Electromagnetism Flashcards
Define electric field strength
The force per unit positive charge
State whether electric field strength is a vector or scaler quantity
Vector quantity
State if electric field strength obeys the inverse square law
Yes it does
Explain how Millikan was able to keep oil droplets stationary in his field of view when looking through a microscope
Millikan applied an electric field between two metal plates between which the oil droplets fell. This produced an upward force to counter the downward force of gravity. By varying the voltage applied to the plates it was possible for him to exactly balance the two forces and keep the drop stationary
X-rays were fired into the chamber that Millikan used. What effect did this have on the oil droplets?
The x-rays fired into the chamber ionised the air. Electrons would attach themselves to the oil droplets thus varying the charge in them
Some of the droplets in Millikans experiment were unnaffected by a change in the strength of the electric field the droplets where in. Why would this be the case?
These droplets would not carry any excess charge
Provide a defenition of the electronvolt
An electron volt is the kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1 volt
State coloumbs law
The force between two point charges is proportional to the product of their charge and inversely proportional to the square of their distance appart
Explain what is meant by a ferromagnetic material
A ferromagnetic material is one which can become permanently magnetised
Describe how ferromagnetic material can become aligned
They will become aligned if placed in a strong magnetic or electromagnetic field
State one difference and one similarity of the magnetic fields around a solenoid and a bar magnet
Similarity - same shape of magnetic field/both have a north and south pole
Difference - magnetic field produced by a current in a solenoid/magnetic field can be switched off in a solenoid
A permenant magnet is dropped on the floor by a teacher. Explain why this might cause its magnetic field to weaken
A knock can cause the dipoles to randomise so reducing the strength of the magnetic field
An unmagnetised iron bar is placed within a solenoid which is attached to a direct current supply and a steady current is passed through the solenoid. The current is then switched off.
a) State what happens to the magnetic field around the solenoid
b) State what will happen to the magnetic field around the iron bar giving reasons for your answer
a) The magnetic field disapears
b) The magnetic field around the iron bar remains as the dipoles have now become aligned creating a permenant magnetic field
An unmagnetised iron bar within a solenoid is attached to an alternating current supply. The iron bar is slowly withdrawn from the solenoid and found to be unmagnetised. Explain this observation
As the current alternates it creates opposing alignment in the dipoles which cancel each other out
An electron, a neutron and a proton all enter a uniform magnetic field at right angles with the same velocity. State and compare how the magnetic field will affect the paths of each particles.
The electron will follow a curved path with a certain radius. The neutron will be unaffected as it has no harge. The proton will follow a curved path in the opposite direction to the electron and with a larger radius as it has more mass.
Charges particles enter the Earths magnetic field. The particles travel with an initial velocity at an angle of theta to the direction of the magnetic field.
a) What name is given to the type of path followed by the charged particle?
b) Why is the path followed by the charged particle this shape?
a) Helical
b) There is a centripetal force acting on the charged particle due to the component of its velocity in the magnetic field. This produces a circular motion. The particle still has a forward component to its velocity which creates the helical path.
A permanent magnet can be made from ferromagnetic material. Explain how ferromagnetic material can be made magnetic and the changes wih take place within it when this happens.
Ferromagnetic material can be made magnetic by placing it within a magnetic or electromagnetic field. The magnetic dipoles in the atom will align themselves with the magnetic field they are in rather than being randomly aligned
A student writes in his notebook “electrostatic force and gravitational force obey the inverse square law with electrostatic force requiring two opposite charges.” Explain what is correct and what is incorrect in his statement
It is correct that both obey the inverse square law but the electrostatic force acts between both like and unlike charges
Why can magnetic field intensity not be described in the same way as gravitational or electrostatic field intensity?
Magnetic fields are not point sources
Which fundamental forces act in the nucleus of an atom?
Strong and weak forces
Why do strong and weak forces not have an effect outside the nucleus of an atom?
They only have a very short range
What is meant by time constant?
The time constant is the time taken to increase the charge stored by 63% of the difference between initial charge and full charge. If the capacitor is discharging it is the time taken for the capacitor to discharge to 37% of its initial charge
Explain what is meant by the term ‘capacitive reactance’ and state the unit is measured in
Capacitive resistance is the opposition to the current flow that a capacitor provides in an a.c circuit. Xc varies inversely with frequency and is measured in ohms
Explain why frequency has an effect on the capacitive reactance of a capacitor
Current stops flowing in a circuit when the capacitor becomes fully charged. The higher the frequency, the more often the direction of current changes the less time there is for the capacitor to become fully charged
Explain what is meant by a ‘back e.m.f’ and how it is produced
A back e.m.f is the potential difference induced across an inductor when the current flowing through it is changing. Its direction is against the e.m.f which is producing the current
State Lenzs Law
Lenzs law states that the induced e.m.f always opposes the change in current which causes it
Explain what is meant by the term ‘inductive reactance’ and state the unit it is measured in
Inductive reactance is the opposition to current flow that an inductor provides in an a.c circuit. Xl varies in proprtion with frequency and is measured in ohms
Describe James Clerk Maxwells contribution to the unification of electric and magnetic fields
Macwell predicted that elecric and magnetic fields would sustain themselves, free from charges and currents, if they took the form of an electromagnetic wave. The electromagnetic wave is formed when an electric field combines with a mgnetic field. The fields are perendicular to each other and to the direction of travel of the wave
