a battery has an emf of 15V. the terminal voltage of the battery is 11.6V when it is delivering 20W of power to an external load resistor R. What is the value of R? and what is the internal resistance of the battery?
Vt=Vb+VL where
Vt - no load voltage = 15V
Vb - Voltage across the battery under load = 11.6 V
VL - load voltage = (15- 11.6 )V
The circuit current is (it is a series circuit)
I=Vt/(Rb +RL)
The power supplied to the load is
PL=VL^2/RL then
RL= PL/VL^2= 20/(15 - 11.6)^2=1.73 ohms
Current I is
I= PL/VL=20/(15 - 11.6)=5.88A
Finally we have
Rb= (Vt - I RL)/I=(15 - 5.88 x 1.73 )/5.88=
Rb=0.821 ohms