Motors andGenerators

2/1.1 Motors use the effect of forces on current-carrying conductors in magnetic fieldsperform a first-hand investigation to demonstrate the motor effect

discuss the effect, on the magnitude of the force on a current-carrying conductor, of variations in:

the strength of the magnetic field in which it is locatedthe magnitude of the current in the conductorthe length of the conductor in the external magnetic fieldthe angle between the direction of the external magnetic field and the direction of the length of the conductor

solve problems and analyse information about the force on current-carrying conductors in magnetic fields using

Magnetic Fields - ReviewMagnetic fields are produced byI electric currents magnetised magnetic materials such as iron

Magnetic Fields - ReviewThe magnitude of the magnetic field produced by an electric current depends on: the magnitude of the current - the greater the current, the stronger the magnetic field

the distance from the conductor - the greater the distance from the wire, the weaker the magnetic field the shape into which the conductor is formed - e.g. a coil

Magnetic Fields - ReviewMagnetic fields are represented by magnetic lines of forcemagnitude of the field - indicated by the spacing of the field lines - closer spacing represents a stronger field direction of the field - indicated by an arrow pointing in the direction in which the north pole of a magnet points in the field

Magnetic Fields - ReviewMagnetic fields are represented by crosses representing a field into the pagearrows - closer spacing stronger B fielddots representing a field out the page

Magnetic Fields - Review

Magnetic Fields - ReviewRemember the directions using the right hand grip rule - but NEVER quote this rule in an exam - it is just a memory aid!

Magnetic Fields - ReviewRemember the directions using the right hand grip rule - but NEVER quote this rule in an exam - it is just a memory aid!

Magnetic Fields - Review

Magnetic Fields - Review

Magnetic Fields - ReviewThe magnetic field close to the end of a bar magnet is uniformBSN

Magnetic Fields - ReviewThe magnetic field inside, and close to the ends of a solenoid is uniformB

2/1.9 First-hand investigation of the motor effectThe motor effect can be demonstrated by placing a magnet near a current carrying wire so that the magnetic field is perpendicular to the direction of the current flow in the wireThe observed result, which demonstrates the motor effect is the deflection of the wire, in a direction perpendicular to the magnetic field and to the direction of the current

2/1.4 Current carrying conductors experience a forceThe direction of the force on a long straight current carrying conductor is given by the left hand FBI rule.NEVER quote this rule in an exam - it is just a memory aid!

Current carrying conductors experience a forceWhat is the direction of the forces acting on the following wires?

2/1.10 Current carrying conductors experience a forceA force is produced on a current carrying conductor in a magnetic field, except when the conductor is parallel to the field.The magnitude of the force depends on and is directly proportional tothe magnitude of the current, Ithe magnitude of the magnetic field, Bthe length of the conductor, l in the fieldthe sine of the angle between the field and the conductorIf the wire is at an angle to the field, the force is reduced by a factor of sin(q)q

The maximum force is exerted when the conductor is perpendicular to the field, and is given by the expression

Write the equation first!F = 2 x 104 x 200 x 103 x 5 x 102F = 2 x 106 newtons2/1.10 Current carrying conductors experience a forceCalculate the maximum force on a conductor of length 5 cm in a magnetic field with an intensity of 2 x 104 T when the current in the wire is 200milliamperes.The force is perpendicular to the current and the field directions

2/1.2 Motors use the effect of forces on current-carrying conductors in magnetic fieldsdescribe qualitatively and quantitatively the force between long parallel current-carrying conductors

solve problems using

k = 2 x 10-7

2/1.2 Force between current carrying conductorsA force is produced between two parallel current carrying conductors The force is a force of repulsion when the currents are in the opposite directions

2/1.2 Force between current carrying conductorsA force is produced between two parallel current carrying conductors The force is a force of attraction when the currents are in the same direction

Force between current carrying conductorsThe force is a force of attraction when the currents are in the same direction (b)The force is a force of repulsion when the currents are in the opposite directions (a)In summary a force is produced between two parallel current c carrying conductors:

Force between current carrying conductorsThe magnitude of the force between the conductors isproportional to the magnitude of the product of the currents in each wireinversely proportional to the distance between the wiresdependent on the magnetic properties of the medium between the wiresThe medium between the wires determines the value of the constant k = 2 x 107 NA2 in air or a vacuumThe magnitude of the force that exists between the two current carrying conductors is given by:

2/1.8 Force between current carrying conductorsCalculate the force between two straight conductors separated by a distance of 1.5 cm with a common length of 35 cm between them when the current in one wire is 200milliamperes and the current in the other is in the opposite direction, with a magnitude of 5000 microamperes.The force isWrite the equation first! Then substitute values F = 4.7 x 109 newtonsREPULSIVE

Force between current carrying conductorsCalculate the force between the side of a square coil consisting of 20 turns carrying a current of 2A and a straight conductor sharing a common length of 25 cm and carrying a current of 3A in the same direction, if the distance between them is 2cm.The force isWrite the equation first! Then substitute values F = 3 x 104 newtons ATTRACTIVE

Force between current carrying conductorsQ1. What is the force between two parallel conductors carrying currents in opposite directions one centimetre apart if the current in one is 10 amperes, in the other is 5 amperes and the common length is 2 m?

Q2. If the distance between the wires was increased to 2 cm, what would be the new force between the wires? (use the fact that force and separation are inversely proportional)The force is one of repulsionWrite the equation first! Then substitute values F = 0.002 newtons

2/1.3 Motors use the effect of forces on current-carrying conductors in magnetic fieldsdefine torque as the turning moment of a force using

describe the forces experienced by a current-carrying loop in a magnetic field and describe the net results of the forces

identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field

solve problems and analyse information about simple motors using

The motor effect - force on a current-carrying wireConsider a wire carrying a current across a magnetic field, B as shownThe net result is called the motor effectThe force on the moving charges in the wire is into the pageThis produces a resulting force on the wire that is also into the page A wire carrying a current in a magnetic field experiences a force due to the movement of charges in the wire.[dont say down]

2/1.3 Torquemagnitude of the force, FThe magnitude of a torque, t depends on theA torque is a force which acts to produce a rotational effect or a moment.

Torque on a current-carrying loopA rectangular loop of wire carrying a current can be placed in a magnetic field so that the force on opposite sides of the loop results in a turning force about an axis between the two sides.The net result is a pair of moments creating a torque which, given a suitable mechanical arrangement, may result in the loops rotation about the axis PQThe force on side WZ is into the page[never say down - it is ambiguous]The force on side XY is out of the page[never say up - it is ambiguous]

Torque on a current-carrying loopSince Force is measured in newtons, and distance is measured in metrestorque is therefore measured in newton metres (N.m)

Torque on a current-carrying loop

2/1.5 Torque on a current-carrying loopThe area of the loop is found by side WZ multiplied by YZ, thus:If more than one loop is involved then the equation becomesThe loop will now begin to rotate. To account for the changing angle of the loop within the magnetic field, the equation becomes: becomes

Torque on simple motorWrite the equation first! Then substitute values

*********************Two parallel conductors carrying an electric current will experience a force between them because each one produces a magnetic field, which acts on the moving charges in the other wire.In the above diagram, two wires are carrying an electric current from top to bottom. The magnetic field shown is that produced by the wire on the left. This magnetic field is a cylindrical field around the wire and only the part of the field in the plane of the page has been shown on the diagram for simplicity. If we now consider the charges carrying the current in the other wire, since they are also moving down, the force on them, and hence on this wire will be to the left or towards the other wire. The same argument can now be applied to the current in the left hand wire as the charges move through the field of the right wire and it will be seen that this results in a force of attraction to the other wire.Having deduced the first result however, the second could also have been deduced from Newton's third law when a force acts on a body, an equal an opposite force acts on the body producing that force.

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