# Internal resistance

1. Jun 2, 2005

### faisal

A battery of e.m.f. 4.5V is connected across a 12 ohm resistor, if the Pd. Across the resistor is 3V calculate
a)the current in the circuit ?
b)internal resistance of the battery?
iv tried my best but i cant do it

2. Jun 2, 2005

### Delta

a) using the resistor, remember ohms law.
b) if the resistor is using 3v what is the internal resistance taking, and also remember that is a series circuit, so what is the same throughout.

3. Jun 3, 2005

### faisal

I Still dont understand heres my answer
a)2/9.5=0.2 amps
b)0.2X 9.5= 1.9v (V=IR)

4. Jun 3, 2005

### faisal

i DONT understand this example given to me
q.1 A 9v battery with an internal resisrance of 0.1 ohms is connected to a 3 ohm resistance. What current flows across the circuit and what is the P.D. across the terminals of the battery
E=I(R+r)
I=E/(R+r)
answer 2.9A (i understand this part)
PD across battery= VR= (9-.29)=8.7V where on earth did 0.29 come from?

5. Jun 3, 2005

### apchemstudent

the 0.29 is the voltage lost to the internal resistance of the battery.

I*internal resistance = 2.9*0.1 = 0.29 Volts.

6. Jun 5, 2005

### Delta

I'm a little puzzled where the 9.5 and the 2 have come from.

If the resistor is 12ohms and is taking 3V, then you can find the current flowing through it, and hence the circuit as all components are in series.

By knowing that there is a supply of 4.5V and the resistor is taking 3V, you can find what PD the internal resistance is taking, and by working out the circuit current in question a), you will find the that internal resistance value.