The capacity of batteries is usually rated in ampere-hours (that is, amperes tim

Question

The capacity of batteries is usually rated in ampere-hours (that is, amperes times hours), abbreviated A hr. For rechargeable NiMH (nickel metal hydride) D cells that provide a potential difference of 1.5V each, this capacity is about 7000 mA hr (where m is milli, the standard prefix for 10A-3). a) Three D cells in series can operate an LED flashlight; a single LED element draws about 100 mW. Assuming ten LEDs in the flashlight, and that the power consumption of the LED and the power yield of the batteries are both constants (they aren't: both degrade over time), how many hours of use is there on a single charge of the three batteries? b) A battery-operated toy car carries a single D cell described above, and drives itself around a track at 0.5 m/s. If it takes 2 N of force to maintain this speed, how long can the battery keep the toy moving on a single charge?

Explanation / Answer

a)p.d of each battery is 1.5v',charge is q=7000mA hr=7Ahr

three cells are in series then total p.d is =3V=3*1.5=4.5v,each cell power p=100mW=10-1w

time t=Vq/p=4.5*7/10-1=315hr

b)a single cell connected to a toy carthen it is used in atime t=Vt/Fv

here V=p.d=1.5, v=velocity=0.5m/s,q=7hr,F=2n

t=1.5*7/(2*0.5)=10.5hr

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