Electromagnetic Induction

I can cause B Can B cause I? YES! We need: Loop (complete circuit) Magnetic Field

I can cause B B can cause I There are 3 ways to induce I: 1) Change B 3) Change the orientation of the loop 2) Change the area of the

All these changes cause ΔΦ ΔΦ – magnetic flux

Example 1 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 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

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 field is zero outside the shaded region. A copper ring slides through the region. For each of the five positions, determine whether an induced current exists and, if so, find its direction.

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?