介電(dian)常數(shu)又稱(cheng)電(dian)容(rong)率(lv)或相對(dui)電(dian)容(rong)率(lv)，是表征電(dian)介質(zhi)或絕緣材料電(dian)性能的一個重要數(shu)據，常用(yong)ε表示(shi)(shi)。介(jie)(jie)(jie)(jie)(jie)(jie)質(zhi)(zhi)在(zai)外(wai)(wai)加電(dian)(dian)(dian)(dian)(dian)(dian)場(chang)(chang)時(shi)會(hui)產生感應(ying)電(dian)(dian)(dian)(dian)(dian)(dian)荷(he)而削弱電(dian)(dian)(dian)(dian)(dian)(dian)場(chang)(chang)，原外(wai)(wai)加電(dian)(dian)(dian)(dian)(dian)(dian)場(chang)(chang)(真空中(zhong)(zhong))與最終介(jie)(jie)(jie)(jie)(jie)(jie)質(zhi)(zhi)中(zhong)(zhong)電(dian)(dian)(dian)(dian)(dian)(dian)場(chang)(chang)比值即為介(jie)(jie)(jie)(jie)(jie)(jie)電(dian)(dian)(dian)(dian)(dian)(dian)常(chang)數。其表示(shi)(shi)電(dian)(dian)(dian)(dian)(dian)(dian)介(jie)(jie)(jie)(jie)(jie)(jie)質(zhi)(zhi)在(zai)電(dian)(dian)(dian)(dian)(dian)(dian)場(chang)(chang)中(zhong)(zhong)貯存靜(jing)電(dian)(dian)(dian)(dian)(dian)(dian)能的(de)(de)(de)(de)(de)相(xiang)(xiang)對(dui)能力，例如一(yi)個電(dian)(dian)(dian)(dian)(dian)(dian)容板中(zhong)(zhong)充(chong)入(ru)介(jie)(jie)(jie)(jie)(jie)(jie)電(dian)(dian)(dian)(dian)(dian)(dian)常(chang)數為ε的(de)(de)(de)(de)(de)物(wu)質(zhi)(zhi)后可使其電(dian)(dian)(dian)(dian)(dian)(dian)容變大ε倍。介(jie)(jie)(jie)(jie)(jie)(jie)電(dian)(dian)(dian)(dian)(dian)(dian)常(chang)數愈(yu)小絕緣性愈(yu)好(hao)。如果(guo)有(you)高(gao)介(jie)(jie)(jie)(jie)(jie)(jie)電(dian)(dian)(dian)(dian)(dian)(dian)常(chang)數的(de)(de)(de)(de)(de)材料放(fang)在(zai)電(dian)(dian)(dian)(dian)(dian)(dian)場(chang)(chang)中(zhong)(zhong)，場(chang)(chang)的(de)(de)(de)(de)(de)強(qiang)度(du)會(hui)在(zai)電(dian)(dian)(dian)(dian)(dian)(dian)介(jie)(jie)(jie)(jie)(jie)(jie)質(zhi)(zhi)內有(you)可觀的(de)(de)(de)(de)(de)下降。介(jie)(jie)(jie)(jie)(jie)(jie)電(dian)(dian)(dian)(dian)(dian)(dian)常(chang)數還(huan)用來表示(shi)(shi)介(jie)(jie)(jie)(jie)(jie)(jie)質(zhi)(zhi)的(de)(de)(de)(de)(de)極(ji)(ji)化(hua)程度(du)，宏觀的(de)(de)(de)(de)(de)介(jie)(jie)(jie)(jie)(jie)(jie)電(dian)(dian)(dian)(dian)(dian)(dian)常(chang)數的(de)(de)(de)(de)(de)大小，反(fan)應(ying)了微觀的(de)(de)(de)(de)(de)極(ji)(ji)化(hua)現(xian)象(xiang)的(de)(de)(de)(de)(de)強(qiang)弱。氣(qi)體電(dian)(dian)(dian)(dian)(dian)(dian)介(jie)(jie)(jie)(jie)(jie)(jie)質(zhi)(zhi)的(de)(de)(de)(de)(de)極(ji)(ji)化(hua)現(xian)象(xiang)比較弱，各種氣(qi)體的(de)(de)(de)(de)(de)相(xiang)(xiang)對(dui)介(jie)(jie)(jie)(jie)(jie)(jie)電(dian)(dian)(dian)(dian)(dian)(dian)常(chang)數都接近1，液體、固體的(de)(de)(de)(de)(de)介(jie)(jie)(jie)(jie)(jie)(jie)電(dian)(dian)(dian)(dian)(dian)(dian)常(chang)數則各不相(xiang)(xiang)同(tong)，而且介(jie)(jie)(jie)(jie)(jie)(jie)電(dian)(dian)(dian)(dian)(dian)(dian)常(chang)數還(huan)與溫度(du)、電(dian)(dian)(dian)(dian)(dian)(dian)源頻率(lv)有(you)關。有些(xie)物質介電常(chang)(chang)數具有復數形式，其實部即為介電常(chang)(chang)數，虛數部分常(chang)(chang)稱為耗散因數。

　　通常將耗(hao)散因(yin)數(shu)與(yu)介電(dian)(dian)常數(shu)之比稱作耗(hao)散角(jiao)正切，其(qi)可表示材料(liao)與(yu)微波(bo)的(de)耦合能力，耗(hao)散角(jiao)正切值越大，材料(liao)與(yu)微波(bo)的(de)耦合能力就(jiu)越強。例如(ru)當電(dian)(dian)磁波(bo)穿過(guo)電(dian)(dian)解質時，波(bo)的(de)速度(du)被減小，波(bo)長也變(bian)短了。

　　介(jie)(jie)質損(sun)(sun)耗(hao)是(shi)指置于交流(liu)(liu)電場中的介(jie)(jie)質，以內部(bu)發熱的形式表現出來的能量(liang)(liang)(liang)損(sun)(sun)耗(hao)。介(jie)(jie)質損(sun)(sun)耗(hao)角(jiao)(jiao)是(shi)指對介(jie)(jie)質施加(jia)(jia)交流(liu)(liu)電壓(ya)時，介(jie)(jie)質內部(bu)流(liu)(liu)過的電流(liu)(liu)相量(liang)(liang)(liang)與電壓(ya)向量(liang)(liang)(liang)之(zhi)間的夾角(jiao)(jiao)的余(yu)角(jiao)(jiao)。介(jie)(jie)質損(sun)(sun)耗(hao)角(jiao)(jiao)正(zheng)切是(shi)對電介(jie)(jie)質施加(jia)(jia)正(zheng)弦波電壓(ya)時，外施電壓(ya)與相同頻(pin)率的電流(liu)(liu)之(zhi)間相角(jiao)(jiao)的余(yu)角(jiao)(jiao)δ的(de)正切值--tgδ.其物理(li)意(yi)義是：每個周期內介質(zhi)損耗的(de)能(neng)量//每個周期內介質(zhi)存(cun)儲的(de)能(neng)量。

　　介(jie)電損(sun)耗角正(zheng)切常用來表征介(jie)質的介(jie)電損(sun)耗。介(jie)電損(sun)耗是指電介(jie)質在交變電場中,由于消耗部分電能而使電介質本身發熱的現象。原因是電介質中含有能導電的載流子,在外加電場作用下,產生導電電流,消耗掉一部分電能,轉為熱能。任何電介質在電場作用下都有能量損耗，包括由電導引起的損耗和由某些極化過程引起的損耗。