http://www.ck12.org Chapter 19. Electromagnetism
19.3 Electrical Power Transfer
Objective
The student will:
- Understand how transformers operate.
Vocabulary
- transformer:A device that transfers electrical energy between two circuits using electromagnetic induction,
usually with a change in voltage.
Introduction
Utility companies have large generating stations that produce potential differences of thousands of volts. The voltage
of a typical electrical outlet in the United States is either 120V or 240V (for larger appliances). By what means, then,
are the large potential differences, generated at power stations, reduced to accommodate consumers?
Transformers
Atransformeris a device that transfers electrical energy between two circuits using electromagnetic induction,
usually with a change in voltage.
A typical transformer is composed of two coils wrapped around an iron ring or iron core. Both coils in the diagram
seem to have the same number of turns. This need not be the case. In fact, it usually is not.Figure19.10 shows a
transformer with a primary coil with four turns and a secondary coil with eight turns.
Faraday’s law of induction states thatV=−N∆Φ∆t.
The voltage in the primary coil is thenVp=−Np∆Φ∆t.
The voltage in the secondary coil isVs=−Ns∆Φ∆t.
The rate of change of the magnetic flux∆Φ∆t is the same for both coils.
We have:
Vp
Vs=
Np
Ns(Transformer Equation A)
Thus, the ratio of the voltages is equal to the ratio of the number of turns in each coil. The ratio is effectively a
statement of how the input voltageVpis related to the output voltageVs.
IfNs>Np, we have a step-up transformer, in which the secondary voltage is greater than the primary voltage.
TakingNp=4 andNs=8 as inFigure19.10, we see that secondary voltage is twice the primary voltage.