learned

-           What remained unclear

-           What is necessary to work on

Where possible the learners could evaluate their own work as well as the work of their classmates using certain assess criteria.

Differentiation – how do you plan to give more support? How do you plan to challenge the more able learners?

Assessment – how are you planning to check students’ learning?

Health and safety regulations

Differentiation can be by task, by outcome, by individual support, by selection of teaching materials and resources taking into account individual abilities of learners (Theory of Multiple Intelligences by Gardner).

Differentiation can be used at any stage of the lesson keeping time management in mind.

Use this section to record the methods you will use to assess what students have learned during the lesson.

Health promoting techniques

Breaks and physical activities used.

Points from Safety rules used at this lesson.

Reflection

Were the lesson objectives/learning objectives realistic? Did all learners achieve the LO?

If not, why?

Did my planned differentiation work well?

Did I stick to timings?

What changes did I make from my plan and why?

Use the space below to reflect on your lesson. Answer the most relevant questions from the box on the left about your lesson. 

Summary evaluation

What two things went really well (consider both teaching and learning)?

1:

2:

What two things would have improved the lesson (consider both teaching and learning)?

1:

2:

What have I learned from this lesson about the class or achievements/difficulties of individuals that will inform my next lesson?