如果加在某定值电阻两端的电压从6V增加到10V，通过该电阻的电流变化了0.2A，则该电阻的阻值和电功率变化值分别是（ ）
A．30Ω 0.8W
B．20Ω 3.2W
C．50Ω 1.6W
D．20Ω 0.8W
电阻两端的电压变化，电阻保持不变，
当电阻两端的电压为6V时，
通过电阻的电流为I1=
，该电阻的电功率为P1=6V R
；(6V)2 R
当电阻两端的电压为10V时，
通过电阻的电流为I2=
，该电阻的电功率为P2=10V R
，(10V)2 R
电流变化量△I=I2-I1=
-10V R
=0.2A，6V R
解得：R=20Ω，
所以该电阻的电功率变化了△P=P2-P1=
-(10V)2 R
=(6V)2 R
-(10V)2 20Ω
=3.2W．(6V)2 20Ω
故选B．
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