Faraday’s Law of Induction- Mutual and Self-Inductance
An introduction
- Electromagnetic induction is the process of generating an electromotive force (emf) in a closed circuit due to changes in the magnetic field.
- Faraday’s Law of Induction states that the emf induced in a circuit is directly proportional to the rate of change of magnetic flux through the circuit.
- Magnetic flux (Φ) is a measure of the magnetic field passing through a given area.
- The equation for Faraday’s Law of Induction is given by:
- ε=−dtdΦ
- Where ε is the emf induced, dΦ/dt is the rate of change of magnetic flux.
Mutual Inductance
- Mutual inductance occurs when the change in current flowing through one circuit induces an emf in another nearby circuit.
- It is denoted by the symbol M.
- The mutual inductance between two circuits is given by:
- M=I1N2Φ21
- Where N2 is the number of turns in the second circuit, Φ21 is the magnetic flux through the second circuit due to the current in the first circuit, and I1 is the current in the first circuit.
Self-Inductance
- Self-inductance occurs when the change in current flowing through a circuit induces an emf in the same circuit.
- It is denoted by the symbol L.
- The self-inductance of a circuit is given by:
- L=INΦ
- Where N is the number of turns in the circuit, Φ is the magnetic flux through the circuit due to the current in the circuit, and I is the current in the circuit.
Induced emf in a Coil
- Consider a coil with N turns and magnetic flux Φ passing through it.
- If the current in the coil changes, the rate of change of magnetic flux induces an emf in the coil.
- The induced emf is given by:
- ε=−NdtdΦ
- Where N is the number of turns in the coil, and dΦ/dt is the rate of change of magnetic flux.
Inductance and Induced emf
- Inductance is a measure of the ability of a circuit or coil to generate an induced emf.
- The larger the inductance, the larger the magnitude of the induced emf for a given rate of change of current or magnetic field.
- Inductance is directly proportional to the number of turns and the rate of change of magnetic flux.
- It is inversely proportional to the current flowing through the circuit.
Induced emf and Faraday’s Law
- Faraday’s Law of Induction relates the induced emf to the rate of change of magnetic flux.
- The negative sign in the equation indicates that the induced emf opposes the change in magnetic flux.
- This is known as Lenz’s law.
- Lenz’s law follows the principle of conservation of energy.
- The induced emf creates a magnetic field in such a way that it opposes the change in the magnetic flux.
Units of Inductance
- The SI unit of inductance is the Henry (H).
- 1 Henry (H) is equal to 1 volt-second per ampere (V·s/A).
- Smaller units of inductance include the millihenry (mH) and the microhenry (μH).
Application of Mutual Inductance: Transformers
- Transformers are devices that use mutual inductance to transfer electrical energy between two circuits.
- They consist of two coils wound around a common magnetic core.
- The primary coil is connected to the input voltage, while the secondary coil is connected to the output load.
- The ratio of the number of turns in the primary and secondary coils determines the voltage transformation ratio.
Application of Self-Inductance: Inductors
- Inductors are passive electronic components that store energy in a magnetic field.
- They consist of a coil of wire wound around a core.
- Inductors are used in various electronic circuits to control the flow of current, filter out unwanted frequencies, and store energy.
- They are commonly denoted by the symbol L and have values measured in Henry (H).
Summary
- Faraday’s Law of Induction states that the emf induced in a circuit is directly proportional to the rate of change of magnetic flux through the circuit.
- Mutual inductance occurs when the change in current in one circuit induces an emf in another nearby circuit.
- Self-inductance occurs when the change in current in a circuit induces an emf in the same circuit.
- Inductance is a measure of the ability of a circuit or coil to generate an induced emf.
- Transformers and inductors are practical applications of mutual and self-inductance, respectively.
Mutual Inductance (Continued)
- The direction of the induced emf depends on the direction of the change in magnetic flux.
- According to Lenz’s Law, the induced emf in the second circuit opposes the change in the magnetic field produced by the first circuit.
- The unit of mutual inductance is Henry (H).
- Mutual inductance can be increased by increasing the number of turns in the second circuit or by increasing the magnetic flux through the second circuit.
Self-Inductance (Continued)
- The self-inductance of a circuit depends on the geometry of the circuit and the magnetic field it creates.
- Inductors are commonly used elements in electronic circuits to store energy in the form of a magnetic field.
- The energy stored in an inductor is given by:
- W=21LI2
- Where W is the energy stored, L is the inductance, and I is the current flowing through the inductor.
Induced emf in a Coil (Continued)
- An alternating current (AC) can produce continuous changes in the magnetic field and induce an alternating emf.
- The equation for the induced emf in an AC coil is given by:
- ε=−NdtdΦ=−NωΦsin(ωt)
- Where N is the number of turns in the coil, dΦ/dt is the rate of change of magnetic flux, ω is the angular frequency, and t is the time.
Back EMF
- Back EMF (electromotive force) is the induced voltage that opposes the change in current in an inductive circuit.
- It is often found in motors and generators.
- Back EMF is directly proportional to the rate of change of current and the self-inductance of the circuit.
- In motors, the back EMF reduces the effective voltage across the motor coil, limiting the current and preventing damage.
AC Circuits and Inductance
- In AC circuits, the inductive reactance (XL) is the opposition to the flow of current due to inductance.
- Inductive reactance is given by:
- XL=2πfL
- Where f is the frequency of the AC current and L is the inductance of the circuit.
- The impedance (Z) of an inductive circuit is the total opposition to the flow of current and is given by:
- Z=R2+XL2
- Where R is the resistance of the circuit.
Time Constants in Inductive Circuits
- In an RL circuit (resistor and inductor in series), the time constant (τ) is the time required for the current to reach approximately 63% of its final value.
- The equation for the time constant in an RL circuit is given by:
- τ=RL
- Where L is the inductance and R is the resistance of the circuit.
Inductors in Parallel
- Inductors connected in parallel have the same voltage across them.
- The total equivalent inductance (LP) of inductors connected in parallel is given by:
- LP1=L11+L21+L31+…
- The equivalent inductance is always less than the smallest individual inductance.
Inductors in Series
- Inductors connected in series have the same current flowing through them.
- The total equivalent inductance (LS) of inductors connected in series is given by:
- LS=L1+L2+L3+…
- The equivalent inductance is always greater than the largest individual inductance.
RL Circuits
- An RL circuit consists of a resistor (R) and an inductor (L) connected in series.
- The time-dependent behavior of the current in an RL circuit is governed by the equation:
- I=e−t/τ
- Where I is the current at time t, and τ is the time constant of the circuit.
- The time constant indicates how fast the current approaches its final value.
Applications of Inductance
- Inductance has various applications in technology and everyday life.
- Some practical applications of inductance include:
- Transformers for power transmission and voltage regulation.
- Inductive sensors and proximity switches.
- Induction heating in cooktops and industrial processes.
- Chokes and filters for noise suppression and signal conditioning.
- Magnetic resonance imaging (MRI) in medical diagnostics.
Applications of Mutual Inductance
- Mutual inductance is commonly used in the following applications:
- Transformers for power transmission and voltage conversion.
- Inductive proximity sensors for detecting metal objects.
- Relays and solenoids for controlling electrical circuits.
- Wireless power transfer systems.
- Inductive coupling in communication systems, such as RFID and wireless charging.
Calculating Induced emf
- To calculate the induced emf in a circuit, we need to determine the rate of change of magnetic flux and the number of turns in the circuit.
- The induced emf can be calculated using the equation:
- ε=−NdtdΦ
- The negative sign indicates that the induced emf opposes the change in flux.
Calculation Example: Induced emf in a Coil
- Suppose we have a coil with 100 turns and a magnetic field changing at a rate of 0.02 T/s.
- The area of the coil is 0.05 m².
- The induced emf can be calculated as follows:
- ε=−NdtdΦ
- ε=−100(0.05)(0.02)
- ε=−0.1V
Induced emf in a Square Loop
- The induced emf in a square loop can be determined by considering the change in magnetic flux through the loop when its orientation changes.
- If the loop area is A and the magnetic field is perpendicular to the plane of the loop, the induced emf can be calculated using the equation:
- ε=−AdtdB
- This equation holds for a uniform magnetic field.
Example: Induced emf in a Square Loop
- Consider a square loop with side length 0.2 m placed in a uniform magnetic field of 0.5 T.
- If the magnetic field changes at a rate of 0.1 T/s, we can calculate the induced emf as follows:
- ε=−AdtdB
- ε=−(0.2×0.2)(0.1)
- ε=−0.004V
Energy Stored in an Inductor
- The energy stored in an inductor can be calculated using the equation:
- W=21LI2
- Where W is the energy stored, L is the inductance, and I is the current flowing through the inductor.
- The energy in an inductor is stored in the form of the magnetic field generated by the flowing current.
Example: Energy Stored in an Inductor
- Suppose we have an inductor with an inductance of 0.05 H and a current of 2 A.
- We can calculate the energy stored in the inductor using the equation:
- W=21LI2
- W=21(0.05)(22)
- W=0.1J
AC Circuits and Inductance
- In AC circuits, the inductive reactance (XL) is defined as the opposition to the flow of current caused by inductance.
- Inductive reactance depends on the frequency of the AC current and the inductance of the circuit.
- The formula for inductive reactance is:
- XL=2πfL
- Where f is the frequency of the AC current and L is the inductance.
Example: Inductive Reactance
- Consider an RL circuit with a 100 mH inductor.
- If the circuit is connected to an AC source with a frequency of 50 Hz, we can calculate the inductive reactance as follows:
- XL=2πfL
- XL=2π(50)(0.1)
- XL=31.42Ω
Recap and Conclusion
- Faraday’s Law of Induction states that the induced emf is directly proportional to the rate of change of magnetic flux.
- Mutual inductance occurs when the change in current in one circuit induces an emf in another nearby circuit.
- Self-inductance occurs when the change in current in a circuit induces an emf in the same circuit.
- Inductance is a measure of the ability of a circuit or coil to generate an induced emf.
- Inductance has various applications in technology and everyday life.
Faraday’s Law of Induction- Mutual and Self-Inductance