How does the energy transfer in E.M.F. differ from that in Potential Difference?

E.M.F. describes the transfer of energy from non-electrical forms (like chemical or mechanical) into electrical energy. Potential Difference describes the transfer of electrical energy into other forms (like heat or light) within circuit components.

Why is the terminal potential difference of a battery usually lower than its E.M.F.?

When a current flows, some voltage is dropped across the battery's internal resistance ($v = Ir$). This 'lost volts' must be subtracted from the total E.M.F. to find the remaining voltage available to the external circuit.

Under what specific condition does the terminal potential difference equal the E.M.F.?

This occurs when no current is flowing through the circuit ($I = 0$), such as in an open circuit. Without current, there are no 'lost volts' ($Ir = 0$), so the measured terminal voltage reflects the full E.M.F.

What is a common mistake when calculating internal resistance from a V-I graph?

Students often forget that the gradient of a $V$ vs $I$ graph is negative ($-r$). The internal resistance is the absolute value (magnitude) of that gradient, not the negative value itself.

Why is it incorrect to assume that terminal potential difference is a constant value for a battery?

Terminal P.D. depends on the current flowing through the circuit ($V = \epsilon - Ir$). As the external load changes, the current changes, which directly alters the amount of voltage lost to internal resistance.

What error occurs if you use a low-resistance voltmeter to measure E.M.F.?

A low-resistance voltmeter allows a small current to flow, which creates 'lost volts' across the internal resistance. This results in a reading that is slightly lower than the true E.M.F.

Define 'Lost Volts' in the context of a DC circuit.

Lost volts refers to the potential difference that develops across the internal resistance of a power source when current flows. It represents energy per unit charge dissipated as heat within the source rather than being delivered to the load.

State the equation relating E.M.F., terminal P.D., current, and internal resistance.

The equation is $\epsilon = V + Ir$, where $\epsilon$ is E.M.F., $V$ is terminal P.D., $I$ is current, and $r$ is internal resistance. It can also be written as $\epsilon = I(R + r)$.

What does the y-intercept of a graph of Terminal P.D. vs. Current represent?

The y-intercept represents the E.M.F. ($\epsilon$) of the power supply. This is because the y-intercept is the point where current $I = 0$, and at zero current, $V = \epsilon$.

How does increasing the external load resistance $R$ affect the 'lost volts'?

Increasing $R$ decreases the total current $I$ in the circuit. Since lost volts is calculated as $Ir$, a lower current results in fewer lost volts and a terminal P.D. closer to the E.M.F.

Library Podcasts

Courses

Referral & Rewards

2. Waves & Electricity

E.M.F. vs. Terminal Potential Difference

Summary

Electromotive force (E.M.F.) and terminal potential difference (P.D.) are two distinct ways of measuring voltage in a circuit. While E.M.F. represents the total energy supplied to the charge by a source, terminal P.D. represents the actual voltage available to the external circuit after accounting for energy losses due to internal resistance.

1. Definition & Core Concepts

Electromotive Force (E.M.F., $\epsilon$ ): This is defined as the total chemical energy converted into electrical energy per unit charge as it passes through a power supply. It is measured in Volts ( $V$ ), where $1 V = 1 J/C$ .
Terminal Potential Difference (V): This is the actual potential difference measured across the terminals of a power supply when it is connected to a circuit. It represents the electrical energy transferred to the external components per unit charge.
Internal Resistance (r): Every power supply has an inherent resistance within it. As current flows through the supply, some energy is dissipated as heat within the source itself, leading to a drop in the available voltage.

Circuit diagram showing a cell with internal resistance r in series with an external load R. A dashed box indicates the power supply boundaries.

2. Underlying Principles

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Graphical Analysis