13.5 Transformers Different electrical devices require different amounts of electrical energy to operate. An electric stove requires a lot of electrical energy while a lamp requires very little. Some devices require different voltages and currents. A computer may require 12 V to operate, so the voltage in your home must be lowered from 120 V to 12 V. Transformers are electromagnetic devices that are used to lower or raise the AC voltage to the value that any electronic device is designed for. Adapters such as cellphone and laptop chargers have transformers as part of their internal circuitry.

13.5 Transformers To understand how a transformer works, recall Faraday’s Ring from Section The ring has a primary and a secondary circuit that are not in direct physical contact, however the current in the primary circuit induces a current in the secondary circuit. A changing magnetic field is required for electromagnetic induction, which can be produced using alternating current. AC in the primary coil is the most critical part of producing AC in the secondary coil of Faraday’s Ring.

13.5 Transformers Transformers have a different number of windings on the primary circuit compared to the secondary circuit. A step-down transformer has fewer windings on the secondary circuit than the primary circuit; Voltage on the secondary side is less than voltage on the primary side. A step-up transformer has more windings on the secondary circuit than the primary circuit; Voltage on the secondary side is higher than voltage on the primary side.

13.5 Conservation of Energy in Transformers According to the law of conservation of energy, energy going into the primary coil must equal energy coming out of the secondary coil. Power in an electrical circuit can be expressed as a product of voltage and current, or P = VI; A change in energy can be expressed as ∆E = P∆t. For primary and secondary sides of the transformer:

13.5 Transformer Equations By grouping current (I) and voltage (V): SP #1 p.607 SP #1 p.608 Voltage (V) in the coil is directly proportional to the number of windings (N): Current (I) is inversely proportional to the number of windings (N): Combining all ratios into one equation:

13.5 Homework Practice # 1, 2 p.608 – Tutorial 1 Practice # 1, 2 p.608 – Tutorial 2 Questions # 1, 5, 7-9 p.609