IB DP Physics: HL复习笔记11.2.3 Step-Up & Step-Down Transformers

• A transformer is

A device that changes high alternating voltage at low current to low alternating voltage at high current, and vice versa

• This is designed to reduce heat energy lost whilst electricity is transmitted down electrical power lines from power stations to the national grid
• A transformer is made up of:
  • A primary coil
  • A secondary coil
  • An iron core
• The primary and secondary coils are wound around the soft iron core
  • The soft iron core is necessary because it focuses and directs the magnetic field from the primary to the secondary coil
  • Soft iron is used because it can easily be magnetised and demagnetised

A step-up transformer has more turns in the secondary coil than primary

• In the primary coil, an alternating current producing an alternating voltage is applied
  • This creates an alternating magnetic field inside the iron core and therefore a changing magnetic flux linkage
• A changing magnetic field passes through to the secondary coil through the iron core
  • This results in a changing magnetic flux linkage in the secondary coil and from Faraday's Law, an e.m.f is induced
• An e.m.f produces an alternating output voltage from the secondary coil
• The output alternating voltage is at the same frequency as the input voltage
• A step-up transformer has more coils in the secondary than the primary and the secondary voltage is larger than the primary voltage
• A step-down transformer has more coils in the primary than the secondary and the secondary voltage is smaller than the primary voltage

Energy losses

• Eddy currents are small currents created in the iron core that come from the changing magnetic fields
  • These current move free electrons within the core causing heating of the core and therefore energy dissipation
• By replacing the solid iron core with a laminated core, power losses are decreased from eddy currents
• If there is flux leakage from the transformer, there could be further eddy currents and power losses in the surrounding metallic structure of the transformer device
• When switching the magnetic field it changes the alignment of the magnetic dipoles, this requires some work which is known as magnetic hysteresis