Internal resistance in Batteries
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.
Equations calculating internal resistance
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.
This is the end of the topic
Drafted by Brandon (Physics)