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By the end of this section, you will be able to:
  • Correlate two nearby circuits that carry time-varying currents with the emf induced in each circuit
  • Describe examples in which mutual inductance may or may not be desirable

Inductance is the property of a device that tells us how effectively it induces an emf in another device. In other words, it is a physical quantity that expresses the effectiveness of a given device.

When two circuits carrying time-varying currents are close to one another, the magnetic flux through each circuit varies because of the changing current I in the other circuit. Consequently, an emf is induced in each circuit by the changing current in the other. This type of emf is therefore called a mutually induced emf , and the phenomenon that occurs is known as mutual inductance ( M ) . As an example, let’s consider two tightly wound coils ( [link] ). Coils 1 and 2 have N 1 and N 2 turns and carry currents I 1 and I 2 , respectively. The flux through a single turn of coil 2 produced by the magnetic field of the current in coil 1 is Φ 21 , whereas the flux through a single turn of coil 1 due to the magnetic field of I 2 is Φ 12 .

Figure shows the cross sections of two coils. In each one, the cross sections of the wire of the coil are shown as two circles, one at the top and the other at the bottom. Dots in the upper circles and crosses in the lower ones indicate the direction of flow of current. Coil 1 has field lines labeled B1 passing from between the two circles, going right. Some of these pass through coil 2, which is smaller than coil 1.
Some of the magnetic field lines produced by the current in coil 1 pass through coil 2.

The mutual inductance M 21 of coil 2 with respect to coil 1 is the ratio of the flux through the N 2 turns of coil 2 produced by the magnetic field of the current in coil 1, divided by that current, that is,

M 21 = N 2 Φ 21 I 1 .

Similarly, the mutual inductance of coil 1 with respect to coil 2 is

M 12 = N 1 Φ 12 I 2 .

Like capacitance, mutual inductance is a geometric quantity. It depends on the shapes and relative positions of the two coils, and it is independent of the currents in the coils. The SI unit for mutual inductance M is called the henry (H)    in honor of Joseph Henry (1799–1878), an American scientist who discovered induced emf independently of Faraday. Thus, we have 1 H = 1 V · s/A . From [link] and [link] , we can show that M 21 = M 12 , so we usually drop the subscripts associated with mutual inductance and write

M = N 2 Φ 21 I 1 = N 1 Φ 12 I 2 .

The emf developed in either coil is found by combining Faraday’s law    and the definition of mutual inductance. Since N 2 Φ 21 is the total flux through coil 2 due to I 1 , we obtain

ε 2 = d d t ( N 2 Φ 21 ) = d d t ( M I 1 ) = M d I 1 d t

where we have used the fact that M is a time-independent constant because the geometry is time-independent. Similarly, we have

ε 1 = M d I 2 d t .

In [link] , we can see the significance of the earlier description of mutual inductance ( M ) as a geometric quantity. The value of M neatly encapsulates the physical properties of circuit elements and allows us to separate the physical layout of the circuit from the dynamic quantities, such as the emf and the current. [link] defines the mutual inductance in terms of properties in the circuit, whereas the previous definition of mutual inductance in [link] is defined in terms of the magnetic flux experienced, regardless of circuit elements. You should be careful when using [link] and [link] because ε 1 and ε 2 do not necessarily represent the total emfs in the respective coils. Each coil can also have an emf induced in it because of its self-inductance (self-inductance will be discussed in more detail in a later section).

Questions & Answers

what is the difference between temperature and heat
Ishom Reply
Heat is the condition or quality of being hot While Temperature is ameasure of cold or heat, often measurable with a thermometer
Temperature is the one of heat indicators of materials that can be measured with thermometers, and Heat is the quantity of calor content in material that can be measured with calorimetry.
2. A brass rod of length 50cm and diameter 3mm is joined to a steel rod of the same length and diameter. What is the change in length of the combined rod at 250°c( degree Celsius) if the original length are 40°c(degree Celsius) is there at thermal stress developed at the junction? The end of the rod are free to expand (coefficient of linear expansion of brass = 2.0×10^-5, steel=1.2×10^-5k^1)
A charge insulator can be discharged by passing it just above a flame. Explain.
Mudassar Reply
of the three vectors in the equation F=qv×b which pairs are always at right angles?
what is an ideal gas?
Justine Reply
What is meant by zero Kelvin ?
Why does water cool when put in the pot ?
when we pour the water in a vessel(pot) the hot body(water) loses its heat to the surrounding in order to maintain thermal equilibrium.Thus,water cools.
when we drop water in the pot, the pot body loses heat to surrounded in order to maintain thermal equilibrium thus,water cool.
types of thermometer?
yemisi Reply
thermometer, Radiation thermometer and vapour pressure thermometer.liquid thermometer use thermometric liquid like mercury ,alcohol etc.
liqid thermometer ,gas thermometer, resitance thermometer,thermo electric thermometer , radiation thermometer andvapour pressure thermometer
calculate the quantity of heat required to rise the temperature of 1gmail of ice _10 to 110
Dargu Reply
A 40cm tall glass is filled with water to a depth of 30cm. A.what is the gauge pressure at the bottom of the glass? B.what is the absolute pressure at the bottom of the glass?
Abdulaziz Reply
A glass bottle full of mercury has mass 50g when heated through 35degree, 2.43g of mercury was expelled. Calculate the mass of the mercury remaining in the bottle
Anjorin Reply
Two electric point charges Q=2micro coulomb are fixed in space a distance 2.0cm apart. calculate the electric potential at the point p located a distance d/2 above the central point between two charges
Abdul Reply
what is wave
Ahmed Reply
A wave is a periodic disturbance which travel with a finite velocity and remains unchanged in type as it travels.
What's a wave motion?
What is charge bodies
Oje Reply
which have free elections
Show that if a vector is gradient of a scaler function then its line around a closed path is zero
Charge bodies are those which have free electons
the melting point of gold is 1064degree cencius and is boiling point is 2660 degree cenciu
Ilyas Reply
is Thomas's young experiment interference experiment or diffraction experiment or both
Ilyas Reply
An aqueous solution is prepared by diluting 3.30 mL acetone (d = 0.789 g/mL) with water to a final volume of 75.0 mL. The density of the solution is 0.993 g/mL. What is the molarity, molality and mole fraction of acetone in this solution?
A 4.0kg mess kit sliding on a fractionless surface explodes into two 2.0 kg parts.3.0 m/s due to north and 0.5 m/s 30 degree north of east. what is the speed of the mess kit
it's a line used to represent a complex electrical quantity as a vector
Gift Reply
Practice Key Terms 3

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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