AS/A-level Physics - E.m.f and Internal Resistance

IN AS/A-level Physics, we all know that Batteries have resistance.

• Resistance comes from electrons colliding with atoms and losing energy.
• In a battery, chemical energy is used to make electrons move. When they move, they collide with atoms inside the battery.
• This resistance is internal resistance!
• Internal resistance is what makes cells warm up.

• The amount of electrical energy the battery produces per unit charge is called the electromotive force (e.m.f)
• The p.d. across the resistance of the components in the circuit is called the terminal p.d.
• If there was no internal resistance, the terminal p.d. would be the same as the e.m.f. However, in real power supplies there is ALWAYS energy lost overcoming internal resistance.
• Energy wasted overcoming internal resistance is called lost volts.

There are any calculations about e.m.f and internal resistance in AS/A-level Physics.

One equation interpereted in different ways:

• ε=V + v
• V= ε - v
• V= ε - IR
• ε= IR - Ir

The last one is given on formula sheet.

• Most power supplies need low internal resistance.
• A car battery needs to supply a really high current, so it needs low internal resistance.
• Generally batteries have an internal resistance of 1Ω.
• High voltage power supplies are an exception.
• High tension (HT) and extremely high tension (EHT) supplies are designed with veryy high internal resistances, so if the short circuit, only a small amount of current will flow, which is much safer!

You can measure internal resistance by changing the value of R in a circuit and measuring I and V,

• you can use a graphical method to work out r. 
• Rearrange the equation to V= -rI + ε, ε is the y intercept and -r is the gradient.

• connect a high resistance voltmeter across the terminals.
•  A small current does pass throught the voltmeter (so value measured will be slightly less than e.m.f.) 
• but it usually isn't significant.

References:

1. https://getrevising.co.uk/revision-notes/emf_and_internal_resistance 
2. https://slidetodoc.com/internal-resistance-batteries-and-cells-and-batteries-come/

This is the end of the topic

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Drafted by Brandon (Physics)