ELECTROMAGNETIC INDUCTION
Very Short Question Answers
Q1. What did the experiments of faraday and Henry show?
Ans. The experiments of faraday and Henry show that whenever the magnetic flux passing through a coil change with time an emf is induced in the coil.
Q2. define magnetic flux.
Ans. Magnetic flux is the number of field lines crossing any surface normal to it.
Magnetic flux $\phi_{B}=\vec{B}\,.\,\vec{A}=BA\,cos\theta $
Q3. State faraday’s law of electromagnetic induction.
Ans. The magnitude of the induced emf in a coil is equal to the rate of change of magnetic flux through the coil.
$$E=\,-\,N\, \frac{\mathrm{d} \phi_{B}}{\mathrm{d} x}$$
Q4. State Lenz’s law.
Ans. The polarity of induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produced it.
Q5. What happens to the mechanical energy (of motion) when a conductor is moved in a uniform magnetic field?
Ans. The mechanical energy expended in moving the conductor is converted into electrical energy and then into thermal energy.
Q6. What are eddy currents?
Ans. When large pieces of conductors are subjected to changing magnetic flux, induced currents are produced in them. Such induced currents are called eddy currents.
Q7. Define inductance.
Ans. When the magnetic flux through a coil change, an electric current is induced in it.
Here, the flux is proportional to the current ϕ_B I . The constant of proportionality in this is called inductance.
(or) inductance is the ratio of the magnetic flux to current.L = Nϕ/I
Q8. What do you understand by self-inductance?
Ans. The e.m.f is induced in a single isolated coil due to change of flux through the coil by varying the current through the same coil is known as self-induction
$$ E= - L \: \frac{\mathrm{d} i }{\mathrm{d} t}$$
where L = self-inductance.
Q9. Current in a circuit falls from 5.0A to 0.0A in 0.1 sec. if an average emf of 200V is induced, give an estimate of the self – inductance of the circuit.
Ans. $di = 5-0 = 5 A,dt = 0.1 sec,E= 200 V$
$ E= - L \: \frac{\mathrm{d} i }{\mathrm{d} t}$
$ L=\frac{E}{ \: \frac{\mathrm{d} i }{\mathrm{d} t}}$
$ L=\frac{200}{ \: \frac{ 5}{0.1}}$
$ L= 4\,H$
Q10. Apair of adjacent coils has a mutual inductance of 1.5 H. If the current in one coil changes from 0 to 20 A in 0.5 s, what is the change of flux linkage with the other coil?
Ans. $ E= - M\: \frac{\mathrm{d} i }{\mathrm{d} t}=\, \frac{\mathrm{d} \phi }{\mathrm{d} t}$
$\therefore\,\, d\phi\,=\,M. \,di$
$ \, d\phi\,=\,1.5(20-0) \, $
$ \, d\phi\,=\, 30 \, wb$
Q11. What did the experiments of faraday and Henry show?
Ans. The experiments of faraday and Henry show that whenever the magnetic flux passing through a coil change with time an emf is induced in the coil.
Q12. define magnetic flux.
Ans. Magnetic flux is the number of field lines crossing any surface normal to it.
Magnetic flux$\phi_{B}=\vec{B}\,.\,\vec{A}=BA\,cos\theta $