Magnetism and Electric Feilds Flashcards
What causes a magnetic field
Due to either:
Moving charges
Permanent magnets
Describe the magnetic field line patterns for a long straigh current-carrying conducter
Use right hand grip rule:
Thumb points in direction of conventional current
Your fingers curl around the wire, showing the direction of the magnetic field lines.
Describe the magnetic field pattern for a flat coil
The lines come out from the end where the current comes out and go into the end which the current goes in
Describe Fleming’s left hand rule
A way to show the relationship in the directions of current, magnetic field lines and the force applied
Define magnetic flux density
The strength of the magnetic field, measured in teslas, hence:
The amount of force applied by unit current and unit length on an object in a magnetic field.
State four factors which effect the magnitude of the force experienced by a wire in an external magnetic field
Current
Length of the wire in the magnetic field
Sin x, where x is the angle between the magnetic field and the current direction
Strength of the magnetic field.
Explain how a charged particle can go travel in circular motion due to a magnetic field
If the force applied to the charged particle due to a magnetic field is at right angles to the particle’s velocity, the force acts as the centrapetal force on the object.
Define a velocity selector
A velocity selector is a device that uses both electric and magnetic fields to select charged particles of specific velocity.
Describe how a velocity selector works
Two parallel horizontal plates are connected to a power supply
They produce a uniform electric field
A uniform magnetic field is also applied perpendicular to the electric field
All particles enter the velocity selector through a narrow slit
In order to be undeflected, only particles with electric force which is equal to its magnetic force will leave through a second narror slit
The desired velocity is chosen by using v= E/B
Define magnetic flux
The product of the component of the magnetic field density perpenicular to the area and the crossectinal area
Define the unit weber
One weber is equal to 1 Tesla unit area (magnetic flux)
What is magnetic flux linkage?
The product of the number of turns in the coil (N) and the magnetic flux
What causes an e.m.f. to be induced in a circuit, when related to magnetism?
When there is a change in the magnetic flux linking the circuit
Define Faraday’s Law
The magnitude of the induced e.m.f is directly proportional to the rate of change of magnetic flux linkage
Define Lenz’s Law
The direction of the induced e.m.f. or current is always such as to oppose the change producing it
Describe how a simple A.C. works
A coil is turned in radical magnetic field
The change in magnetic flux linkage causes an e.m.f to be induced
As e.m.f. ∝ -∆ø, plotting a graph of magnetic flux linkage against time’s negative gradient will equal the e.m.f.
As the change in magnetic flux linkage varies with cos(x) where x is the angle made with the vertical, and the coil is turned at a constant speed, the rate of change of magnetic flux linkage changes in relation to a cos graph, so the gradient will give a sin graph, giving an alternating e.m.f., so an alternating current
Define a transformer
An appliance which changes alternating voltages to higher or lower values
Suggest why a transformer is made from a laminated soft iron core?
Soft Iron: can easily be magnitised and de-magnitised
Laminated: minimise currents in the core itself, so minimising loses due to heating
What causes an electric field?
Any object with a charge
Define electric field strength
The force experienced per unit positive charge at that point
Describe how an object with an electric field can be modelled
A uniformly charged sphere is often modelled as a point charge at its centre
What is the use of electric field lines?
They are used to map electric fields and travel from positive to negative
Define coulomb’s law
Any two point charges exert an electrostatic force on each other which is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
Give the similarities between electric and gravitational fields
Both inversley proportional to the separation2
Both a point mass/charge which produces a radial field
Field strenght is both a force per unit of something
1
Describe how the electric field strength of two parallel plates can be determined
The voltatge across the plates can be measured and divided by the distance between the plates in metres.
Define electric potential
The work done per unit charge in bringing a positive charge from infinity to that point.
What is the electric potential at infinity?
Zero
Derive an equation for the capacitance of an isolated sphere
C=Q/V
Q=4πε0RV
Combining gives: C=4πε0RV/V
So C=4πε0R
Describe how the work done for a point charge can be worked out
Calculate the area under the force-distance graph for that point charge
Define electric potential energy
The work done on a particle to bring it from infinity to a certain point in an electric field
