Electricity Flashcards
Electric Force
exists between charges.
same charge, repulsive force; different charge, attractive force.
the magnitude of the force depends not only on the magnitudes of the charges but also the square of the distance between them.
Charge
Charge (q) is given in units of coulombs (C).
The formula describing the magnitude of the force of the repulsion or attraction between two charged objects is called Coulomb’s law:
F = k (q1q2/r2), where k is Coulomb’s constant (k = 8.988 x 109 N m2/C2
Universal Law of Conservation of Charge
The universe has no net charge. Net charge is created by separating electrons from protons.
Gravitational forces vs. electrical forces
Both types of forces change inversely with the square of the distance between the centers of mass or charge.
Gravitational forces are always attractive while electrical forces may be either attractive or repulsive.
Coulomb forces are usually of a far greater magnitude than gravitational forces, and, unless the masses are very large, gravitational forces are negligible.
Center of charge
is a point from which the charge generated by an object or system of objects can be considered to originate.
Both mass and charge create fields. A field is…
some type of distortion or condition in space that creates a force on a charge (or mass, if it is a gravitational field; or magnet, if it is a magnetic field).
Any field can be represented by lines of force. Lines of force point…
in the direction of the field (positive to negative for electric fields, towards the mass creating the field for gravitational fields). The relative distance between lines indicates the strength of the field; the closer the lines, the stronger the field.
Lines of force must begin on a positive charge and end on a negative charge.
Electric Field
An electric field (E) is defined as the electrostatic force per unit charge. E is a vector pointing in the direction of the field and has units of N/C or V/m.
The force on a charge (q) in an electric field (E) is F = qE
Potential Energy (U) of a charge in an electric field
is the force multiplied by the displacement: U = qEd
Voltage (V)
is the potential for work by an electric field in moving any charge from one point to another.
V = Ed
units of volts (V) or J/C
Work done by a field is path independent.
equipotential surfaces
In an electric field, a surface normal to the field that describes a set of points all with the same potential.
electric dipole
created by two opposite charges with equal magnitude.
Moving electricity
If the charges are experiencing a potential difference, charges move from areas of higher potential to areas of lower potential. A potential difference, or voltage, is said to produce a flow of charge, or current.
Conductivity
When charge moves along an object (usually in the form of electrons), that object is said to be conducting electricity.
All substances conduct electricity to some extent, and all substances resist movement of charge to some extent (superconductors excluded).
Good conductors, such as metals, allow electrons to flow relatively freely.
Poor conductors (good resistors) hold electrons tightly in place and are represented by network solids such as diamond and glass.
Current
Moving charge
units: amps (A) or C/s
Circuit
a cyclical pathway for moving charge
Resistance (R)
the quantitative measure of an object of a particular shape and size to resist the flow of charge.
measured in ohms
Resistance = resistivity (p) x L/A, so if the length of a wire is doubled or its cross sectional area is cut in half, its resistance is also doubled.
Ohm’s Law
Electric current (i) = potential difference (voltage)/resistance (R).
Battery
adds energy to a circuit.
rated by electromotive force (EMF) - fancy word for voltage.
Capacitor
used to temporarily store energy (in the form of separated charge) in a circuit.
In a parallel plate capacitor, two plates made from conductive material are separated by a very small distance. the amount of charge that can be stored is directly proportional to the area of each plate.
C = Q/V
Capacitance is increased in a parallel plate capacitor by:
- decreasing distance between plates
- increasing area of plates
- increasing dielectric constant
Effects of putting circuit elements is series or parallel
putting circuit elements in series is like increasing the length of that circuit element.
putting circuit elements in parallel is like increasing the width, or area.
Resistors in series: Reff = R1 + R2 + …
Resistors in parallel: 1/Reff = 1/R1 + 1/R2 + …
Capacitors are opposite to resistors
energy (U) stored in any shape capacitor
U = 1/2 QV or U = 1/2 CV2 or U = 1/2 Q2/C
dielectric constant, k
refers to the substance between the plates of a capacitor. The substance between the plates must be an insulator, or it would conduct electrons from one plate to the other and charge could not be built up.
A dielectric acts to resist the creation of an electric field.
