8Electromotive force and internal resistance of current source.

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Demonstration.  

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?

Simple demonstrations can show qualitatively that:

• Cells supply electrical energy.
• Lamps (and other components) transfer electrical energy to other forms.
• Voltage is linked to work done by an electric field.

Circuit 1: The reading of V₁ in the first circuit shows how much energy is lost in the bulb - it is turned into heat and light. The reading of V₂ will be 0, as almost no energy is lost in the connecting wires.

Circuit 2: the battery is supplying energy to two bulbs. If the bulbs are identical the energy lost in one will be the same as the energy lost in the other.
Using the figures in the diagram there will be a 3 V drop across each bulb.

Circuit 3: one bulb is much more powerful than the other, there is much more energy used in it and so the voltage drop across it is large compared with the other bulb.

Discussion: 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.

·         If a given amount of mass travels down the hill, say one truck load, the energy lost depends on the height of the hill; the higher the hill the more energy is lost. The height is measured vertically from the top to the bottom of the hill.

·         If a given amount of electricity flows from a battery, say one coulomb, the energy lost depends on the voltage of the battery; the greater the voltage the more energy is lost. Voltage (properly called potential difference) is measured between two places in the circuit.

(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:

ε=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 sub circuit

V= We/ 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.

Sample:

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."

REFLECTION

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.