# Circuit analysis - two sources of emf

Hello all!

I'm confused about an explanation that my textbook gives, and I'm wondering if someone can help me understand it. The passage is as follows:

A circuit consists of a 12 V battery with negligible internal resistance, in parallel with a charging unit with an emf of 15 V and internal resistance of 0.5 ohms. These two components provide power for all the car's electrical equipment.

The car's battery maintains an emf of 12 V across it, because of its negligible internal resistance. Hence, the terminal p.d. of the charging unit must also be 12 V. This implies that 3 V must be "lost volts" across the internal resistance of the charging unit. Since its internal resistance is 0.5 V, there must be a current of 6 A supplied by the charging unit.

I'm confused specifically with the underlined statement - why does the p.d. of the charging unit have to be 12 V as well? Is it because the battery is actually powering the charging unit? Why then does it have an emf of 15 V?

Any insight would be appreciated.

Miss E.

jedishrfu
Mentor
Welcome to PF!

Please take some time to read the site guidelines. We ask that all homework questions be formatted using the homework template so we can gauge your understanding.

With respect to your question, the charging unit is trying to charge the battery. A car battery is used primarily to start the car and after that the engine powers a generator to keep the battery charged and to supply the necessary amperage to power the cars electrical system. There should be no drain on the battery.

Last edited:

## Homework Statement

Hello all!

I'm confused about an explanation that my textbook gives, and I'm wondering if someone can help me understand it. The passage is as follows:

A circuit consists of a 12 V battery with negligible internal resistance, in parallel with a charging unit with an emf of 15 V and internal resistance of 0.5 ohms. These two components provide power for all the car's electrical equipment.

The car's battery maintains an emf of 12 V across it, because of its negligible internal resistance. Hence, the terminal p.d. of the charging unit must also be 12 V. This implies that 3 V must be "lost volts" across the internal resistance of the charging unit. Since its internal resistance is 0.5 V, there must be a current of 6 A supplied by the charging unit.

I'm confused specifically with the underlined statement - why does the p.d. of the charging unit have to be 12 V as well? Is it because the battery is actually powering the charging unit? Why then does it have an emf of 15 V?

Any insight would be appreciated.

Miss E.

Kirchoff's laws

## The Attempt at a Solution

n/a

Thank you, I've moved the question to the proper forum.

haruspex
Homework Helper
Gold Member
2020 Award
why does the p.d. of the charging unit have to be 12 V as well?
Let the terminals of the charging unit be A and B, those of the battery be C and D, with A connected to C, B connected to D.
You know the potential difference between C and D, independently of current. You can take the resistances of AC and BD to be very low. If the p.d. between A and B exceeds that between C and D, what does that tell you about the currents in AC, BD?

NascentOxygen
Staff Emeritus