Physics – Formulae Sheet
17. CURRENT ELECTRICITY (1). Resistance of a uniform conductor of length L, area of cross-section A and resistivity along its length, R
A
(2). Current density j
di ds
(3). Conductance G
i4) Drift velocity v d
eE i t. m neA
(6) Resistivity is
m
(7) A o di g to Oh (9) ne
ne2 t s la
1 . R
(5). Current i neAv d
1 where is resistivity. j E and V iR
(8) Mobility of free electrons
vd E
(10). Thermal resistivity of material is T 0 [1 (T T0 )]
(11). Potential difference across a cell during discharging V ir (12). Potential difference across a cell during charging
R R r
V ir
(13) For n cells in series across load R, current through load
i
n R nt
(14). For n identical cells in parallel across load R, current through load i
n nR r
(15). Wheatstone bridge network For balanced Wheatstone bridge
R1 R2
R3 R4
(16). If unknown resistance X is in the left gap, known resistance R is in the right gap of meter bridge and balancing length from left end is l then X
R 100
(17) Potentiometer 27