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Phy A.C. root mean square 急!!!
發問:
rms voltage 同 rms current 乜唔係可以睇成 係 D.C既constant voltage 同 current 咁姐係話 rms votage 同 rms current 係CONSTANT then 我個問題就係 若係RC Series Circuit,比左個rms voltage of capacitor 同rms voltage of resistor , 找個 total pd across capacitor 同 resistor , 點解仲要用phasor diagram , 點解唔可以直接加埋????仲有, 我好唔明, 有年al 考過呢個問題, marking... 顯示更多 rms voltage 同 rms current 乜唔係可以睇成 係 D.C既constant voltage 同 current 咁姐係話 rms votage 同 rms current 係CONSTANT then 我個問題就係 若係RC Series Circuit,比左個rms voltage of capacitor 同rms voltage of resistor , 找個 total pd across capacitor 同 resistor , 點解仲要用phasor diagram , 點解唔可以直接加埋???? 仲有, 我好唔明, 有年al 考過呢個問題, marking 到寫話因為個rms votage of capacitor 同 rms votage of resistor 唔同phase ,所以就唔洗加埋 , 但rms 既votage 唔係一個constant 咩,, 仲有phase 可言???唔係應該用唔到phasor diagram??? 求求幫忙清下d concept, thx 更新: 但係 V(rms) = peak voltage / root 2 , Vc(rms) against time 同 Vr(rms) aganist time 都係橫線,即係CONSTANT 係原則上冇佢地冇PHASE difference , 個phasor diagram of Vc(rms) and Vr(rms) WHY 仲係差90度?? thx
最佳解答:
THe "constant volgae or constant current" concept of rms values are only applicable to power consumption. It is by definition that the rms current (or voltage) produces the same heating effect produced by the same value of dc current (or voltage). In a RC series circuit, it is correct that the supplied rms voltage is not equal to the direct sum of rms voltages across the capacitor and resistor.The peak-to-peak voltages of the RC series circuit can be written as, (Vs)^2 = (Vc)^2 + (Vr)^2 where Vs, Vv and Vr are the peak-to-peak voltages of the power source, capacitor and resistor respectively. Multiply both sides of the equation by(1/2), we have (Vs)^2/2 = (Vc)^2/2 + (Vr)^2/2 but [Vs/square-root(2) equals to Vs(rms), thus (Vs)^2/2 = [Vs(rms)]^2. Similarly for Vc and Vr, hence, we have: [Vs(rms)]^2 = [Vc(rms)]^2 + [Vr(rms)]^2 Clearly, the rms voltages could not be added up directly. The reason is because of there is a phase difference of 90 degrees (pi/2 radians) between Vc and Vr. 2011-01-17 09:06:42 補充： First, I have to correct a typo. Vs, Vc and Vr are the peak values, NOT the peak-to-peak values. The rms values is just a physical concept. It denotes the value of ac current as if it were constant such that it could produce the same heating effect as the same value of dc current....(cont'd) 2011-01-17 09:07:44 補充： ... Hence, be aware that in reality, the ac current does vary with time. In the actual fact, phase and current value are two different physical quantities. A constant value doesn't implies that there is no phase diffence....(cont'd) 2011-01-17 09:07:59 補充： ...There is no theory whatsoever that there is no phase difference between rms current of different circuit components.
Phy A.C. root mean square 急!!!
發問:
rms voltage 同 rms current 乜唔係可以睇成 係 D.C既constant voltage 同 current 咁姐係話 rms votage 同 rms current 係CONSTANT then 我個問題就係 若係RC Series Circuit,比左個rms voltage of capacitor 同rms voltage of resistor , 找個 total pd across capacitor 同 resistor , 點解仲要用phasor diagram , 點解唔可以直接加埋????仲有, 我好唔明, 有年al 考過呢個問題, marking... 顯示更多 rms voltage 同 rms current 乜唔係可以睇成 係 D.C既constant voltage 同 current 咁姐係話 rms votage 同 rms current 係CONSTANT then 我個問題就係 若係RC Series Circuit,比左個rms voltage of capacitor 同rms voltage of resistor , 找個 total pd across capacitor 同 resistor , 點解仲要用phasor diagram , 點解唔可以直接加埋???? 仲有, 我好唔明, 有年al 考過呢個問題, marking 到寫話因為個rms votage of capacitor 同 rms votage of resistor 唔同phase ,所以就唔洗加埋 , 但rms 既votage 唔係一個constant 咩,, 仲有phase 可言???唔係應該用唔到phasor diagram??? 求求幫忙清下d concept, thx 更新: 但係 V(rms) = peak voltage / root 2 , Vc(rms) against time 同 Vr(rms) aganist time 都係橫線,即係CONSTANT 係原則上冇佢地冇PHASE difference , 個phasor diagram of Vc(rms) and Vr(rms) WHY 仲係差90度?? thx
最佳解答:
THe "constant volgae or constant current" concept of rms values are only applicable to power consumption. It is by definition that the rms current (or voltage) produces the same heating effect produced by the same value of dc current (or voltage). In a RC series circuit, it is correct that the supplied rms voltage is not equal to the direct sum of rms voltages across the capacitor and resistor.The peak-to-peak voltages of the RC series circuit can be written as, (Vs)^2 = (Vc)^2 + (Vr)^2 where Vs, Vv and Vr are the peak-to-peak voltages of the power source, capacitor and resistor respectively. Multiply both sides of the equation by(1/2), we have (Vs)^2/2 = (Vc)^2/2 + (Vr)^2/2 but [Vs/square-root(2) equals to Vs(rms), thus (Vs)^2/2 = [Vs(rms)]^2. Similarly for Vc and Vr, hence, we have: [Vs(rms)]^2 = [Vc(rms)]^2 + [Vr(rms)]^2 Clearly, the rms voltages could not be added up directly. The reason is because of there is a phase difference of 90 degrees (pi/2 radians) between Vc and Vr. 2011-01-17 09:06:42 補充： First, I have to correct a typo. Vs, Vc and Vr are the peak values, NOT the peak-to-peak values. The rms values is just a physical concept. It denotes the value of ac current as if it were constant such that it could produce the same heating effect as the same value of dc current....(cont'd) 2011-01-17 09:07:44 補充： ... Hence, be aware that in reality, the ac current does vary with time. In the actual fact, phase and current value are two different physical quantities. A constant value doesn't implies that there is no phase diffence....(cont'd) 2011-01-17 09:07:59 補充： ...There is no theory whatsoever that there is no phase difference between rms current of different circuit components.
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