Transformers

A transformer changes the size of an alternating voltage. It is used to make electricity safer for homes and more efficient for long-distance transmission.

Construction of a Simple Transformer

• Soft iron core: carries the magnetic field easily and loses magnetism quickly.
• Primary coil: many turns of insulated wire connected to the input a.c. supply.
• Secondary coil: many turns of insulated wire where the output a.c. is taken.

How It Works (Principle)

• A.c. in the primary makes a changing magnetic field in the iron core.
• This changing field cuts the secondary coil and induces an a.c. voltage in it (electromagnetic induction).
• Transformers need a.c. to work. Direct current (d.c.) does not change, so it will not induce a steady output.

Step-up and Step-down

• Step-up: secondary has more turns than primary; output voltage increases.
• Step-down: secondary has fewer turns; output voltage decreases.

Turn–voltage relationship (ideal transformer):

$$\frac{V_p}{V_s} = \frac{N_p}{N_s}$$

Where $V_p$, $V_s$ are the primary and secondary voltages, and $N_p$, $N_s$ are the number of turns.

Power in an Ideal Transformer

If it is 100% efficient: $$V_p I_p = V_s I_s$$ Increasing voltage means decreasing current for the same power, and vice versa.

Transformers in Power Transmission

• Power stations use a step-up transformer to raise voltage for the grid. High voltage means lower current.
• Lower current reduces energy loss in cables because $P_{loss} = I^2 R$.
• Near homes, a step-down transformer lowers voltage to safe values.

Key Points and Misconceptions

• More turns on the secondary gives a higher secondary voltage.
• Transformers transfer energy; they do not create energy.
• They work with a.c., not steady d.c.
• Soft iron is used because it is easy to magnetise and demagnetise.