Electromagnetism Flashcards
(34 cards)
What is an electric field?
The region that surrounds electrically charged particles, in which a force is exerted on other electrically charged particles.
Define electric field strength. (E)
The electrical force acting on a unit positive charge.
Define Electric Potential. (V)
The work done in moving a unit positive charge from infinity to a point in an electric field.
Describe Millikan’s experiment for determining the charge on an electron.
He sprayed negatively charged oil drops into an electric field.
He then adjusted the voltage until the electric force balanced the weight of an oil drop.
Using a microscope, he determined the diameter and hence the volume of the oil drop.
From this, and the density of the oil, determined the mass and weight of the oil drop.
From the electric force he then calculated the charge on the oil drop.
Millikan found that the charge on the oil drop was always a multiple of a basic unit of charge: 1.6 x 10-19 C
Define the Electronvolt. (eV)
The energy acquired when 1 electron is accelerated from rest through a potential difference of 1 volt.
What is 1eV equal to?
1eV = 1.60x10-19 J
What is the value of the Permittivity of free space? (ε₀)
ε₀ = 8.85x10-12 F/m
What is the value of the Permeability of free space? (μ₀)
μ₀ = 4π x10-7 H/m
State Coulomb’s Law in words.
The electrostatic force between two point charges is directly proportional to the product of the magnitude of the charges and inversely proportional to the square of the distance between them.
F=QQ/4 πε0r^2
Explain what it is meant by a Ferromagnetic material.
A ferromagnetic material is one which can become permanently magnetised as their magnetic dipoles can be made to align.
Describe how domains in a ferromagnetic material can become aligned.
They will become aligned if placed in a strong magnetic or electromagnetic field.
What materials exhibit a magnetic effect called Ferromagnetism?
Iron, nickel, cobalt and steel exhibit ferromagnetism.
Explain the meaning of the term ‘uniform electric field.’
The force per unit positive charge is constant at any point in the field.
(V=Ed)
Define Potential Difference
Potential difference is the work done in moving a unit positive charge in a uniform field, across a distance.
Explain why a moving charged particle in a magnetic field follows a helical path. (Enters at an angle other than 90 degrees).
This is due to the two components of the particle’s velocity— one perpendicular to the magnetic field (v sin θ) creating the circular motion, and a component parallel to the field (v cos θ) creating the pitch in the helix.
What is it meant by the time constant of an RC circuit?
The time constant gives the time taken for a capacitor to charge to 63%.
After how many time constants is an uncharged capacitor said to be fully charged?
5 time constants (5τ)
(This is the same for discharging a full capacitor)
Define Capacitive Reactance.
The opposition (of a capacitor) to changing current.
Define Inductive Reactance.
Is the opposition (of an inductor) to changing current.
Is capacitive reactance inversely proportional to frequency.
Yes, capacitive reactance is inversely proportional to frequency.
Complete the sentence: ‘Inductive reactance is ___ ___ to frequency.’
Inductive reactance is directly proportional to frequency.
What is Lenz’s Law?
Lenz’s law states that the induced EMF always opposes the change in current that opposes it.
What are some implications of Lenz’s law? (6)
Lenz’s law is used in electromagnetic brakes and induction cooktops.
It is also applied to electric generators, AC generators.
Metal detectors
Braking systems on train
Card Readers
Microphones
Define ‘back EMF’
A back EMF is the potential difference induced across an inductor when the current flowing through it is changing.
