Motors and Generators - Physics

Topics: Magnetic field, Electromagnetism, Magnetism Pages: 11 (2998 words) Published: July 31, 2013
1. Motors use the effect of forces on current-carrying conductors in magnetic fields

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 located

The magnitude of the force is proportional to the magnetic field strength. Thus, an increase in magnetic field strength will cause an increase in the force on the wire and a decrease in magnetic field strength will cause a decrease in force on the wire.

The magnitude of the current in the conductor

The magnitude of the force is also proportional to the current (I). Thus, an increase in current will result in an increase in force whilst a decrease in current will lead to a decrease in force.

The length of the conductor in the external magnetic field

The magnitude of the force is also proportional to the length (L).Thus, an increase in the length of the wire within the field will result in an increase in force on the wire, whilst a decrease in the length of the wire in the magnetic field will lead to a decrease in force.

The angle between the direction of the external magnetic field and the direction of the length of the conductor.

The force is at its maximum when the current carrying conductor is at right angles to the field (sin90=1), and is zero when the conductor is parallel to the field (sin0=0). The magnitude of the force is proportional to the components of the magnetic field that is at right angles to the current carrying conductor (sin90 =1).

The above points can be mathematically expressed as:

F = nBILsin(0)

Where:
F= magnitude of the force (N)
N = number of turns of wire
B= magnetic field strength (T)
I= current (amps)
L = current length (m)
Sinθ= angle between the magnetic field and current.

Describe qualitatively and quantitatively the force between long parallel current-carrying conductors : F/l =kll/d

The force (F) per unit length (l) between two wires carrying currents i1 and i2 separated by a distance (d) in a vacuum is given by: [F/I= K i1i2/d]
Two parallel wires each carrying a current, will exert a force on the other. This happens because each current produces a magnetic field (as shown in Oersted’s experiment). Therefore each wire will find itself carrying a current across the magnetic field produced by the other wire and hence experiences a force

the magnetic field strength at a distance, d, from a long straight conductor carrying a current, I, can be found using the formula: [B= kL/d]
Where k= 2x10^-7Na-2
the magnitude of the force experienced by a length ,L, of a conductor due to an extended magnetic field is: [F/I= K i1i2/d]
or rearranged to make F the subject:
[F= i2l (ki1/d)]
The size of the force is proportional to the two currents and the common length of the wires •The size of the force is inversely proportional to the perpendicular distance between the two wires, meaning that if they are further away from each other, the force is less.

if both currents are in the same direction, then the conductors will attract •if both currents are in opposite directions then the conductors will repel

Define torque as the turning effect of a force using: t = fd

Torque is the turning effect of a force acting on an object which causes it to rotate. [T=Fd]

If torque is applied at an angle less then 90 or 90 degrees to the point of application then the formula T = Fd sinθ is used.

Where T= torque (N.m)
F= force (N)
d= distance from rotational axis (m)
0 = angle to the point of application

From the formula above torque depends on both the size of the force and how far from the turning point it is applied. The further a force is from the turning point, the greater the effect it has.

[t=nBIAcosθ]
Where T= torque (N.m)
n= number of coils
B= magnetic field strength (T)
I=...
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