Short circuiting a cell If resistance of the wire AB = R is zero & the switch S is closed, then the cell is said to be short circuited. Then we can write & In fact this is the maximum value of current that can be supplied by a cell or a battery. i.e. terminal […]
Simple circuit 1. Current delivered by cell: 2. Power dissipated by a load is PR = I2R 3. Power dissipated by a int. resistor is Pd, r = I2r 4. Power generated by a cell is 5. t.p.d. across cell & load is same & is V = VA – VB = ξ – I […]
Kirchoff’s laws 1st or junction rule: ΣI = 0 at any isolated junction to conserve current. 2nd or mesh rule: ΣV = 0 for any closed mesh to conserve energy.
Selection of cells Leclanche cell for intermittent supply of current. Daniel cell for constant EMF source. Lead accumulator for strong current ~ 100 A due to its low r. Lead accumulator also called acid cell for highest EMF (= 2.05 V) & minimum internal resistance (≈0.01 Ω). Dry cell for weak current ~ 10 mA.
Secondary cells Secondary cells or accumulators are those which can be recharged. e.g. lead accumulator & alkali cell. Due to smaller internal resistance a secondary cell gives more current than a primary.
Primary cells Primary cells are those which can’t be recharged. e.g. Simple voltaic cell (ξ = 1.08 V), Daniel cell (ξ = 1.12 V), Leclanche cell (ξ = 1.45 V) & Dry cell etc (ξ = 1.50 V).
Standard cell A cell is said to be Standard if its EMF is precisely defined. e.g. Weston – Cadmium cell with an EMF E = 1.0183 V at 20 0 C.
Internal resistance Is the obstruction to the free motion of positive & negative ions of the electrolyte by the viscosity of the electrolyte used in the cell. It is zero for an ideal cell. For a freshly prepared cell the internal resistance is very small & its value increases as the cell is put to […]
^Significant figures All accurately known digits plus the first uncertain digit of a measurement is called significant digits or significant figures. e.g. if in a measurement, mass of a body lies between 5.6 kg & 5.7 kg & we estimate it as 5.62 kg then the first two digits 5 & 6 are certain while […]
EMf of a cell It is defined as the work done by the cell force in moving a unit positive charge inside the cell from the negative to the positive terminal against the potential difference between the cell terminals when no current is drawn from it. i.e. mathematically EMF of a cell depends upon Nature […]
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