Faradays Law

Wire in a magnetic field

# of 29 Boardworks Ltd 20071Boardworks GCSE Additional Science: Physics Motors and GeneratorsTeacher notesThis virtual experiment can be used as an introduction to the motor effect or as a revision exercise.

The direction of the force acting on a wire in an electromagnetic field can be reversed by:The direction of the force is therefore relative to both the direction of the magnetic field and the current. reversing the currentreversing the magnetic fieldChanging the direction of the force

# of 29 Boardworks Ltd 20072Boardworks GCSE Additional Science: Physics Motors and GeneratorsThe Left Hand Rule( for motors) shows what happens when charged particles (such as electrons in a current) enter a magnetic fieldThis rule is also called Fleming's Left Hand Rule, after English electronics pioneer John Ambrose Fleming First finger represents the direction of the Field. The Second finger represents the direction of the Current [conventional current, positive(+) to negative(-). The Thumb represents the direction of the Thrust or resultant Motion. FBI - moving from thumb to second finger.The thumb is the force FThe first finger is the magnetic field BThe second finger is the of current I

3And there is a force on a current carrying wire due to a magnetic field+q

What direction is the force on this particle?

Fm-force on one particle4The number of charges in the segment is nAl, where n is the number of charges per unit volume.Total magnetic force on the wire of length l: Fmag = (qvd x B) nAl Remember that current I = nqvdA. Therefore: Fmag = I(l x B), where l is a vector in the direction of the current. The magnitude of l equals the length of the wire segment.This equation only works for a straight segment of wire in a uniform external magnetic field.The magnetic field produced by the current itself is also ignored. The wire cannot produce a force on itself.

LF = B I L [Newtons]B = Density of the magnetic flux in Teslas I = Induced current in AmpsL = Length of conductor in field in metresExample 1If a conductor of length 0.4m carrying a current of 10.6A is placed in a magnetic field with a flux density of 0.03T, determine the force experienced by this conductor in newtons.F = 0.03 x 10.6 x 0.4 = 0.1272 NBIFThe Motor Effect

Force

North pole

South poleTorque on a current loop in a uniform magnetic field

b/2sin bPerp. Distance=(b/2)sin pheta7Max. Torque on a current loop in a uniform magnetic field

bottom viewside view

Torque=f x perp. distance8Application: Galvanometer

Application: DC Motor

Application: DC motor commutator

ProblemA small circular coil consisting of 20 turns of wire lies in a region with a uniform magnetic field whose magnitude is 0.50 T. The arrangement is such that the normal to the plane of the coil makes an angle of 60 with the direction of the magnetic field. The radius of the coil is 4.0 cm, and the wire carries a current of 3.0 A. What is the magnitude of the torque exerted on the coil? T=NIABsin 600 =0.13 Nm12

What is electromagnetic induction?When current flows through a wire held in a magnetic field, a force is created that moves the wire.The opposite is also possible: if a wire is moved across a magnetic field, a current is produced. This is called electromagnetic induction.Induction also occurs if a magnet is moved in a coil of wire, or if a coil of wire rotates in a magnetic field.In all these methods of inducing a current, the wire and magnetic field move perpendicular to each other. If they move parallel to each other, no current is induced.

# of 29 Boardworks Ltd 200713Boardworks GCSE Additional Science: Physics Motors and GeneratorsFaradays LawWhen the magnet moves, a current is induced as if there was a source of emf (like a battery) in the circuit!Faradays Law

Faradays Law of InductionAn induced emf is produced by a changing magnetic field.Lenzs Law An induced emf is always in a direction that opposes the original change in the flux that caused the emf.

Units: [Volts]How can we change the flux?

How can we change the flux?

How can we change the flux?Change flux by:Change areaChange angleChange field

InductionLenzs LawInductionThe direction of the induced current in a loop is determined from Lenzs law.The law states that: An induced current has a direction such that the magnetic field due to the current opposes the change in the magnetic flux that induces the current.

It is possible to predict the direction of the induced current produced by a generator if the direction of the force (or motion) or the magnetic field are known. Flemings right-hand rule is used to do this.Flemings right-hand ruleFirst finger = magnetic FieldseCond finger = Current

thuMb = Motion

24Boardworks GCSE Additional Science: Physics Motors and GeneratorsTeacher notesStudents should be made aware that the direction of a magnetic field is usually taken as being N-S rather than S-N, and the direction of current is the direction of conventional current, i.e. positive to negative.The current generates the force FB26Right hand rule2730. In Figure P31.30, the bar magnet is moved toward the loop. Is Va Vb positive, negative, or zero? Explain.

Figure P31.30A generator is a device that converts mechanical energy into electrical energy. It is the opposite of an electric motor.Power stations use generators to produce electricity on a large scale. Mechanical energy is provided by rotating turbines that can be powered by:What are generators?

high-pressure steam in coal, oil, gas and nuclear power stationswind in wind turbinesfalling water in hydroelectric power stations

# of 29 Boardworks Ltd 200729Boardworks GCSE Additional Science: Physics Motors and GeneratorsPhoto credit: Clipper Windpower / National Renewable Energy Laboratory

Rotating Generators and Faradays Law

0

For N loops of wire

Induced emf in a Rotating LoopThe induced emf in the loop is

This is sinusoidal, with emax = NABw

How can the size of an induced current be increased?In a power station generator, an electromagnet is often used as this can provide a stronger magnetic field than is possible with a permanent magnet.Increasing the size of the induced currentincrease the speed at which the coil rotatesincrease the strength of the magnetic fieldincrease the number of turns in the coilincrease the total area of the coil.

# of 29 Boardworks Ltd 200732Boardworks GCSE Additional Science: Physics Motors and GeneratorsPhoto credit: David Parsons / National Renewable Energy LaboratoryThe generator of Niagara Mohawks Dunkirk steam station in New York. This coal-fired power plant produces 600,000 kilowatts of 60 cycle power.Rotating Generators

A 2.00-cm by 1.50-cm rectangular coil has 300 turns and rotates in a region that has a magnetic field of 0.400 T. (a) What is the maximum emf generated when the coil rotates at 60 rev/s? (b) What must its angular speed be to generate a maximum emf of 110 V?