Electric and Magnetic Fields Flashcards
What is an inverse square law?
The electrostatic force between two charged spherical objects
What happens between two like and opposite charges
Like- repels opposite- attract
What is electric field strength and units? and what type of unit
E, at a point is the force per unit charge at that point Nc^-1, vector
Talk about field lines
The close they are, the stronger the field, they point towards the negative charge
Which direction is the field around a negative point charge directed
Inwards towards the charge, and around a spherical positive charge distribution it is directed radially outwards
What is an example of a non radial field and where is it found
Uniform electric field, it is found in the space between two parallel plates when there is a potential difference applied between the plates
What is a capacitor
A device that stores energy by separating charge. Placing charge on the capacitor in a potential difference
Units of capacitance? And what is definition.
A farad, 1F, 1CV^-1, it is a huge unit and normally ranges from mewf to pf. Amount of charge stored per volt. C=Q/V
When is a capacitor fully charged
When the potential difference across it becomes equal to the emf of the source
E
Energy is stored in the capacitor because work is done as charge moves through the net potential difference in the circuit. This becomes electrostatic potential energy
How do we measure the work done
The shaded area under the graph of potential difference between the plates v against the charge on the plates q. Area of triange half base times height so w=0.5QV and using C=Q/V you can change to different formulas when you make q and v subjects.
How do you discharge a capacitor? and what happens
Move the switch, as the charge decreases, so does the potential difference across the capacitor and hence also the current
What is the discharging process an example of?
Exponential decay
What is RC
Time constant of the circuit
What does a wire carrying a current have around it and what happens when wire is wound into a long solenoid.
a circular magnetic field and when wound into a long solenoid there is a uniform field inside . Parallel equally spaced field lines
What are lines representing the magnetic field called
lines of magnetic flux
3 things about the lines
Arrows indicate the forces that would act on a magnetic north pole
The lines are continuous and must not cross each other
The closer the lines, the stronger the magnetic field
What do the number of lines passing through a unit area perpendicular to the field represent
the flux density beta, and is measured in tesla, T. Flux omega is measured in webers, wb.
When will there be a maximum force in f=bilsintheta and what is the magnetic field strength B defined as?
when the current is at right angles to the direction of the field, the force on one metre of wire carrying a current of one amp at right angles to the magnetic field.
Charged particle beams- A charge q, moving with speed v through a magnetic field also experiences a magnetic force F, as long as there is a component of the field perpendicular to the direction of charge movement.
Again, when max force?
Maximum force will be when the charge movement is ar right angles to the direction of the field. F=BQvSintheta
When is an emf generated
When any conductor experiences a changing magnetic flux
What causes the changing magnetic flux
May be due to relative movement between the conductor and a magnetic field (dynamo effect).
Or because the conductor is in a region of varying magnetic field strength (the transformer effect)
What is flux linkage
For a conducting coil, the size of the effect increases with increasing turns of wire, knows as flux linkage.
N Omega, where n is the number of turns. The flux through the coil depends upon the angle of the normal to the plan of the coil to the field direction.
What is Faraday’s law
States that the magnitude of the emf, is directly proportional to the rate of change of flux linkage.
What is Lenz’s law
States that the induced emf must cause a a current to flow in such a direction as to oppose the change in flux linkage that produces it, otherwise energy would appear from nowhere.
What do the fingers represent in the right hand rule.
Thumb- current direction
Curled fingers- Point in the direction of the field
Explain the dynamo effect
As the coil rotates, the magnetic flux linkage goes from zero (when the coil is parallel with the field) to a maximum (when the coil is
Explain the transformer effect
If the current in a coil changes, then so does the magnetic field produced . If the changing magnetic flux links with another coil then there will be an emf induced in that coil. The coil in which current is changing is referred to as primary. The other coil is called the secondary
How do you ensure maximum flux linkage between the two coils
They are mounted on a soft iron core. This becomes magnetized when current flows in the primary, and flux is channeled to the secondary.
Explain eddy currents
Any metal object placed in a region of changing magnetic field will have an emf induced in it. This emf causes current to circulate in the object . We call such currents eddy currents. The eddy currents in a transformer dissipate energy, so can lead to inefficiency. In situations where relative movement is involved, the eddy currents cause braking.
Talk about field strength in a radial field and a uniform field and explain why, also equation for electric field strength
Raidal field: Depends on how far you are from the charge, as you go further away from a point charge q, the field lines get further apart and the field strenght decreases
Unifrom field: the field strength is the same everywhere. The field lines are parallel so always same distance apart so the field strength depends on the potential difference V and distance d E=V/d (Vm^-1)
E=F/Q force per unit charge
Formula for field strength
E=kQ/r^2 where k=1/4pieE
The time taken to charge and discharge a capacitor depend on what two things?
The capacitance
The resistance
What is a magnetic field and example
A region where a force is exerted on a magnetic material. There is a magnetic field around a wire carrying electric current.
What is magnetic field strength or magnetic flux density a measure of and what are the units and what type of quantity.
Is a measure of the strength of the magnetic field per unit area. Its a vector quantity with both a direction and magnitude and is measured in teslas
What is flux linkage
The product of magnetic flux and the number of turns on the coil. Wb- weber
Talk about the process of a wire carrying a current in a magnetic field experiencing a force
A force acts if a current carrying wire cuts magnetic flux lines
If the current is parallel to the flux lines, no force acts
The direction of the force is always perpendicular to both the current direction and the magneti
Fleming’s left hand rule finger meanings
First finger- direction of the uniform magnetic field
seCond finger- direction of the conventional current
thuMb- direction of the force in which motion takes place
