6.7 Electromagnetic Induction - Lenz Law

Question 1

What is Lenz Law?

Answer 1

‘the induced emf is always in such a direction as to oppose the change in flux that caused it’

Question 2

What is Lenz Law example of ?

Answer 2

Conservation of energy and Newton’s Third Law.

Question 3

How is Lenz Law an example of conservation of energy?

Answer 3

As when there is a change in flux linkage energy si transferred to induce emf. (work is done on charge carriers)

Question 4

How is Lenz Law an example of Newton’s Third Law?

Answer 4

As every action will try to be opposed by the creation of magnetic field in the wire.

Question 5

What is the equation for Lenz Law?

Answer 5

E = - nΦ / t

Question 6

What does the minus sign represent?

Answer 6

It shows the direction of emf and how the magnetic field created will try to exert a force to oppose change in magnetic flux / magnetic flux linkage.

Question 7

What is the induction rule, what does each finger represent?

Answer 7

the new right hand rule
THUMB = motion
FIRST FINGER = field
SECOND FINGER = direction of induced emf

Question 8

What would be direction of the current from this rule?

Answer 8

The same as direction of emf, second finger

Question 9

How do we use this rule for? What do we use it for?

Answer 9

We use this rule to work out direction of induced emf, then we can use fleming’s left hand rule to work out direction of resistance to motion.

Question 10

What is this resistance to motion created by?

Answer 10

Magnetic field of the emf induced wire, it is either magnetic attraction or repulsion.

Question 11

What would happen when a north pole of a bar magnet is approaching a coil of wire?

Answer 11

There would be and emf and therefore current induced. This creates a magnetic field around the coil in such a way that the closest side of coil to magnet will act as a north pole to oppose the motion.

Question 12

What would happen when withdrawing the magnet?

Answer 12

The current would backtrack and so the bar magnet will be attracted to the coil, as the coil of wire will be acting as a south pole.

Question 13

Using the practical from the book, for capturing induced emf what set up would we use?

Answer 13

A coil connected in series to a data logger that has a resistor between its poles.

Question 14

What do we do in this activity?

Answer 14

We drop a bar magnet through a coil and use the data logger to capture the pd across a time frame.

Question 15

What would an emf over time graph look like?

Answer 15

Start at 0, then increase until reaching peak, then drop in a straight line peak on the negative y-axis and increase to 0.

Question 16

In what case would the 2 peaks (in the negative and positive direction ) equal?

Answer 16

As long as the bar magnet is dropped through the centre of the coil.

Question 17

Why does the emf become negative?

Answer 17

When magnet starts to exit the coil , this happens as current backtracks changing the magnetic flux / magnetic flux linkage.

Question 18

What would be the conclusion of the peaks not being equal?

Answer 18

The bar magnet wasn’t dropped centrally.
or
The magent accelerates as it falls, making speed grater with distance covered so that time is less.

Question 19

Why wouldn’t the magnet normally fall at 9.8ms^-1?

Answer 19

As induced emf and therefore current in the coil produce a magnetic field of its own, after emf has been induced by the change in magnetic flux/ magnetic flux linkage. This magnetic field will act in a way to repel the action of the magent pushing it out of the coil initially and later trying to keep it in, according to Lenz Law. These produce an upward force stopping magnet from falling at 9.8ms^-1.

Question 20

What would happen to the emf time graph if the number of coils was increased?

Answer 20

emf would have double at each stage as magnetic flux/magnetic flux linkage is greater.