6.7 Electromagnetic induction

Question 1

What is the equation for the emf induced in a circuit including magnetic flux

Answer 1

e = -N x (change in phi divided by change in time)

Question 2

Why is there a minus sign in the equation and what law does this relate to

Answer 2

if an induced emf was connected in a circuit, it could create an induced current around the circuit which would set up a magnetic field. the minus sign is because the induced magnetic field would always oppose the change in magnetic flux that is causing the induced emf. This is called lenz’s law

Question 3

What is lenz’s law an example of

Answer 3

The law of conservation of energy

Question 4

Recap what a solenoid is

Answer 4

A long spring like coil, of length many times its diameter and it often has many turns every centimetre. A current in a solenoid produces a magnetic field along its axis

Question 5

What is the size of B over the internal cross section of a solenoid

Answer 5

The size of B is uniform; B = mu0ni where mu0 is a constant with a value of 1.26 x 10^-6 NA^-2

Question 6

What are some real life uses of solenoids

Answer 6

Solenoids can be used in MRI scanners. Huge magnetic field strengths of around 2T are produced in the solenoids. The patient is slid into the solenoid for examination. The solenoid is made of superconducting coils at liquid helium temperatures and they carry very large currents.

Question 7

How much could an MRi scanner cost a hospital

Answer 7

Over 1 million

Question 8

Why are patients told to remove jewellery before going into a MRI scanner

Answer 8

Since there are extremely strong magnetic fields inside the scanner, the metal jewellery will be attracted to the field and this could injure the patient

Question 9

When a solenoid is connected to an alternating current supply how does the current vary

Answer 9

I = -I0sin theta

Question 10

What will happen when a coil is placed inside the solenoid connected to the ac supply

Answer 10

Any change in the current in the solenoid will now produce an induced emf in the coil in accordance to faraday’s law, since there will be a change in the magnetic flux linking the coil

Question 11

What will happen if the current of the solenoid in the circuit is alternating

Answer 11

There will be an alternating magnetic field in the solenoid which will produce an alternating induced emf in the coil according to faraday’s law