11TH GRADE · OSCILLATIONS AND WAVES
Electromagnetic vibrations and alternating current
Oscillatory circuit, reactance and transformer.
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Basic Concepts
Oscillatory circuit
A circuit of a capacitor and a coil in which electromagnetic oscillations occur.
Energy periodically transfers from the electric field of the capacitor to the magnetic field of the coil and back.
AC current
Electric current that periodically changes magnitude and direction.
For sinusoidal current, the effective values are usually used: they give the same thermal effect as the corresponding direct current.
Transformer
A device for changing alternating voltage using electromagnetic induction.
The transformer only works when the magnetic flux is changing; in an ideal model, a gain in voltage is accompanied by a decrease in current.
Physical quantities
Capacitor charge
Instant charge of the capacitor plate.
Unit: pendant ·
Charge amplitude
The largest modulus of charge of the capacitor.
Unit: pendant ·
Current amplitude
The largest current module in the circuit.
Unit: ampere ·
Inductive reactance
Coil resistance to alternating current.
Unit: ohm ·
Capacitance
Resistance of a capacitor to alternating current.
Unit: ohm ·
Voltage amplitude
The largest module of alternating voltage.
Unit: volt ·
EMF amplitude
The largest module of the variable EMF.
Unit: volt ·
Primary voltage
Voltage on the input winding of the transformer.
Unit: volt ·
Secondary voltage
Voltage on the output winding of the transformer.
Unit: volt ·
Primary current
Current strength in the input winding of the transformer.
Unit: ampere ·
Secondary current
Current strength in the output winding of the transformer.
Unit: ampere ·
Number of turns of the primary winding
The number of turns in the input winding of the transformer.
Unit: dimensionless quantity
Number of turns of the secondary winding
The number of turns in the output winding of the transformer.
Unit: dimensionless quantity
Lesson formulas
Thomson's formula
The period of natural oscillations of the circuit is determined by inductance and capacitance.
Charge fluctuation
The charge of the capacitor changes harmoniously with the phase.
Current amplitude in the circuit
The amplitude of the current is equal to the amplitude of the charge multiplied by the cyclic frequency.
Inductive reactance
Inductive reactance increases with frequency and inductance.
Capacitance
Capacitance decreases with increasing frequency and capacitance.
Unit of reactance
Inductive and capacitive reactance are measured in ohms.
Energy of the oscillatory circuit
The total energy of the circuit is transferred between the electric field of the capacitor and the magnetic field of the coil.
Transformation ratio
The voltage ratio of an ideal transformer is equal to the ratio of the number of turns.
Ideal transformer currents
In an ideal transformer, an increase in voltage is accompanied by a decrease in current.
Related formulas
The same formulas in the reference, with symbols and units.
Tasks on the topic
Statements, answer options and the walkthrough.