Electric And Magnetic Fields Flashcards
Electric field definition
A region where a charged particle experiences a force
Electric field strength equation and scalar or vector?
E=F/Q
E is a vector pointing in the direction that a positive charge would move
Equation for force (attraction or repulsion) between two point charges in a vacuum
Coulomb’s law
F= Q1Q2/ 4pie0r^2
Equation for electric field between two parallel plates
E=V/d
Definition of capacitance (equation)
C=Q/V
Equation for energy stored by a capacitor
W=1/2QV
Other versions of W=1/2QV
W=1/2CV^2
W=1/2Q^2\ C
What is a point charge
If a charged object is uniformly sphere you can assume all its charge is at its centre
Have radial fields
Coulomb’s law directions for F
If the charges are opposite, the force is attractive so F is negative
If the charges are alike then the force is repulsive and F is positive
Coulomb’s law- type of law?
Inverse square law
Further apart the charges, the weaker the force between them
Inverse square law for electric field strength in a radial field/ point charge
E is inversely proportional to r^2
Field strength decreases as you go further away from Q
Electric potential
The electric potential energy that a unit positive charge (+1C) would have at that point
Depends on how far a point is from the charge creating the electric field and the size of that charge
Electric potential equation for radial field
V=Q/4pie0r
Sign of V in the electric potential equation in a radial field
V is positive when Q is positive and F is repulsive
V is negative when Q is negative and F is attractive
Finding change in V between two points from a graph of E agains r
Area under graph
Finding E from a V against r graph
Gradient of a tangent
As E=V/r
Equipotential definition
Lines joining points where the electric potential is equal
No work is done when you travel along an equipotential
Equipotentials for radial and uniform fields
Radial: surfaces where any point on the surface is the same distance from the centre of the charge
Uniform: equipotentials are flat planes
Capacitors
Definition
How it works
Electric component that stores electrical charge
Made of two conducting plates separated by an air gap/ insulating material
When it’s connected to a power source, positive and negative charge build up on opposite plates, creating a potential difference
Creates a uniform field between the plates
Deriving W=1/2QV, capacitors
Work is done removing negative charge from one plate and depositing it onto the other.
This energy comes from the electrical energy of the battery, QV
Energy stored by a capacitor is equal to the work done by the battery
So energy stored is area under VQ graph (straight line through origin as p.d across capacitor proportional to charge stored on it)
Deriving W=1/2CV^2 from W=1/2QV
Substitute Q for CV because Q=CV
Deriving W=0.5Q^2/ C from W=1/2Q^2/ C
Substitute V for Q/C because V=Q/C
Magnetic flux definition and equation
number of field lines per unit area
Measure of the strength of a magnetic field
=BA (B= magnetic flux density and A is area perpendicular to a magnetic field)
Units Wb
Magnetic flux density definition
The force on one meter of wire carrying a current of one amp at right angles to the magnetic field
Vector
Tesla
How to find the direction of a magnetic field around a current carrying wire
Flemings right hand rule
Thumb: current
Curled fingers: direction of magnetic field
How to find direction of a force on current carrying wire in an external magnetic field
Flemings left hand rule
Thumb: force
First finger: magnetic field
Second finger: current
Magnitude of force when the current carrying wire is parallel to the magnetic field
0N
F=BIl
Current perpendicular to magnetic field
Force proportional to current, length of wire, flux density
What causes the force on a current carrying wire in an external magnetic field
Subsequent equation for wire at an angle to the field
Component of the magnetic field perpendicular to the wire
F=BIlsin0
(0 is theta)
F=Bqv and F=Bqvsin0
Forced act on charged particles in magnetic fields
The force on a current carrying wire in a magnetic field that is perpendicular to the current is given by F=BIl
Electric current is rate of flow of charge, I=Q/t
A charged particle which moves a distance,l, in time, t, has velocity l/t
Combining these equations gives the force acting on a single charged particle with q charge moving through a magnetic field where it’s velocity is perpendicular to the magnetic field
Inducing emf
Relative motion between a conducting rod and a magnetic field
Electrons in rod will feel a force causing them to accumulate at one end
Induces an emf across the ends of the rod
Induced whenever magnetic flux passing through a conductor changes
Moving coil towards/ away from poles of a magnet
Factors affecting emf induced
Number or coils
Size of magnetic flux
Flux linkage
Faradays law
The induced emf is directly proportional to the rate of change of flux linkage
Faradays law equation
Magnitude of induced emf= flux linkage change/ time taken
Lenz’s law
The induced emf is always in such a direction as to oppose the change that caused it
It will produce a force that opposes the motion of the conductor- a resistance
Flemings left hand rule to find direction of induced emf
Alternating current
One that changes direction with time
