Magnetic Fields 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.
Describe a bar magnet
It has ends which are referred to as 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 does a magnetic field line represent?
The line along which a north pole would move in the field. Every field line has a starting point and an end point (north to south)
What is the motor effect?
The force acting on a current-carrying conductor due to a magnetic field. Only when the conductor is placed at a non 0 angle to the field lines. The force is perpendicular to the conductor and field lines. Force is greatest when conductor is perpendicular to the field and 0 when it’s parallel to the field.
Fleming’s left hand rule
Applies to current-carrying conductor in magnetic field. Thumb is direction of force. First finger is direction of field lines. Second finger is direction of current.
What is magnetic flux density?
Magnetic field strength. Symbol B. The magnetic force per unit length per unit current on a current carrying conductor at right angles to the field lines. Unit Tesla (T).
When does F=BIl apply?
Only when B and I are at right angles.
How does a couple arise on a coil in a magnetic field?
For a rectangular current-carrying coil of wire, two sides are perpendicular to the field. They each experience a horizontal force in opposite directions at right angles to the field lines. The pair of forces acting on both sides form a couple as they don’t act along the same line. The coil rotates. The torque is equal to Fd where d is the perpendicular distance between the line of action of the forces.
Why do moving charged particles in a magnetic field move in circles?
The direction of force on the charged particle is perpendicular to its motion and its direction changes as the direction of motion changes. This acts as the centripetal force. Remember that the direction of current is the direction a positive particle moves.
How is F=BQv derived?
Represents force on a moving charged particle in a magnetic field moving perpendicular to the direction of the field. Start with F=BIl. I=Q/t. L=vt. Sub in.
What is the force acting on a charged particle moving parallel to a magnetic field?
0N
Why does a magnetic field do no work on a moving charged particle?
Its direction of motion changes but not its speed so the kinetic energy of the particle is unchanged by the magnetic field.
How does a cyclotron work?
It consists of two hollow D shaped electrodes (dees) in a vacuum chamber. A uniform magnetic field is applied perpendicular to the plane of the dees and a high frequency alternating voltage is applied between the dees. Charged particles are directed into one of the dees neat the centre and forced on a circular path by the magnetic field so they emerge from one dee to another. As they enter the other dee, the alternating voltage reversed so they accelerate into this dee. The process repeats and each time it crosses dees it gains speed and the radius of orbit increases. It emerges once the radius equals the cyclotron radius.
What is the Tesla?
The unit of magnetic flux density equivalent to 1N/m/A
How to investigate the force on a current carrying wire in magnetic field
Place wire perpendicular to the magnetic field between parallel magnets in metal cradle on top pan balance. Connect wire to variable dc voltage supply in series with ammeter. With 0 current, set balance reading to 0. Vary the voltage of the supply to vary the current and record mass on balance. Graph of magnitude of mass reading against current. Gradient, g and length used to find B.
