 # Cells, EMF and Internal Resisance

• A cell is device which converts chemical energy into electrical energy.
• The EMF of a cell is defined as the work done in moving unit positive charge in the whole circuit.
• The EMF of a cell is depends only on nature of electrodes and electrolyte.
• Internal resistance is the resistance offered by electrolyte between anode and cathode.
• Internal resistance depends on area and size of plates and distance between the plates. Electrolyte of temperature.
• The terminal potential difference is the work done in moving unit positive charge from anode to cathode.
• The voltage dropped across the terminals of internal resistance is called lost volt.
• Terminal potential difference V = E – ir.
• EMF developed in the cell against the emf of the cell called Back EMF.
• \tt i = \frac{E_{1} + E_{2}}{r_{1} + r_{2} + R}
• Terminal potential difference across first cell V1 = E1 – ir1.
• Terminal potential difference across second cell V2 = E2 – ir2.
• When "n" cells of emf E and internal resistance "r" are connected in series. Then current \tt i = \frac{nE}{R+ nr}
• When out of ‘n’ cells ‘m’ cells are connected wrongly effective emf = (n – 2m) E.
• If two cells of different EMF are connected in parallel \tt E_{eq} = \frac{E_{1} r_{2} + E_{1}r_{2}}{r_{1} + r_{2}}
• Current “i” when E1, E2 connected in parallel \tt i = - \frac{E_{1}r{2} + E_{2} r_{1}}{r_{1}r_{2} + R(r_{1} + r_{2})}
• When ‘n’ cells each of emf ‘E’ and internal resistance ‘r’ are connected in parallel. Current \tt I = \frac{E}{R+ (\frac{r}{m})}
• If R <<  \tt \frac{r}{m} Then \tt I = \frac{mE}{r}
• If R >>  \tt \frac{r}{m} Then \tt I = \frac{E}{R}
• "n" cells in series such ‘m’ cells are in parallel. Then Net resistance of circuit \tt R + \frac{nr}{m}.
• Current in the above \tt i = \frac{nmE}{mR + nr}.

### View the Topic in this video From 0:44 To 35:44

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1. Relation between ε, V and R
The voltage across R is
V = IR