A transformer is a device that changes alternating voltage using coils of wire and a shared magnetic field. It matters because electric power is easier and safer to transmit when voltage can be raised or lowered at different points in the grid. Power stations use step-up transformers to send electricity long distances with less energy loss.
Homes and devices use step-down transformers to reduce voltage to useful levels.
Key Facts
- Ideal transformer voltage ratio: Vs/Vp = Ns/Np
- Step-up transformer: Ns > Np, so Vs > Vp
- Step-down transformer: Ns < Np, so Vs < Vp
- Ideal power relation: Pp = Ps, so VpIp = VsIs
- Changing magnetic flux induces voltage: E = -N ΔΦ/Δt
- Transformers require changing current, so they work with AC rather than steady DC
Vocabulary
- Primary coil
- The primary coil is the input winding connected to the source of alternating voltage.
- Secondary coil
- The secondary coil is the output winding where the changed voltage is induced.
- Magnetic flux
- Magnetic flux measures how much magnetic field passes through a surface or coil.
- Iron core
- The iron core guides and strengthens the changing magnetic field linking the two coils.
- Turns ratio
- The turns ratio is the comparison Ns/Np that determines how the output voltage compares with the input voltage.
Common Mistakes to Avoid
- Using the current ratio the same way as the voltage ratio is wrong because ideal transformer current changes inversely, so Ip/Is = Ns/Np.
- Forgetting that transformers need AC is wrong because a steady DC current produces no continuously changing magnetic flux after the initial switch-on moment.
- Assuming a step-up transformer creates extra energy is wrong because raising voltage lowers current in an ideal transformer, keeping power approximately constant.
- Ignoring coil turn counts is wrong because the voltage change depends directly on the ratio of secondary turns to primary turns.
Practice Questions
- 1 A transformer has 200 turns on the primary coil and 800 turns on the secondary coil. If the input voltage is 12 V AC, what is the output voltage for an ideal transformer?
- 2 An ideal step-down transformer changes 240 V AC to 24 V AC. If the primary coil has 1200 turns, how many turns should the secondary coil have?
- 3 A phone charger uses a transformer to reduce wall voltage before electronics convert it further. Explain why this transformer must be connected to an alternating current source rather than a steady direct current source.