Magnetic Fields Flashcards
What is Faraday’s Law?
The magnitude of the emf induced in a conductor is directly proportional to the rate of change of magnetic flux linkage.
What is the motor effect?
When 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.
What is magnetic flux density, B?
The force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines. (Unit is the tesla T)
F = BIL (only applied when B and I are at right angles)
Define the tesla.
The tesla is the unit of B, magnetic flux density and is equal to 1 Nm-1 A-1
Describe the path a charged particle entering a uniform magnetic field at right angles would take?
A circular path.
BQv = mv^2/r therefore r = mv/BQ
What is Lenz’s law?
The direction of the induced current is always such as to oppose the change that causes the current. (Conservation of energy)
Why do charged particles which enter magnetic fields at right angles follow circular paths?
- A charged particle experiences a force due to the magnetic field.
- The particle follows a circular path because the direction of the force on it is perpendicular to its direction of motion (and to the field).
- This force always acts towards the centre of curvature of the circular path.
What is the force on a charged particle moving in a magnetic field?
F = BQv where the field is perpendicular to velocity.
What is the equation for magnetic flux and it’s unit?
Magnetic flux = BA (unit Wb = 1Tm^2)
State the equation for magnetic flux linkage and the unit.
Magnetic flux linkage = BAN (unit Wb turns)
What is the main characteristic of a step-up transformer?
A step up transformer has more turns on the secondary coil than on the primary coil.
(A step down transformer has more turns on the primary coil)
Name three features of a transformer that increases its efficiency?
1) low resistance windings to reduce power wasted due to the heating effect of the current.
2) a laminated core consisting of layers of iron separated by layers of insulator to reduce heating effect and production of eddy currents.
3) a core of “soft iron” which is easily magnetised and demagnetised. Reduces power wasted through repeated magnetisation and demagnetisation of the core.
What happens to voltage and current in a step up transformer?
- voltage is increased “stepped up”
- current is decreased “stepped down”
What happens to voltage and current in a step down transformer?
- voltage is decreased “stepped down”
- current is increased “stepped up”
What condition must be met for a transformer to work?
Transformers only work if there magnetic flux through them is changing, as they won’t work with steady dc.
