Chapter 5: Electrostatic and Magnetism Flashcards
Charges
One, the proton, has a positive charge; the other, the electron, has a negative charge. While opposite charges exert attractive forces, like charges—those that have the same sign—exert repulsive forces.
While many of the particles we discuss in electrostatics are very, very tiny, do not forget that they still do have mass. We can use equations such as the kinetic energy
equation when solving problems with charged particles, and the MCAT will sometimes require us to do just that.
Static electricity
Static charge buildup or static electricity is more significant in drier air because lower humidity makes it easier for charge to become and remain separated.
The SI unit of charge is the coulomb, and the fundamental unit of charge is
e = 1.60 × 10−19
The fundamental unit of charge is e = 1.60 × 10−19 C. A proton and an electron each have this amount of charge; the proton is positively charged (q = +e), while the electron is negatively charged (q = −e).
Coulombs Law
Fe= Kq1 q2/r2
where Fe is the magnitude of the electrostatic force, k is Coulomb’s constant, q1 and q2 are the magnitudes of the two charges, and r is the distance between the charges.
Coulomb’s constant (also called the electrostatic constant) is a number that depends on the units used in the equation/ 8.99 x 10^9,
Electric Field
Electric fields are produced by source charges (Q). When a test charge (q) is placed in an electric field (E), it will experience an electrostatic force (Fe) equal to qE.
E=Fe/q=kQ/r^2
where E is the electric field magnitude in newtons per coulomb, Fe is the magnitude of the force felt by the test charge q, k is the electrostatic constant, Q is the source charge magnitude, and r is the distance between the charges. The electric field is a vector quantity, and we will discuss the process of determining the direction of the electric field vector in a moment
Electric potential energy
U= kQq/r
If the charges are like charges (both positive or both negative), then the potential energy will be positive. If the charges are unlike (one positive and the other negative), then the potential energy will be negative.
Remember that work and energy have the same unit (the joule), so we can define electric potential energy for a charge at a point in space in an electric field as the amount of work necessary to bring the charge from infinitely far away to that point.
Change in U= W=Fdcos = Fr x 1= kQq/r
Electric potential energy is the work necessary to move a test charge from infinity to a point in space in an electric field surrounding a source charge.
The electric potential energy of a system will increase when two like charges move toward each other or when two opposite charges move apart. Conversely, the electric potential energy of a system will decrease when two like charges move apart or when two opposite charges move toward each other.
Electric potential energy
V= U/q = change in v/change in t
Electric Dipoles
The electric dipole, which results from two equal and opposite charges being separated a small distance d from each other, can be transient (as in the case of the moment-to-moment changes in electron distribution that create London dispersion forces) or permanent (as in the case of the molecular dipole of water or the carbonyl functional group).
V= kq (r2-r1)/ r1r2 = Kqd /r1 cos
The product of charge and separation distance is defined as the dipole moment (p) with SI units of C · m: p=qd
Thus, the net torque on a dipole can be calculated from the equation
τ = pE sin θ
For a dipole at some angle in an external electric field, there will be translational equilibrium, but not rotational equilibrium. This is because the forces are in opposite directions (left and right in Figure 5.4), but the torques are in the same
direction (clockwise for both).
Dia, Para, and Ferromagnetic
Any moving charge, whether a single electron traveling through space or a current through a conductive material, creates a magnetic field. The SI unit for magnetic field strength is the tesla (T).
Diamagnetic materials are made of atoms with no unpaired electrons and that have no net magnetic field.
Paramagnetic materials will become weakly magnetized in the presence of an external magnetic field, aligning the magnetic dipoles of the material with the external field.
Ferromagnetic materials, like paramagnetic materials, have unpaired electrons and permanent atomic magnetic dipoles that are normally oriented randomly so that the material has no net magnetic dipole. However, unlike paramagnetic materials, ferromagnetic materials will become strongly magnetized when exposed to a magnetic field or under certain temperatures
Magnetic Field
For an infinitely long and straight current-carrying wire, we can calculate the magnitude of the magnetic field produced by the current I in the wire at a perpendicular distance, r, from the wire as:
B= μ0 I/ 2pi r
where B is the magnetic field at a distance r from the wire, μ0 is the permeability of free space (1.3 x 10^-6), and I is the current.
Circular loop is
B= μ0 I/ 2r NO PI
Straight wires create magnetic fields in the shape of concentric rings. To determine the direction of the field vectors, use a right-hand rule. (This is one of two right-hand rules used in magnetism.) Point your thumb in the direction of the current and wrap your fingers around the current-carrying wire. Your fingers then mimic the circular field lines, curling around the wire.
Magnetic force
Fb=qvb sinO
where q is the charge, v is the magnitude of its velocity, B is the magnitude of the magnetic field, and θ is the smallest angle between the velocity vector v and the magnetic field vector B.
Remember that sin 0° and sin 180° equal zero. This means that any charge moving parallel or antiparallel to the direction of the magnetic field will experience no force from the magnetic field
Parts of the right-hand rule for magnetic force MOVING CHARGE:
Thumb—velocity (indicates direction of movement, like a hitchhiker’s thumb)
Fingers—field lines (fingers are parallel like the uniform magnetic field lines)
Palm—force on a positive charge (you might give a “high five” to a positive person)
Back of hand—force on a negative charge (you might give a backhand to a negative
person)
Force on a Current-Carrying Wire
FB = ILB sinθ
where I is the current, L is the length of the wire in the field, B is the magnitude of the magnetic field, and θ is the angle between L and B. The same right-hand rule can be used for a current-carrying wire in a field as for a moving point charge; just remember that current is considered the flow of positive charge.
