constant and it will cease to induce an emf or a current in the circuit on the right.
If the current on the left-hand side were an alternating instead of a direct current, it
would induce an undiminishing alternating current on the right-hand side. Why? The
magnetic field due to the changing left-side current would constantly change in strength
and orientation. The constantly changing magnetic field would continuously induce an
emf and current in the circuit on the right, and the light bulb would stay lit.
Mutual induction
When current is constant, no induction
occurs29.15 - Transformers
Transformer: A device used
to increase or decrease an
alternating potential
difference.
The way in which electricity is transmitted and
distributed requires conversions of potential
differences across an enormous range of values.
Long distance power transmission lines typically
operate at about 350,000 V, while city power lines
near a home function at 15,000 V. Before the
electricity enters a house, the potential difference between a current-carrying and a
“neutral” wire is further reduced to 120 V. All of these currents are alternating, and
transformers are used to convert potential differences in alternating currents. (One
reason why alternating current is used in power systems is the ease with which
transformers can change its potential difference.)
The illustration in Concept 1 shows a transformer: two coils of wire wrapped around an
iron core. The wires are insulated so that no current flows directly between them. The
iron core allows a magnetic field to be efficiently transmitted from one coil to the other:
Less than 5% of the energy transformed in a typical transformer is dissipated as heat. In
this section, we focus on the characteristics of an ideal transformer, one in which no
energy is lost.
The wire wrapped around the iron core on the left-hand side is called the primary
winding. In our scenario, it is directly attached to an AC generator. The current passing
through this coil generates a magnetic field. Since the current continually changes, so
does the magnetic field it creates.
The field passes through the second coil on the right. This coil is called the secondary
winding. The change in magnetic flux through this coil induces an emf that causes a
current in the secondary circuit. The secondary circuit powers a resistive component,
such as a light bulb, that is called the load. The transformer changes the emf from that
created by the AC generator to that required by the load.
There are a different number of loops, or turns, in the primary and secondary windings.
This is crucial to the operation of the transformer. The ratio of the number of primary
loops to the number of secondary loops is called the turns ratio.
The transformer “transforms” the potential difference created by the generator: The
potential difference across the secondary winding differs from that across the primary
winding. In the transformer to the right, the potential difference decreases. This is a
step-down transformer. Step-up transformers increase potential difference. The
transformer symbol that is used in circuit diagrams is shown in Concept 2.
The change in the potential differences depends on the turns ratio. The proportion in
Equation 1 states that the ratio of the potential differences equals the turns ratio. To put
it another way: If the number of loops on a given side is reduced, the potential
difference on that side decreases.
We derive this “turns equation” below using Faraday’s law. The loops in each coil are assumed to span an equal surface area, and the same
amount of magnetic field passes through each one. This means the ratio of the changes in flux in the primary and secondary windings is
proportional solely to the number of loops in each coil.
Power transmission lines conduct AC current at high voltages.
Transformers convert this current from one voltage to another.Transformers
Increase or decrease potential
difference
AC creates changing magnetic field
Induces potential difference on rightTransformer
Symbol used in circuit diagrams