Faraday's Law

Faraday's Law

 

For a conductor moving in a magnetic field, the factors that affect the induced emf are:

• how quickly the magnetic field is changing
• the number of turns or loops of conductor in the field

This leads to Faraday's Law which states:

 

The induced e.m.f. is directly proportional to the rate of change of flux linkage (or rate of flux cutting)

 

A plane of wingspan 30m flies through a vertical field of strength 5 x 10^-4T. Calculate the emf induced across wing tips if its velocity = 150ms^-1.

Answer:
Calculate the area swept out each second by the wings. Multiply that by the field strength, B and you have got the flux swept out in a second.

 

So each second, 2.25Wb of flux is swept out.

 

This method leads us to a simpler equation for the emf induced by flux cutting:

E = Blv

Where:
B = magnetic flux density, T
l = length of the conductor cutting the field, m
v = speed at which the conductor cuts the field, m/s

Remember:it is only the motion perpendicular to the field that induces an emf.

 

 

Electromagnetic Induction

Electromagnetic Induction

 

So magnetic field lines are also known as flux and field strength is how close the field lines are together. The closer the lines the stronger the field. Magnetic field strength is also known as flux density.

 

Magnetic flux is the amount of field lines per area and is measured in Wb/m^2 (Tesla)

 

Magnetic flux density is the amount of field lines and is measured in Webers (Wb)

 

When you move a conductor (a metal wire is a conductor) inside a magnetic field, you create and e.m.f.

 

e.m.f is known as electromotive force. Its definition is:

 

A difference in potential that tends to give rise to an electric current.

 

Voltage is also known as potential difference and the difference between voltage and e.m.f is that when a circuit is open there is a maximum potential difference, when a circuit is closed a current flows round the circuit which gives rise to internal resistance and resistance across the load. When the circuit is closed there is a voltage drop.

 

If you swipe a metal bar through a magnetic field the movement creates an emf across the bar ends. This happens because as you swipe the bar through the field the electrons in the bar will experience a force and accumulate at the ends of the bar. One end will be positively charged and the other will be negatively charged. You then are creating a potential difference between the bar, an emf.

 

Any of the following would induce more emf:

• A longer bar would sweep more area of field
• A stronger field would mean that you swept through more lines of field in the same distance
• A faster sweep would mean you swept out more field per second

 

If you have a coil of wire and you move the coil inside a magnetic field you also create an e.m.f. and the amount of e.m.f. induced depends of the amount of flux passing through the coil and the number of turns on the coil. The product of these is called flux linkage.

 

What is the flux linkage in a coil of 15 turns and area of 25cm^2 in a field strength of 5T?

 

Answer:

 

Flux=Flux density x Area

 

Flux Linkage= Flux x number of coils

 

You need to work out the flux

 

So 5 x 25x10^-4 (you need it in metres not cm)  then multiply this by amount of turns...

 

5 x (25x10^-4) x 15= 0.1875Wb

 

Magnetic Field Questions

Magnetic Field Questions

 

Q1) Describe why a current-carrying wire at right angles to an external magnetic field will experience a force?

 

Q2) Write down the equation you would use to find the force on a current-carrying wire.

 

Q3) A square loop of wire carrying a current of 3A is within a magnetic field of strength 2x10^5T. Each side is 4cm long. Side A is at right angles to the magnetic field. Side B runs parallel to the magnetic field.

 

a)What is the force on side B of the loop.

 

b) Calculate the magnitude of the force on side A of the loop.

 

 

Answers are coming soon....