An electronics technician is analyzing how electromotive force (EMF) and terminal voltage … — Armed Services Vocational Aptitude Battery (ASVAB)

An electronics technician is analyzing how electromotive force (EMF) and terminal voltage behave across various operational states of a DC power source. Match each electrical condition on the left with its correct analytical relationship or physical description on the right.

Open-circuit cell voltageEquals the full internal electromotive force ( $\mathcal{E}$ ) because zero current produces no internal voltage drop ( $I r = 0$ ).
Loaded cell terminal voltageEquals the source electromotive force minus internal resistance drop ( $\mathcal{E} - I r$ ), resulting in a lower potential difference across terminals.
External battery charging voltageMust exceed the source electromotive force ( $\mathcal{E} + I r$ ) to reverse current direction into the positive terminal.
Fundamental definition of the voltRepresents one joule of energy transferred per coulomb of electric charge ( $1\text{ V} = 1\text{ J/C}$ ).

Answer

Open-circuit cell voltage matches the full internal electromotive force without internal drop; loaded cell terminal voltage matches the electromotive force minus internal resistance drop; external battery charging voltage matches the electromotive force plus internal resistance drop; and the fundamental definition of the volt matches one joule of energy per coulomb of charge.

Electromotive force (EMF,

\mathcal{E}

) is the total energy per unit charge supplied by a source. Under no-load conditions (

I = 0

), the terminal voltage equals EMF. When delivering current (

I > 0

) to a load, internal resistance produces a drop (

I r

), lowering terminal voltage to

\mathcal{E} - I r

. During charging, the external supply must exceed EMF by

\mathcal{E} + I r

to force current inward. Fundamentally, one volt is defined as one joule of energy per coulomb of charge (

1\text{ V} = 1\text{ J/C}

Step-by-Step Solution

Analyze open-circuit state for a DC voltage source

Terminal voltage equals internal electromotive force (

V = \mathcal{E}

)

When no current flows (

I = 0\text{ A}

), no voltage is dropped across internal resistance (

I r = 0\text{ V}

Evaluate terminal voltage during active discharge under load

Terminal voltage drops to

V = \mathcal{E} - I r

Current passing through the battery's internal resistance consumes a portion of the source EMF.

Determine required external voltage for charging a cell

Applied charging voltage must be

V = \mathcal{E} + I r

The charger must provide higher potential than the cell's EMF to drive current in reverse against internal resistance.

State the fundamental physical definition of potential difference and electromotive force

1\text{ V} = 1\text{ J/C}

Voltage is defined as electrical potential energy (work in joules) per unit charge (coulombs).

Key Concept

Electromotive Force (EMF) vs. Terminal Voltage and Internal Resistance Relations

Estimated Time:1m 30s

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