1) A household 120V bulb is rated 75W. howmuch is the current via the bulb at 12
ID: 1870192 • Letter: 1
Question
1) A household 120V bulb is rated 75W. howmuch is the current via the bulb at 120V, and how much is the resistance of the bulb's filament at this current?
2) How much energy does a 60W bulp dissipate in one hour, in Joules? One day?
3) How much is the resistance of 50 ft of household 14 gauge Cu wire (1.63 mm in diameter), assumin that the resistivity of Cu is 1.70x10^-8 Ohm m. If a current of 15 A is flowing in this wire, how much is the voltage drop on it?
4) In the previous problem, how muc is the speed of electron in Cu wire, assuming that Cu has 8.5x10^28 free electron per cubic meter?
Explanation / Answer
1.
Current
I=P/V =75/120 =0.625 A
Resistance
R=V/I =120/0.625
R=192 ohms
2.
Energy dissipated in t=1 hour
E=Pt =(60/1000)*1=0.06 KWH
in t=1 day
E=(60/1000)*24=1.44 KWH
3.
1 ft =0.3048 m
Area A=pi*d2/4 =pi*(1.63*10-3)2/4
A=2.087*10-7 m2
Resistance
R=pL/A=(1.7*10-8)*(50*0.3048)/(2.087*10-7)
R=0.124 ohms
Voltage drop
V=IR =15*0.124=1.862 Volts
4.
Speed of electrons
Vd=I/nAq =15/(8.5*1028)(2.087*10-7)(1.6*10-19)
Vd=5.286*10-4 m/s
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