Cambridge A Levels A2 Physics Chapter 21 Alternating Current
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Course
Cambridge A Levels A2 Physics
Institution
Chapter 21 Alternating Current: 12 pages
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Chapter 21 Alternating Current
21.1 Characteristics of Alternating Current
Alternating Current (a.c.)
A current which periodically reverses direction (various from positive to negative) and
changes its magnitude continuously with time.
Direct Current (d.c) Alternating Current (a.c)
Steady magnetism along the wire Cause of the direction Rotating magnet along the wire
of flow of electrons
Frequency of d.c = 0 Frequency Frequency of a.c = 50/60 Hz
D.C flows in one direction in the circuit Direction A.C reverses its direction while
flowing in a circuit
Constant magnitude Magnitude Magnitude varies with time
Electrons move steadily in one direction Flow of electrons Electrons keep switching directions
(forward only) (forward & backward)
Cell / battery Source A.C generator and mains
▪ The direction of an alternating current varies every half cycle.
During half of the cycle, the current is positive, and on the other half of the cycle, the current is
negative.
▪ The variation of the current and potential difference with time is sinusoidal (a sine curve).
▪ In a wire with a.c, the free electrons within the wire move back and forth with simple harmonics
motion (s.h.m).
, Alternating Current
𝐼 = 𝐼0 sin 𝜔𝑡
I: current at time t (A)
I0: peak value of the alternating current (A)
: angular frequency of the supply (rad s-1)
t: time (s)
Alternating Voltage
𝑉 = 𝑉0 sin 𝜔𝑡
V: potential difference at time t
V0: peak value of the alternating voltage
In a wire carrying a.c, the electrons move backwards and forwards with s.h.m.
T: Period of the current; time taken for one complete cycle of the alternating current.
f: Frequency of the current; the number of complete cycles per unit time.
1
𝑓=
𝑇
Angular Frequency of Current
2𝜋
𝜔= 𝜔 = 2𝜋𝑓
𝑇
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