How does the definition of EMF differ from the definition of Potential Difference (PD) regarding energy?

EMF is the energy converted from non-electrical forms to electrical energy per unit charge. PD is the energy converted from electrical energy to other forms (like heat or light) per unit charge as it passes through a component.

Under what specific condition is the terminal potential difference of a battery equal to its EMF?

This occurs when no current is flowing through the circuit (an open circuit). Since $V = \varepsilon - Ir$, if $I = 0$, then $V = \varepsilon$.

Why does the terminal voltage of a battery decrease when the current drawn from it increases?

Increasing current increases the 'lost volts' ($Ir$) across the internal resistance. Since the terminal voltage is $V = \varepsilon - Ir$, a larger $Ir$ term results in a smaller $V$.

What is the most common mistake when using the formula $V = IR$ in a circuit with internal resistance?

Students often use the EMF ($\varepsilon$) instead of the terminal voltage ($V$) when calculating the current through the external load. $V$ is the voltage actually across the load, whereas $\varepsilon$ is the total voltage of the system.

Why is it incorrect to say that a 12V battery always provides 12V to a circuit?

The 12V rating is the EMF, which is only available at the terminals when no current flows. Once a circuit is connected, internal resistance causes a voltage drop, making the actual delivered voltage less than 12V.

What happens to the 'lost volts' if a battery is short-circuited?

In a short circuit, the external resistance is nearly zero, so the current becomes very high. Consequently, almost all of the EMF is consumed as 'lost volts' ($Ir$) within the battery itself, leading to rapid heating.

Define 'Internal Resistance' in the context of a power supply.

Internal resistance is the inherent opposition to the flow of current within the power source itself, caused by the materials and chemical reactions inside the cell.

What are 'Lost Volts'?

Lost volts refer to the potential difference dropped across the internal resistance of a power source when current flows. It is calculated as $v = Ir$ and represents energy wasted as heat inside the source.

State the full circuit equation for EMF involving both external and internal resistance.

The equation is $\varepsilon = I(R + r)$, where $\varepsilon$ is the EMF, $I$ is the current, $R$ is the external load resistance, and $r$ is the internal resistance.

How can the internal resistance of a cell be determined from a graph of Terminal Voltage ($V$) vs. Current ($I$)?

The internal resistance $r$ is equal to the negative of the gradient (slope) of the $V-I$ graph. The y-intercept of this graph represents the EMF ($\varepsilon$).

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Electromotive Force

Summary

Electromotive Force (EMF) is the total energy supplied by a power source per unit of electric charge. It represents the conversion of non-electrical energy (such as chemical or mechanical) into electrical energy, establishing the maximum potential difference available before internal losses occur.

1. Definition & Core Concepts

Electromotive Force (EMF) is defined as the work done by a source in driving a unit charge around a complete circuit. It is measured in Volts (V), where $1\text{ V} = 1\text{ J/C}$ .
Despite the name, EMF is not a force in the mechanical sense; it is a measure of energy transfer per unit charge.
In an open circuit (where no current flows), the potential difference across the terminals of the power source is exactly equal to its EMF.
The symbol for EMF is typically the Greek letter epsilon ( $\varepsilon$ ).

Circuit diagram showing a power supply with EMF (ε) and internal resistance (r) in series with an external load resistor (R).

2. Underlying Principles

Energy Conservation: The total energy supplied by the source (EMF) must equal the sum of the energy used in the external circuit and the energy dissipated within the source itself.
Internal Resistance ( $r$ ): Real power sources possess inherent resistance due to their chemical or physical makeup. When current flows, some energy is converted to heat inside the source.
Lost Volts: The potential drop across the internal resistance is referred to as 'lost volts' ( $v = Ir$ ). This voltage is not available to the external components of the circuit.

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions