Revision: Electromagnetic Induction and Alternating Current Physics HSC Science Class 12 Tamil Nadu Board of Secondary Education

The phenomenon in which electric current is generated in a conductor or closed coil due to a varying magnetic field is called electromagnetic induction.

Eddy currents are circulating currents induced in the bulk of a conductor when the magnetic flux linked with it changes.

They are named after the whirlpool-like eddies formed in flowing water.

The phenomenon of electromagnetic induction in which a change in current through a coil induces an EMF in the same coil, opposing the change in current, is called self-induction.

The self-inductance of a coil is defined as the ratio of the total magnetic flux linkage to the current flowing through it.

An a.c. generator is a device which converts the mechanical energy into the electrical energy using the principle of electromagnetic induction.

The transformer is a device used for converting low voltage into high voltage and high voltage into low voltage. It works on the principle of electromagnetic induction.

An electrical device which converts low alternating voltage at high current to high alternating voltage at low current (or vice versa) — i.e., a device which reduces or increases the voltage in an AC circuit through mutual induction — is called a transformer.

The Power Factor is the ratio of True Power (measured in Watts) to Apparent Power (measured in Volt-Amperes) in an AC circuit.

Power factor (cos Φ) = `"True power"/"Apparent power"`

In the expression Pav = V_rmsI_rms cos⁡ ϕ, the quantity cos φ is called the power factor.

OR

Power Factor is the cosine of the phase angle (φ) between the voltage and current in an AC circuit. It equals the ratio of true (average) power to apparent power.

The current flowing in a purely inductive or purely capacitive circuit for which cos φ = 0 and no power is dissipated even though the current is flowing is called wattless current.

OR

Wattless Current (also called reactive current) is the component of current in a purely inductive or purely capacitive circuit that flows without consuming any power.

NΦ_B ​= LI

Where:

• N = number of turns
• Φ_B​ = magnetic flux linked per turn
• I = current flowing through the coil
• L = coefficient of self-inductance (self-inductance)

\[\frac{V_s}{V_p}=\frac{N_s}{N_p}=k\]

Where:

• V_s = Secondary (output) voltage (V)
• V_p = Primary (input) voltage (V)
• N_s = Number of turns in secondary coil
• N_p = Number of turns in primary coil
• k = Transformation ratio (turns ratio)

\[\frac{I_p}{I_s}=\frac{N_s}{N_p}=\frac{V_s}{V_p}\]

\[\frac{V_s}{V_p}=\frac{N_s}{N_p}=\frac{I_p}{I_s}=k\]

 P_input = P_output

P = VI = \[\frac{V_mI_m}{2}[\cos\phi-\cos(2\omega t+\phi)]\]

Where:

• V_m​ = Peak voltage
• I_m​ = Peak current
• ϕ = Phase angle between voltage and current
• ω = Angular frequency
• t = Time

\[\cos\phi=\frac{P_{av}}{P_{apparent}}=\frac{P_{av}}{V_{rms}\cdot I_{rms}}\]

P_av ​= VI cos ϕ = I²Z cos ϕ = \[\frac {V_m​I_m}{2}\]​​cos ϕ = V_rms​I_rms​ cos ϕ

A transformer is based on the principle of mutual induction, i.e., whenever the magnetic flux linked with a coil changes, an emf is induced in the neighbouring coil. For an ideal transformer there is no loss of power, so P_input = P_output​. On the basis of winding, transformers are of two types — step-up and step-down.

• Electricity and magnetism are interrelated.
• Electric current can produce a magnetic field.
• A changing magnetic field can produce electric current.
• The production of electric current by a varying magnetic field is called electromagnetic induction.
• Michael Faraday showed in 1831 that a moving magnet can produce current in a conductor.
• The current produced due to electromagnetic induction is called induced current.
• Generators, transformers, induction motors, and wireless chargers work on electromagnetic induction.