Electrostatics & Magnetism Flashcards
Having trouble remembering the difference between electric force, electric fields, and electric potential? Use these cards to master the topic of electrostatics and magnetism as tested on the Chemical & Physical Foundations section of the MCAT.
What unit measures charge, and what sign/s can it be given?
- coulomb (C)
- It can have either a positive sign (for current or protons) or a negative one (for electrons).
The MCAT is primarily concerned with the movement of current, which is conventionally positive.
What characteristics define charges that are attracted to each other?
Opposite charges
In other words, a positive charge is attracted to a negative one and vice versa.
The attractive force varies proportionally with the magnitude of the charges.
What characteristics define charges that repel each other?
Like charges
In other words, a positive charge will be pushed away from another positive charge; the same will happen for two negative charges.
The repulsive force varies proportionally with the magnitude of the charges.
What must be true of the total charge in a closed system?
Total charge in a closed system must remain constant. This is the law of conservation of charge.
5 protons and 4 electrons collide with sufficient energy to create 4 neutrons and 1 proton. How do the total charges before and after the collision differ?
Total charge is the same before the collision as it is afterwards; in both cases, it is +1.
Initially, the total charge is (+5) + (-4) = +1. After the collision, the charge is 4(0) + (+1) = +1. This fits with the law of conservation of charge.
Define:
conductance
The ability of a material to transfer charge.
Conductance is generally higher in metals and lower in nonmetals.
What property defines a good conductor?
A conductor must contain many electrical charges that are relatively moveable within the medium.
Common conductors include metals with high atomic masses, such as silver, copper, and gold.
Calcium contains three times as many moveable electric charges than iron. Which metal is a better conductor?
Calcium is a better conductor; specifically, it has three times the conductance of iron.
The more charges contained within a material, and the more easily these charges can move, the higher the relative conductance.
Define:
insulation
It is the degree to which a material resists the transfer of charge.
Good insulators are poor conductors, and vice versa.
Insulation is relatively high in polymers, amorphous crystals, and ionic solids. Insulation is low in most other substances.
What property defines a good insulator?
An insulator has few free electrical charges within the medium, and those charges are difficult to move.
Common insulators include glass, quartz, rubber and teflon.
Rubber contains 10,000 times as many moveable electric charges as paraffin. Which substance is a better insulator?
Paraffin is a better insulator; specifically, it has a resistance that is 10,000 times that of rubber. Resistance refers to a substance’s ability to hinder charge flow.
The fewer moveable charges a material has, the greater its relative resistance, and the better it is at insulating.
What equation gives the relationship for the force between two charged particles separated by a given distance?
Coulomb’s law, it is defined as:
F = Kq1q2 / r2
Where:
K = the Coulombic constant, measured in Nm2/C2
q1 and q2 = charges on particles (C)
r = distance between charges (m)
In Coulomb’s Law, what is indicated by a positive value for F?
It indicates that the charges are the same sign, and will experience a repulsive force.
In Coulomb’s Law, what is indicated by a negative value for F?
It indicates that the charges are opposite in sign, and will experience an attractive force.
The distance between two positive charges is changed from r to 2r. How is the force between the charges affected?
The force between the charges will decrease to 1/4 of its original value.
According to F = Kq1q2 / r2, force is inversely proportional to the square of distance. Doubling the distance will reduce the magnitude of the force to one-quarter of its original value.
A positive test charge is placed next to a stationary positive charge. What direction will the electrostatic field lines point?
The field lines point away from the source charge.
By convention, all positive source charges have field lines with arrows pointing outwards, since a positive test charge will be repulsed.
A positive test charge is placed next to a stationary negative charge. What direction will the electrostatic field lines point?
The field lines point toward the source charge.
By convention, all negative source charges have field lines with arrows pointing inwards, since a positive test charge will be attracted.
Describe the motion of a positive test charge that is placed exactly at the midpoint between two equal, positive stationary charges.
The test charge will remain immobile.
The two positive charges repel the test positive charge equally; in other words, each charge cancels the other’s force vector. There will be no net force on the test charge.
What direction will a positive charge move when introduced into a uniform electric field pointing directly upwards?
The charge will also move directly upwards.
Positive charges always follow the electric field.
What direction will a negative charge move when introduced into a uniform electric field pointing directly upwards?
The charge will move directly downwards.
Negative charges always move exactly opposite compared to the electric field.
What direction will a neutron move when introduced into a uniform electric field pointing directly upwards?
Assume the neutron is initially stationary.
The neutron will not move at all.
An uncharged, or neutral, particle will not be accelerated in any direction by an electric field.
What direction will a proton move when introduced into a uniform electric field pointing towards the right?
The proton will move toward the right.
Protons are positive, and positive charges always move in the same direction as the field lines.
What direction will an electron move when introduced into a uniform electric field pointing towards the left?
The electron will move toward the right.
Electrons are negative, and negative charges move in the opposite direction compared to the field lines.
What concept describes the set of equal-energy positions where a positive test charge can be placed, when adjacent to a stationary charge?
Equipotential lines, as their name implies, give the positions where a test charge has the same potential with reference to a source.
All charges produce equipotential lines that form closed, non-overlapping loops around them.
Define:
electric dipole
It occurs when positive and negative charge are separated in an object or system.
The simplest example of a dipole is a linear object with one positive and one negative end.
Describe the motion of a dipole placed into an electric field.
The dipole will rotate so that the positive end is aligned with the electric field and the negative end is exactly opposite to the field.
Assuming that the dipole is neutral overall, it will not undergo any translational motion in the field.
