7_Electromotive force and internal resistance of current source.

To investigate the relationship between electromotive force and source voltage

Analysis
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;

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?

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.

EMF (electromotive force) VS Potential difference

Definition:

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.

Open Circuit

ε= Vterminal




















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

Close Circuit

Vlost = 𝜺𝜺− Vterminal = Ir
𝜺𝜺= Vterminal + Vlost
 

















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.

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

Answer and Explanation:

False
If there is no current flow, the emf of the battery is simply equal to the potential difference across its terminal. This is equivalent to the total voltage that the battery can provide assuming that the battery is ideal or has no internal resistance. However, there is no perfect battery which means that all batteries or cells always have its own resistance called the internal resistance. This internal resistance opposes current flow inside the battery causing a thermal energy to be wasted within itself. Thus, when current flows through the circuit, the terminal voltage that can be measured across the terminal of the battery is always lesser than its emf. And so, the electromotive force (emf) is given by:
ϵ=V+Ir
where
V is the terminal voltage
I is the current through the circuit
r is the internal resistance