How to find internal resistance

How to find internal resistance

The current source has internal resistance. It appears because there are forces which counteract the third-party forces returning charges on a source pole contrary to Coulomb forces. By the nature they remind friction forces. It is possible to calculate internal resistance having used the law of Ohm for a complete chain.

It is required to you

  • - current source;
  • - tester;
  • - consumer.


1. Learn the electromotive force of a source of current (EMF). Usually it is specified on the source or documentation to it. If there is no it, measure it independently. For this purpose take a tester, tune it into voltage measurement. Surely make sure that it has high resistance. Prisoyedinimte a tester to current source clips. It will show value as close as possible to EMF as the current proceeding through it it will be insignificant is small.

2. After that attach to a current source the adequate consumer who is calculated on tension which is generated by this source. For example, if it is necessary to calculate the internal resistance of the battery, connect to it a bulb on 3.5 volts, or the corresponding resistor, but not the household iron. And, on the contrary, at measurement of internal resistance of the powerful generator, connect on it the corresponding loading. Not including a source, measure the consumer's resistance, having measured it by the tester switched to an ohmmeter operating mode.

3. If the tester does not work in the ohmmeter mode, arrive differently. Connect the consumer to a current source. Turn on the measuring device in an operating mode of the ampermeter for measurement of current, and include it in a chain consistently to the consumer and a source. Measure the current proceeding on network in amperes. As all devices are attached consistently, current will be identical in all chain.

4. After that measure voltage drop on the consumer. For this purpose switch a tester for voltage measurement in volts. Attach it parallel to the consumer. Learn voltage drop on the consumer. Find its resistance R, having divided U tension into the current of I (R=U/I). Receive result in Ohms.

5. Calculate the internal resistance of a source of current of r, having divided EMF into current in a chain of I, and having taken away resistance of the consumer of R (r= EDS/I-R) from result. Receive result in Ohms.

Author: «MirrorInfo» Dream Team