ELECTROMAGNETIC INDUCTION

YES!We need:

Loop (complete circuit) Magnetic Field

I can cause B

Can B cause I?

I can cause B B can cause I

There are 3 ways to induce I: 1) Change B

2) Change the area of the 3) Change the orientation of the loop

All these changes cause ΔΦ

ΔΦ – magnetic flux

cosBA

• Wire is 16cm long• B1 = 4.0mT

• Find magnetic flux if:• A) it’s a square• B)it’s a rectangle with one

side = 2.0m•

EXAMPLE 1

EXAMPLE 1

• Wire is 16cm long• B1 = 4.0mT

• Find magnetic flux if:• A) the square is

perpendicular to the field• B)the square makes a 60o

angle with the field•

cosBA

LENZ’S LAW

The induced emf resulting from a changing magnetic flux has a polarity that leads to an induced current whose direction is such that the induced magnetic field opposes the original flux change.

BLAH – BLAH – BLAH …

Change of B creates current Current will have its own B around it

INDUCED B always FIGHTS THE CHANGE:

• If the flux is growing, it will fight it ( )

• If the flux is dying, it will help it ( )

There is a constant magnetic field directed into the page in the shaded region. The fieldis zero outside the shaded region. A copperring slides through the region.

For each of the five positions, determine whetheran induced current exists and, if so, find itsdirection.

HOW STRONG IS I INDUCED?

What does current depend on? (Remember this guy

Where would V come from if there is no battery? Just the wire….

No battery = no ‘terminal voltage’ – only emf

FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION

The average emf induced in a single coil is

The Emf Induced by a Changing Magnetic Field

A coil of wire has an area of 0.0015 m2. A magnetic field is perpendicular to the surface. Initially, the magnitude of the magnetic field is 0.050 T and 0.10s later, it has increased to 0.060 T.Find the average emf induced in the coil during this time.

What would be the total emf if we had 20 coils?

FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION

The average emf induced in coil with N loops is

MOTORS, GENERATORS, TRANSFORMERS

ME - EPE

• In a generator (AC or DC) a loop of wire is rotated in a constant magnetic field (Mechanical energy delivered by a person / falling water, etc.)

• Rotation of the loop causes ΔΦ (change of magnetic flux)

• ΔΦcauses emf (with changing polarity in AC-generator and constant polarity in DC generator

or x ?

or ?

direction of I?

EPE - ME

• In a motor, wires with current in a constant magnetic field are moved by magnetic forces.

or x ?

direction of I?

direction of Fm ?

TRANSFORMERS

• Two coils (uh-huh!): a primary and a secondary coil.

• Emf on the primary coil induces emf on the secondary coil.

EXAMPLE 1

A step-down transformer providing electricity for a residential

neighborhood has exactly 2680 turns in its primary. When the

potential difference across the primary is 5850 V, the potential

difference at the secondary is 120 V. How many turns are in the

secondary?

EXAMPLE 2

A step-down transformer has 525 turns in its secondary and 12 500

turns in its primary. If the potential difference across the primary is

3510 V, what is the potential difference across the secondary?