Electric Fields, Magnetic Fields And Electromagnetism Flashcards
State Coulomb’s Law
The force between two charges is proportional to the product of the charges and inversely proportional to the square of their separation.
How is an electric field created?
Electric fields are created around any object that has charge. Other charges in this field experience an electrostatic force.
State features of field lines and what they show
Direction of arrow - shows the direction a positive charge would move in the field.
Density of field lines - indicates field strength.
Define electric field strength
Electrostatic force per unit positive charge.
What is the direction of electric field strength the same as?
The direction of force on a positive charge in the field.
(-) attraction
(+) repulsion
What are features of a uniform field?
Constant field strength and constant direction at all points.
How is a uniform electric field produced?
A uniform electric field is produced between two oppositely charged, parallel plates.
What is a useful equation for a charge from when it starts at rest?
QV = 1/2 mv^2
Define electric potential
The work done in bringing a unit charge from infinity to a point. Potential is zero to infinity.
How can a magnetic field be created?
By a permanent magnet - made from iron, steel, cobalt, nickel or neodymium.
By moving charges - electric currents in wires produce a magnetic field.
Define a solenoid
A coil of wire with a length much greater than the diameter of the coils.
Define a flat coil/Helmholtz coil
A coil of wire that has length much less than the diameter of the coil.
What is the field pattern for a solenoid?
Like a bar magnet but inside the solenoid it is very strong and uniform. (Lots of straight parallel lines close together)
What is the field pattern of a flat coil?
Strong and uniform through the centre.
Define magnetic flux density
The force exerted per unit current per unit length where current is at 90° to the field.
What are the units of magnetic flux density?
Tesla (T)
Define a Tesla
A flux density of one tesla produces a force of 1 Newton on a wire of length 1 metre carrying a current of 1 Ampere at 90° to the field.
What is a useful equation for an object experiencing circular motion due to a magnetic field?
BQv = mv^2 / r
What is the direction of current for Flemming’s left hand rule the same as?
Conventional current, which is the direction a positive charge moves. For a negative charge, current flows in the oppose direction.
What is an equation for when electric and magnetic forces due to fields are equal?
BQv = EQ
v=E/B
Define electromagnetism
The production of an EMF due to relative motion between a magnetic field and a conductor, in the form of a wire or a coil.
If the conductor circuit is complete a current is also produced.
What is the word equation for an induced EMF?
Induced emf = - rage of change of magnetic flux linkage
(-) due to Lenz’s Law
What is the equation for magnetic flux?
Magnetic flux (Φ) = magnetic flux density B x Area A
When is Φ=BA ?
When the field is at 90° to the area.
What are the units for magnetic flux?
Weber (Wb)
Define magnetic flux
The product of the magnetic flux density and the cross-sectional area when the magnetic field is at 90° to the area.
Φ = BAcosθ for different values of θ
θ=0°, B is perpendicular to area, Φ=BA
θ=90°, B is parallel to area, Φ=0
What is the equation for magnetic flux linkage?
Magnetic flux linkage NΦ = number of turns N x magnetic flux Φ
What are the units for magnetic flux linkage?
Weber or Weber turns (Wb)
Define magnetic flux linkage
The product of the magnetic flux and the number of turns of the coil.
Define Lenz’s Law
When an electric current is induced it will always flow in a direction to oppose the charge producing it.
Explain how Lenz’s Law is seen in Farady’s experiments
When a force opposes motion so work must be done to produce electrical energy.
How is magnetic flux density shown?
By how dense the field lines are.
When is a transformer a step-up transformer?
Ns > Np
Number of turns on the secondary coil > number of turns on the primary coil
When is a transformer a step-down transformer?
Ns < Np
Number of turns on the secondary coil < number of turns on the primary coil
What is the equation for a 100% efficient transformer?
Power in = power out
Vp x Ip = Vs x Is
State Faraday’s law of electromagnetic induction
Induced emf = - rate of change of flux linkage
