Magnetic Fields and Induced EMFs

Electromagnetic Induction  Just as a magnetic field can be formed by a current in a circuit, a current can be produced using an external magnetic field.  The process of inducing a current in a circuit with a magnetic field is called electromagnetic induction

But How???  Remember, a moving charge in a magnetic field experiences a force.  Well… a charge that is still will also experience a force if the magnetic field is moving around it.  The moving magnetic field causes the force on charges which will cause them to go around the circuit with a current.

Electromagnetic Induction  A current can only be produced if the magnetic field is changing, or what physicist call, a magnetic flux  There are three ways that a magnetic flux can be produced…

Induced Current method #1  The circuit is moved into or out of a magnetic field.  This is relative, so the magnetic field could be moving, the circuit could be moving, or both could be moving.

Induced Current method #2  The circuit is rotated in the magnetic field.  The strength of a magnetic field is dependent on the angle to the circuit.  By rotating the circuit, the strength of the magnetic field is changing and therefore a current can be induced.

Induced current #3  The intensity of the magnetic field is varied.  This change in the strength of the magnetic field will cause a current to be induced.

Faraday’s Law  The emf produced is proportional to the change in the magnetic flux per unit of time and the number of loops in a circuit. Or

Magnetic Flux  Φ is the magnetic flux, or how much magnetic field is going through an area.  Picture a hula-hoop in the rain…

Hula-hooping in the rain  The harder the rain, the more water that will go through the hoop.  The bigger the hula-hoop, the more water that will go through the hoop.  Magnetic flux is the same as this. It is a factor of how much magnetic field there is and how much room does it have to flow through.

What if you turn the hoop?  Let’s say you are hula-hooping around your arm. Can any rain get through?  The angle also determines the amount of magnetic field that can get through.

Magnetic Flux  Therefore… Where, A is the area of the circuit loop, and B is the magnetic field strength

Faraday’s Law again  In total, Faraday’s Law is…

Summary  The things that effect electromagnetic induction according to Farday’s law are:  1) The angle between the magnetic field and the circuit  2) The strength of the magnetic field  3) The area of the loop of the circuit  4) The number of loops in the circuit  5) How quickly the magnetic field changes

 simulation simulation

Lenz’s Law  What direction is the induced current?  Lenz’s Law states:  “The magnetic field of the induced current opposes the change in the applied magnetic field”.

Right-hand rule #1 Again  Remember, a current can induce a magnetic field of it’s own.  The current that it induces will oppose the direction of the magnetic field flux.

Example  A circuit of wire is pulled through a magnetic field pointed out of the smart board. What is the direction of the induced current?

Solution  The current induced will be such that it will create a magnetic field to go back into the smart board.  Therefore, the direction of the current will be clockwise

Door Bells  Ever wonder why there is a small light behind the button of a door bell??

Door bell continued  Pushing the door bell disrupts the current in the left circuit.  This disruption causes there to be a decrease in the magnetic field in the left circuit.  According to Farday’s law, this decrease in the left circuit, causes a current to be induced in the right circuit.

Door bell continued  The induced current causes a magnetic field in the right circuit directed through the middle to the coil.  The iron bar is pushed outwards and hits the chime.

Door Bell