Alternating current (a.c.) repeatedly changes direction, so the magnetic field it produces keeps changing. That is why transformers need a.c.: only a changing field in the core induces a voltage in the secondary coil.
This lesson follows using the transformer ratio and is part of induction and transformers.
What is alternating current?
In direct current (d.c.) the charge flows in one direction only. In alternating current the direction reverses again and again, so the voltage rises to a peak, falls through zero, reaches a peak in the opposite direction and returns.
One complete pattern is a cycle.
The period T is the time for one cycle, and the frequency f is the number of cycles per second. They are linked by f = 1 ÷ T. Many countries, including Malaysia, use a mains frequency of 50 Hz.
Why does a transformer need a changing supply?
The link is induction. A current in the primary coil creates a magnetic field in the iron core. If the current is steady, the field is steady, so no field lines are cut in the secondary and no voltage is induced.
If the current alternates, the field alternates too. The secondary coil continuously has a changing field through it, so an alternating voltage is induced there at the same frequency.
Worked example
An oscilloscope trace of a fictional supply is shown. (Invented example data.)
The time base is 5.0 ms per division and the voltage scale is 2.0 V per division. One complete cycle spans 4.0 divisions horizontally. The peak of the wave is 3.0 divisions above the centre line.
Step 1, period: T = 4.0 × 5.0 ms = 20 ms = 0.020 s.
Step 2, frequency: f = 1 ÷ T = 1 ÷ 0.020 = 50 Hz.
Step 3, peak voltage: 3.0 × 2.0 V = 6.0 V.
Step 4, cycles in 2 seconds: 50 × 2 = 100 cycles.
Step 5, check: 100 cycles in 2 s is the same as 50 per second, matching the frequency. Units are seconds, hertz and volts.
The mistake to watch for
Mistaken answer: “A battery can be connected to a transformer, because the current flows through the primary coil, so a voltage is induced in the secondary.”
The student linked current to voltage but forgot that the current must change.
The correction: a battery gives a steady current and a steady field. A voltage appears in the secondary only for an instant when the circuit is switched on or off. For a continuous output, the primary supply must alternate.
Check yourself
Try these, then open each answer.
1. A wave has a period of 0.040 s. Find the frequency.
Show answer
f = 1 ÷ 0.040 = 25 Hz.
2. A supply has a frequency of 100 Hz. Find the period in milliseconds.
Show answer
T = 1 ÷ 100 = 0.010 s = 10 ms.
3. Explain in two sentences why a d.c. supply cannot keep a transformer working.
Show answer
A steady direct current produces a steady magnetic field, so there is no change of field through the secondary coil. With no change, no voltage is induced except briefly when the circuit is switched on or off.
Where this leads next
The next step is to use these ideas to explain why power is sent at high voltage: calculating a transmission loss. The induction practice set then mixes all the skills.
Turning a fact into a clear reason is a habit that grows with feedback on your own sentences. That is what our teachers offer in online one-to-one Physics tuition.