Magnetic Fields 1 Flashcards
What is a magnetic field?
A force field surrounding a magnet or current-carrying wire which acts on any other magnet or current-carrying wire placed in the field.
Where are the magnetic fields of a bar magnet strongest?
At the ends: the north-seeking and south-seeking poles (according to which direction, north or south, each end points when the magnet is free to align itself with the horizontal component of the Earth’s magnetic field).
What’s a line of force (magnetic field line) of a magnetic field?
A line along which a north pole would move in the field.
What is Earth’s magnetic field caused by?
Circulation currents in the molten iron in the Earth’s core.
In which direction does a magnetic compass point?
It points TO the Earth’s north pole (currently in Northern Canada).
What’s the motor effect?
A current-carrying wire placed at a non-zero angle to the lines of force of an external magnetic field experiences a force due to the field.
The force is perpendicular to the wire and to the lines of the force. (Use Fleming’s left hand rule)
When a current flows, the section of the wire in the magnetic field experiences a force that pushes it out of the field. The magnitude of this force depends on : (4)
the current, the strength of the magnetic field, the length of the wire, and the angle between the lines of force of the field and the current direction.
The force is:
—– when the wire is at right angles to the magnetic field.
—– when the wire is parallel to the magnetic field.
greatest
zero
What’s the magnetic flux density, B, of the magnetic field?
(also called strength of the magnetic field)
Units?
The force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines.
Unit: Tesla (T).
Therefore for a wire of length L carrying a current I in a uniform magnetic filed B at 90° to the field lines, the force F on the wire is given by F = BIL.
DONT NEED TO KNOW
For a straight line wire at angle θ to the magnetic field perpendicular to the wire, magnitude of force on wire?
F = BILsinθ
The couple on a coil in a magnetic field:
Consider a rectangular current-carrying coil in a uniform horizontal magnetic field. The coil has n turns of wire and can rotate about a vertical axis.
Draw this.
The long sides of the coil are vertical. Each wire down each long side experiences a force BIL, where L is the length of each long side. Each long side ∴ experiences a (direction) force F = —— in —- directions —— to the field lines.
Each long side ∴ experiences a horizontal force F=BILn in opposite directions at right angles to the field lines.
The pair of forces acting on the long sides form a couple as the forces…?
Are not directed along the same line.
The torque of the coupe = Fd, where d is the perpendicular distance between the line of action of the forces on each side.
If the plane of the coil is at angle θ to the field lines, then d = ?
d = wcosθ, where w is the width of the coil.
Therefore the torque =
torque = Fwcosθ = BILnwcosθ = BIAncosθ, where the coil area A = Lw
If θ = 0 (i.e. the coil is parallel to the field), the torque =?
If θ = 90 (i.e. the coil is perpendicular to the field), the torque =?
Torque = 0, as cos 0 = 0
Torque = BIAn, as cos 90 = 1
The electric motor:
The simple electric motor consists of?
A coil of insulated wire which spins between the poles of a U-shaped magnet.
Draw this.
When a direct current passes round the coil:
1 - forces
2 - effect
the wires at opposite edges of the coil are acted on by the forces in opposite directions.
The force on each edge makes the coil spin about it axis.
Current is supplied to the coil via?
Via a split-ring commutator.
What does it do? Why?
The direction of the current round the coil is reversed by the split-ring commutator each time the coil rotates through half a turn.
This ensures the current along an edge changes direction when it moves from one pole face to the other. The result is that the force on each continues to turn the coil in the same direction.
In a practical electric motor, several evenly spaced —– coils are wound on —– —–.
Evenly spaced armature coils are wound on an iron core.
Each coil is connected to its own section of the commutator.
The result is that each coil in sequence experiences a — when its connected to the voltage supply, so the armature is …
torque
repeatedly pushed round.
Because the iron core makes the field —, each coil is in the —- of the field (ie θ = 0) for most of the time.
radial
plane
As a result, the torque is steady and the motor runs more smoothly. In addition, the iron core can make the field much stronger so the torque of the motor is much…
greater
