Electric fields are created electric charges, in which the field strengths represent the force experienced per unit positive charge. Electric field lines represent:
• Direction of force, through the arrows represented (positive repels if a charge which is relatively more negative, which will is attract)
• Strength, which is represented by the gap between each field lines, where closer the lines stronger the force
Uniform fields; occur between charged parallel plates. Charged plates can be created in a circuit, where E∝V. As the distance between the charge plates increase, the strength of the electric field decreases, hence E∝1/d. joining these two proportionalities together we get the formula for uniform electric field strength.
Radial fields, occur in a point charges. Similar to gravitational field strength, E∝1/r2. Furthermore, E∝Q and one final factor that affect E is the medium (e.g. medium can have and vary charge, like water) knowing this E∝1/4πε0. Joining all these probabilities gives us the formula of radial electric field strength.
The force these electric fields exert on another charged particle is given by multiplying the electric field strength by the charge of the new particle or plate. This is because F=EQ (derived from E=F/Q). Coulomb’s law is the Force experience between two point charges (radial electric force).
In a uniform electric field, the motion of a charged particle will:
•Accelerate vertically in direction of force acting on the particle
• if particle is positive accelerate towards the negative and vice versa.
• The acceleration is the force in a uniform electric field (where Q is the particle), divided by mass (F=MA)
• Travel at a constant horizontal velocity. The velocity is to be calculated before or given.
Magnetic field is a region where particles with magnetic properties and moving, charged particles with experience a force.
A magnetic field is induced by two types of magnets
• Permanent magnets with North and South pole that has a magnetic field around it that couldn’t “turn off”
• Electromagnets (temporary) magnets that produce a magnetic field by the flow of current. This can be turned off” if current flow is stopped.
Magnetic flux density, B (magnetic field strength/ intensity of field lines) is the force (F) acting per unit current (I) in a wire of a unit length (l), which is perpendicular to field and wire (see electric field in wire). This is measured in tesla (T) where one tesla is 1 NA-1m-1.
The force acting on a particle travelling in a perpendicular direction to field is given by F=BQv. This is because
I= Q/t and IL= QL/t but L/t= v hence IL=Qv. Since sin (θ) = 1, F=BIL = F=BQv
Mass spectrometry uses deflection of charged particles, using electromagnets to identify the mass of a sample.
• Smaple being ionised will then be accelerated towards an electromagnet.
• This exerts a magnetic force on the different particles.
• Particles of larger mass accelerate less (F=ma) relative to particles of lighter mass.
• This degree of deflection can be measured to identify the mass of the samples.
Knowing that a current flow can induce a magnetic field, it is also possible to induce an emf (electron motive force - voltage (energy make current (electron) flow)). This is done by some way of “sweeping” a conductor across a magnetic field, where three thing affect the magnitude of EMF induced
• Magnetic flux density- as more field lines greater the magnitude emf induced
• Area swept by conductor- as greater the area the more field lines “swept”
• Speed of the swiping- more time the field lines are swept
• Number of coils in conductor- more coil more of the same area swept, hence it multiplies
Magnetic flux (ΦB) is the product of magnetic flux density and the area swept by a conductor (field lines swept by conductor, which are perpendicular to each other). It is measured in Weber (Wb), where one weber NmA-1.
Magnetic flux linkage (Φ) is the product of magnetic flux and the number of turns in a coil. The number of turns in a coil increases the area being swept, essentially multiplying the emf induced.
Hence the only factor left is the speed of the “sweeping” , therefore if we divide magnetic flux linkage by time, we get the emf induced, this is Faraday’s law which states “emf induced is equal to the rate of change in flux linkage”.
Lenz’s Law states “emf is induced in a direction which opposes whatever causes the induction”,
• the emf induced follows the right hand rule (rather than the left hand rule)
• create a force that opposes the motion of the conductor, by making current flow in the opposite direction.
• This alters faradays law by making the value for E (emf) negative.
AC generator creates an alternating current, to supply electricity for households around the country.
• It is created by a taut coil being rotated around a uniform magnetic field.
• When the motion of rotation is perpendicular to the field, it induces a current to oppose the motion
• this will decrease as the motion will become more and more parallel (where current become 0).
• As the coil continue to rotate, notice the will now be in the opposite direction to before
• this will induce a current in the opposite direction.
AC generator is used to supply electricity as it is more efficient, because resistance can be reduced by transforming electricity by:
• Step- up- number of turns of secondary coil is greater than primary. This increases voltage, decreases current (conservation of energy)
• Step-down- number of turns on primary is greater than secondary. This decreasing voltage, increase current
In order to transform a.c electricity, a transformer is used. It uses electromagnetism and an electromagnetic induction to
• Induce a magnetic field on the iron core from the primary coil
• induce an emf on the secondary coil from the magnetic field
Transforming electricity is done through changing the turns-ratio.
This is the end of the topic
Drafted by Brandon (Physics)