Untitled Flashcards Set

Conductor

are materials in which electric charges move freely (i.e., they have low internal resistance)

Semiconductors

are materials that lie in between conductor and insulator. if controlled amounts of foreign atoms are added to semiconductors, their electrical properties can be changed by orders of magnitude

Semiconductor examples

Germanium, silicon

Insulators

are materials in which electric charges do not freely move (i.e., they have high internal resistance)

Insulator Exmaples

Glass, rubber

The earth

infinite reservoir of or for electrons

Conductor being grounded

When a conductor is connected to the Earth (e.g., con ducting wire or copper pipe)

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Robert Millikan (1909) made a discovery about an electric charge

Millikan Oil drop Expierment: IF an object is charged, it charge is always in a multiple of (e)

Charge is said to be

quantized

Conductor Examples

copper, aluminum, silver

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Example of a conductor being grounded

Lightning Rods

What does Coulomb's law state about electric charges?

Two electric charges experience a force between them.

How is the force between two electric charges related to the separation distance?

The force is inversely proportional to the square of the separation distance r.

What is the relationship between the force and the magnitudes of the charges in Coulomb's law?

The force is proportional to the product of the magnitudes of the charges, q1 and q2.

What type of force is experienced between charges of opposite sign according to Coulomb's law?

The force is attractive.

What type of force is experienced between charges of the same sign according to Coulomb's law?

The force is repulsive.

Coulombs' Constant

8.9875 × 109 N m2/C2

What type of force does Coulomb's law represent?

A field force law

Does Coulomb's law require physical contact between particles?

No, there is no physical contact between the particles.

What type of law is Coulomb's law in relation to distance?

An inverse-square law

How does the strength of the electric force change with distance according to Coulomb's law?

The strength of the electric force falls off as the inverse of the distance squared.

What does the principle of superposition state in a system with charged particles?

The Coulomb force exerted on one particle is the summation of all of the Coulomb forces between that particle and the rest of the particles in the ensemble.

What are the component Coulomb forces in the x, y, and z directions represented as?

Fi is the component Coulomb forces in the x direction, Fj is the component forces in the y direction, and Fk is the component forces in the z direction.

Electric Field with a proton & Electron

An electric charge emits an electric field which always points away from a positive charge and points towards a negative charge

If q is positive

(i.e., q > 0), then E is positive and points radially away from the charge

If q is negative

(i.e., q < 0), then E is negative and points radially inward towards the charge

Electrostatic Equilibrium

No net motion of charge occurs within a conductor.

The electric field is zero everywhere inside the conductor.

Any excess charge on an isolated conductor resides entirely on its surface.

The electric field just outside a charged conductor is ⊥ to the conductor's surface.

On an irregular shaped conductor, charge accumulates at its sharpest points.

electric flux

is a measure of the number of E-field lines that crosses a given area. ΦE =EA

ΦE =EA

An E-field whose lines penetrate a cross-sectional (or surface) area A ⊥ to A has an electric flux ΦE given by

ΦE = E · A = EA cosθ

if the E-field lines lie at an angle θ with respect to the normal line of area A, the electric flux is given by the more general formula.

The angle θ is the angle between that normal line and the E-field direction

Gauss's Law

The electric flux through any closed surface is equal to the net charge Q inside the surface divided by the permittivity of free space ϵ◦.

The electrostatic force is conservative

which means that the work done by the force in a closed loop is zero.

The electrostatic force is conservative, which means that the work it does on a particle depends only on the initial and final position of the particle, and not on the path followed.

This work done in displacing the particle from one position to another is independent of the path taken, and it can be associated with a potential energy.

potential difference

between points A and B is defined as the change in potential energy (final minus initial values) of a charge q moved from A to B divided by the charge.

Potential difference is not the same as potential energy =⇒ it is a potential energy per unit charge

Electric potential (V ) is a scalar quantity

Electric potential is measured in volts V

The electric potential of a point charge

V=ke q/ r .

electron volt

as the energy that an electron (or proton) gains when moving through a potential difference of one volt.

equipotential surface

A surface on which all points are at the same potential

The potential difference of any 2 points on an equipotential surface is zero.

No work is required to move a charge at constant speed on an equipotential surface

⃗ E is always ⊥ to an equipotential surface.

capacitance

C of a capacitor is the ratio of the magnitude of the charge on either conductor (e.g., plate) to the magnitude of the potential difference between the conductors

C ≡ Q /∆V

measured in farads (F)

1 F ≡1C/V

What is the capacitance of a parallel-plate capacitor?

C =ϵ◦ A/ d

How is Coulomb's constant related to the permittivity of free space?

The permittivity of free space determines how electric forces behave between two-point charges in a vacuum.

How do capacitors add in a parallel circuit? And in a series circuit?

Capacitors can be added in a parallel circuit by summing their individual capacitances.

In a series circuit, the equivalent capacitance is calculated by taking the reciprocal of the sum of individual capacitances

Describe how the internal energy of a capacitor changes as the voltage changes within it.

As the capacitor is being charged, the electrical field builds up. The voltage V is proportional to the amount of charge which is already on the capacitor. The first charge placed on a capacitor experiences a change in voltage (\Delta V=0), since the capacitor has zero voltage when uncharged. The final charge placed on a capacitor experiences (\Delta V=V), since the capacitor now has its full voltage (V) on it

How does dielectric affect the capacitance, charge, potential difference, and internal energy of a capacitor? Why isn't the air a good dielectric?

When a dielectric is added, the capacitance increases, and when it is removed, the capacitance decreases2. The dielectric enhances the electric field, boosting the capacitor's ability to store charge3. However, air is not a good dielectric because it does not enhance the electric field as effectively as other dielectrics1.

dielectric

is any type of insulating material (e.g., rubber, plastic, inserted between the plates of a capacitor to affect capacitance.

electric current

is defined as the rate at which charge flows through a surface.

What direction does current flow?

The direction of current is defined to be the direction at which a positive charge would flow through a wire.

In metals, it is electrons that flow and not positive charges =⇒ the electrons flow in the opposite direction of the current!

Moving charge (whether positive or negative) through a conductor is known as a mobile charge carrier

Drift speed

Although the electron makes a zigzag path through the wire, on average, it continues to move down the electric field (remember in the opposite sense) at an average speed

Ohm's Law

Resistance that remains constant over a wide range of applied voltage differences such that the voltage difference is linearly dependent on current: ∆V =IR

Materials that obey this law are called ohmic =⇒ con ductors are ohmic.

Materials that do not obey this law are called non-ohmic =⇒ semiconductors are non-ohmic.

How is resistance different from resistivity? What is meant by Resistance is futile?

Resistance measures how much an object hinders electric current, while resistivity reflects the inherent ability of the material itself to resist current.

Resistance is futile is a phrase that may be a play on the word "resistance" in electrical contexts, suggesting that it is unnecessary or ineffective

How does resistance and resistivity change with temperature?

Resistivity (resistance per unit area) increases with higher temperature for metals.

The resistance of the conductor is proportional to the temperature. Thus, the increase in temperature increases conductor resistance

thermistors/thermocouples

that measure temperature from resistance in their circuit are called

Superconductor

There is a class of metals and compounds whose resistance virtually goes to zero below a certain temperature, Tc, called the critical temperature

Power

is the amount of work exerted over an interval of time

P=W/delta t

work is equal to the change of potential energy

W =∆PE =q∆V or P =q∆V/∆t

If we have a current of charges ∆q across a voltage difference ∆V, we can rewrite

P =I∆V

the unit for power (W = watt) must be equal to amperes times volts, or

1 W = A·V

Using Ohm's law (Eq. III-9), we can also express Eq

P =I2R= (∆V)2/R

The power delivered to a conductor of resistance R is often referred to as

I2R loss.