Question

when to use V=E-Ir and V = E+Ir???​

Answer 1

V=E-Ir and V=IR

you should have elaborated,

it has a sound as if a cell of emf E and internal resistance r is connected to a load resistance R such that a current I is flowing

E will be the open circuit potential difference between the electrodes of the cell

it would be incorrect to infer that I = E/R because the cell has its own internal resistance and it is supplying a current I to a circuit,

in this case, the potential difference across the cell terminals drops to

V = E-IR

this potential difference gets applied to the load resistance R causing a current

I = V/R

Answer 2

There are basically 2 cases as follows ==>

1. During charging-

When the cells are being charged the formula used is V=E+Ir which can be further written as V-E=Ir

2. During discharging-

When the cells are being discharged the formula used is V=E-Ir which can be further written as E-V=Ir

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