Why does a torch become dimmer as its battery gets old, even though the electromotive force has hardly changed?

The internal resistance rises as the cell discharges, so for the same lamp a larger share of the electromotive force is lost inside the cell and the terminal potential difference across the lamp falls.

Why must a supply intended to deliver large currents have a low internal resistance?

The lost volts are Ir, so with a large current a high internal resistance would produce a large drop in terminal potential difference and would also dissipate a great deal of energy inside the supply itself.

Why is a car battery dangerous if its terminals are accidentally connected by a spanner?

Its internal resistance is very small, so the short-circuit current E/r is enormous; the spanner and the battery heat rapidly and may cause burns, an explosion of the battery or a fire.

State the expression for the greatest current a source can deliver, and explain why it is a maximum.

The greatest current is I = E/r, obtained when the external resistance is zero, because from E = I(R + r) any external resistance can only make the total resistance larger and the current smaller.

CCEA A-Level Physics 1210

Physical Quantities, Scalars and Vectors

Topic overview Physical quantities and SI base units Prefixes and standard form Scalars and vectors Resolving vectors and finding resultants Coplanar forces in equilibrium

Moments and Equilibrium

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Linear Motion and Dynamics

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Newton's Laws of Motion

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Linear Momentum and Impulse

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Current, Charge and Potential Difference

Topic overview Electric current and the coulomb Potential difference and energy transfer Potential difference, power and current Electromotive force Comparing electromotive force and potential difference

Resistance and Resistivity

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Internal Resistance and Potential Dividers

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Waves

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Internal Resistance and Potential Dividers

Consequences of internal resistance for external circuits

3 things to know
Unit AS 1

Overview

Recall questions on why headlamps dim when the starter turns and why an old battery gives a dim torch, the greatest current a source can deliver, the power delivered to the load against that wasted internally, why efficiency falls as the external resistance falls, and why a school extra-high-tension supply has a deliberately large internal resistance.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Write down expressions for the power delivered to the external circuit and the power wasted inside a source.

Key term

Why do a car's headlamps dim briefly when the starter motor is operated?

Key term

Why does the efficiency of energy transfer from a source fall as the external resistance is reduced?

Questions from this subtopic

Each one with the model answer underneath.

1
Why do a car's headlamps dim briefly when the starter motor is operated?
Model answer
The starter motor draws a very large current from the battery, so the lost volts Ir become significant and the terminal potential difference falls, leaving less potential difference across the headlamps.
2
Why does a torch become dimmer as its battery gets old, even though the electromotive force has hardly changed?
Model answer
The internal resistance rises as the cell discharges, so for the same lamp a larger share of the electromotive force is lost inside the cell and the terminal potential difference across the lamp falls.
3
Why must a supply intended to deliver large currents have a low internal resistance?
Model answer
The lost volts are Ir, so with a large current a high internal resistance would produce a large drop in terminal potential difference and would also dissipate a great deal of energy inside the supply itself.
4
Why is a car battery dangerous if its terminals are accidentally connected by a spanner?
Model answer
Its internal resistance is very small, so the short-circuit current E/r is enormous; the spanner and the battery heat rapidly and may cause burns, an explosion of the battery or a fire.
5
State the expression for the greatest current a source can deliver, and explain why it is a maximum.
Model answer
The greatest current is I = E/r, obtained when the external resistance is zero, because from E = I(R + r) any external resistance can only make the total resistance larger and the current smaller.
6
Write down expressions for the power delivered to the external circuit and the power wasted inside a source.
Model answer
The power delivered externally is I²R and the power wasted internally is I²r, where I is the current, R the external resistance and r the internal resistance, and the two together equal the total power EI produced by the source.

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