1、Basic Physical Quantities of Circuits1 currentSI unit:AThe directional movement of charges creates current.The direction in which the positive charge moves is defined as the positive direction of the current.Current is the time rate of change of charge.ddqit=other commonly used unit:mA,kAmagnitude a
2、nd direction do notchange with timeotherwisemagnitude and direction change periodically with time and the average value is zerodirect current(DC) Itime-varying current ialternating current(AC)The current must be on a line, and arrows are usually used to indicate the direction.iababi2 voltageSI unit:
3、VThe positive direction of the voltage is from high potential to low one.ddWuq=other commonly used unit:mV,kVThe voltage between two points a and b in an electric circuit is the energy (or work) needed to move a unit charge from a to b;abumathematically,potential： voltage between a certain point and
4、 the reference point.（ zero potential）The potential refers to one point, while the voltage refers to two.au buThe voltage between any two points a nd b in the circuit is equal to the difference between the potential at point a and the potential at point b.a b a bu u u=Is the voltage between two poin
5、ts relevant to the choice of reference point?Do you know the relationship between voltage and electromotive force? Practice problem： Which of the following solutions is wrong?cab1.5V1.5VAnswer： DABab 1. 5Ve =bc 1. 5Ve =CDac 3Ve =ac 3Vu =3 powerSI unit:Wmore commonly used equationother commonly used unit:kW,MWPower is the time rate of providing or absorbing energy.mathematically,ddWpt=
