Physics chpt 20 & 21

ben franklin named the direction in which he thought electricity flows

"positive direction of current" but we realized later that electrons move the opposite way in a wire. THE CONVENTION IS AWAY FROM POSITIVE TERMINAL.

what creates a current?

"voltage sources" batteries and generators; basically devices that drive a current and create a potential difference; When a voltage source is connected to a conductor, it applies a potential difference V that creates an electric field. The electric field in turn exerts force on charges, causing current.

resting potential of a neuron

-90mV according to physics 2e

speed of electrical signals carried by currents

10^8 m/s speed of sounds: 343 m/s light: 299,000 km/s

what are free electrons

Atoms in a metallic conductor are packed in the form of a lattice structure. Some electrons are far enough away from the atomic nuclei that they do not experience the attraction of the nuclei as much as the inner electrons do. These are the free electrons. They are not bound to a single atom but can instead move freely among the atoms in a "sea" of electrons. These free electrons respond by accelerating when an electric field is applied. Of course as they move they collide with the atoms in the lattice and other electrons, generating thermal energy, and the conductor gets warmer. In an insulator, the organization of the atoms and the structure do not allow for such free electrons.

What is the difference between a conductor in static electricity vs moving electricity?

By defintion, in static electric conductors at equilibrium, no electric field is produced. On the other [moving] hand, it is important to realize that there is an electric field in conductors responsible for producing a current. An electric field is needed to supply energy to move the charges.

how the conventional direction of current (the way positive charges would flow) varies

Depending on the situation, positive charges, negative charges, or both may move in a current. i. Van de Graaff - can produce a purely positive current of protons ii. In metal wires - electrons flow iii. Nerves and ionic solutions - both flow, like Na+ and Cl-

As batteries are depleted, r (__________________) increases.

Internal resistance

emf = Ir + IR1 + IR2 ...

Kirchoff's second rule, stating that the emf equals the sum of the IR (voltage) drops in the loop.

How electrical signals carried by currents travel extremely fast despite the fact that the individual charges that make up the current move much more slowly on average

Most electrical signals carried by currents travel at speeds on the order of 108m/s, a significant fraction of the speed of light. Interestingly, the individual charges that make up the current move much more slowly on average, typically drifting at speeds on the order of 10^−4m/s. The high speed of electrical signals results from the fact that the force between charges acts rapidly at a distance. Thus, when a free charge is forced into a wire, as in Figure 20.5, the incoming charge pushes other charges ahead of it, which in turn push on charges farther down the line. The density of charge in a system cannot easily be increased, and so the signal is passed on rapidly. The resulting electrical shock wave moves through the system at nearly the speed of light. To be precise, this rapidly moving signal or shock wave is a rapidly propagating change in electric field. When charged particles are forced into this volume of a conductor, an equal number are quickly forced to leave. The repulsion between like charges makes it difficult to increase the number of charges in a volume. Thus, as one charge enters, another leaves almost immediately, carrying the signal rapidly forward.

explain how depolarization and repolarization occurs in the neuron

Na+ finally rushes in since its attracted to the negativity in the membrane, which depolarizes the cell The depolarization causes the cell to quickly be impermeable to Na+ , K+ rushes in quickly repolarizes the cell and it is now back at resting potential.

most important ions in nerves

Na+, K+, Cl- Na+ and Cl-: the membrane at rest is permeable to them. Their diffusion (and coulomb forces preventing them from going too close together) creates the 2 difference electrical layers in and outside the cell membrane... this creates the resting potential at 70 to 90mV respective to the extracellular fluids at rest.

T or F: all axons are myelinated

Not all axons are myelinated, so that cross talk and slow signal transmission are a characteristic of the normal operation of these axons, another variable in the nervous system. Grey matter = unmyelinated FALSE!

resistance

Ohms (Ω) R = V/I so 1 Ohm is 1 Volt/Ampere zig zag lines resistance usually increases with temperature Ohmic materials - resistance R that is independent of I and V. Non-Ohmic material - superconductor (no resistance) Resistance is related to the shape of an object and the material of which it is composed.

electrical power

P = IV Unit: Watts (since work is joules, power is work/time, thus work is joules/time aka Watts)

Precisely how are voltage, current, and resistance related to electric power?

P = IV (always relevant) I = V^2/R (resistor only) implies voltage has a big influence (its squared!) especially when resistance is kept constant (but remember, the temperature would actually increase with usage) AND the lower resistance = more power delivered P = I^2R (resistor only) implies

The time dependence of power for an AC current

P = IV (reminder) the time dependence of power is P=I_0V_0sin^2*2πft the product is always non-negative fluctuates between zero and I0V0�0�0. Average power (P_ave) is (1/2)I_0*V_0

electrical potential energy equation

PE = qV (q is electric charge and V is voltage)

Why are the chemicals able to produce a unique potential difference?

Quantum mechanical descriptions of molecules, which take into account the types of atoms and numbers of electrons in them, are able to predict the energy states they can have and the energies of reactions between them. In the case of a lead-acid battery, an energy of 2 eV is given to each electron sent to the anode. Voltage is defined as the electrical potential energy divided by charge: V=P_E/q. An electron volt is the energy given to a single electron by a voltage of 1 V. So the voltage here is 2 V, since 2 eV is given to each electron. It is the energy produced in each molecular reaction that produces the voltage. A different reaction produces a different energy and, hence, a different voltage.

resistance affects by length and cross section/diameter(A) (uniform)

R increases proportionally with length R and A are inversely related (the more A, the less R)

benefits of the arrangement of myelination and nodes of Ranvier (gaps)

Since myelin is an insulator, it prevents signals from jumping between adjacent nerves (cross talk). Additionally, the myelinated regions transmit electrical signals at a very high speed, as an ordinary conductor or resistor would. There is no action potential in the myelinated regions, so that no cell energy is used in them. There is an IRIR signal loss in the myelin, but the signal is regenerated in the gaps, where the voltage pulse triggers the action potential at full voltage. So a myelinated axon transmits a nerve impulse faster, with less energy consumption, and is better protected from cross talk than an unmyelinated one.

who won? Tesla & Westinghouse vs Edison

Tesla won because AC had better high-voltage transmission efficiency and can be used with transformers. Edison thought AC was super dangerous and was a proponent for DC but in reality some high-voltage (the static electricity ones) are harmless. In reality high voltage is definitely dangerous if moving though.

drift velocity definition

The average velocity of the free charges is called the drift velocity, v_d, and it is in the direction opposite to the electric field for electrons. in metals, the free charges are free electrons. The distance that an individual electron can move between collisions with atoms or other electrons is quite small. The electron paths thus appear nearly random, like the motion of atoms in a gas. But there is an electric field in the conductor that causes the electrons to drift in the direction shown (opposite to the field, since they are negative). The drift velocity v_d is the average velocity of the free charges. Drift velocity is quite small, since there are so many free charges. If we have an estimate of the density of free electrons in a conductor, we can calculate the drift velocity for a given current. The larger the density, the lower the velocity required for a given current.

Why is it that a continuous power input is required to keep a current flowing?

The free-electron collisions transfer energy to the atoms of the conductor. The electric field E does work on electrons/charges to make 'em move. electrons. The work is transferred to the conductor's atoms, possibly increasing temperature. Thus a continuous power input is required to keep a current flowing. An exception, of course, is found in superconductor.

"can't let go" effect

The muscles that close the fingers are stronger than those that open them, so the hand closes involuntarily on the wire shocking it. This can prolong the shock indefinitely. It can also be a danger to a person trying to rescue the victim, because the rescuer's hand may close about the victim's wrist. Usually the best way to help the victim is to give the fist a hard knock/blow/jar with an insulator or to throw an insulator at the fist. Modern electric fences, used in animal enclosures, are now pulsed on and off to allow people who touch them to get free,

T or F: the resistivity of all materials depends on temperature

The resistivity of all materials depends on temperature. Some even become superconductors (zero resistivity) at very low temperatures. Increased heat leads to more R since there are more vibrations thus collisions.

terminal voltage

The voltage output of a device, measured across its terminals V = emf - Ir

Combinations of Series and Parallel

These are commonly encountered, especially when wire resistance is considered. wire resistance is in series with other resistances that are in parallel. Combinations of series and parallel can be reduced to a single equivalent resistance using the technique illustrated

T or F: voltages are created across the membranes of almost all types of animal cells

True; Such voltages (like -90mV for a resting neuron) are created across the membranes of almost all types of animal cells but are largest in nerve and muscle cells. In fact, fully 25% of the energy used by cells goes toward creating and maintaining these potentials.

T or F: collisions occur within a conductor, which leads to the conductor getting warmer.

True; the collisions happen with the free electrons within the conductor.

Equation for voltage output of a device is _______________. For a battery to be recharged by passing a current through them in the opposite direction to the current they supply to a resistance, the voltage output of the battery must be greater than the emf of the battery to reverse current through it.

V = emf - Ir

ohm's law

V=IR or the discovery by Ohm that the voltage and current are directly proportional in a metal wire. I = V/R Not universally valid, aka it can fall apart in plenty of circumstances.

voltage across the capacitor

Vc=Q/C q is charge c is capacitance

collision

When two objects collide, the kinetic energy of the colliding particles can be converted into other forms of energy, including thermal energy or heat. During a collision, the kinetic energy of the particles involved can be transformed into vibrational or rotational energy, which in turn increases the internal energy of the objects. This increase in internal energy manifests as an increase in temperature, and we perceive it as the generation of heat. It's important to note that while collisions can generate heat, heat itself is not a product of collisions. Heat is the transfer of thermal energy from a region of higher temperature to a region of lower temperature. Collisions are just one mechanism through which this transfer of thermal energy can occur. Other mechanisms include conduction, convection, and radiation.

1 OHM

Will let let 1v generate 1Amp of current

T or F: the emf is equivalent to the number of IR drops in the loop

Yes since the energy supplied by the source must be transferred into other forms by devices in the loop, which in turn each will have its own particular voltage drop. The emf is equivalent to the potential difference when the current isn't moving so i guess it makes sense cuz the potential difference is comprised of the IR drops

RC circuit

a circuit that contains both a resistor and a capacitor

simple circuit

a circuit with a single voltage source and a single resistor The wires connecting the voltage source to the resistor can be assumed to have negligible resistance, or their resistance can be included in R.

voltage sources and EMF

a device that creates a potential difference can supply current if connected to a resistance the potential difference creates an electric field that exerts force on charges, thus causing current (electromotive force EMF) EMF is NOT A FORCE at all, it is a special type of potential difference. It is the potential difference of a source when no current is flowing.

short circuit

a low resistance path between terminals of a voltage source. When the insulation on wires gets worn through, there is undesired contact between the 2 wires at very high voltage Since the resistance of the short, r, is very small, the power dissipated in the short, P=V^2/r, is very large. the materials within the surrounding materials can quickly ignite or melt when the kW is too high. One particularly insidious aspect of a short circuit is that its resistance may actually be decreased due to the increase in temperature (for example, with semi conductors and their unique characteristic that α is negative)

The Wheatstone bridge

a null measurement device for calculating resistance by balancing potential drops in a circuit

Why AC for power distribution?

a smaller # of big powerful energy supply plants is more economical (costs less money to build) than a larger number of mediocre ones it is actually more energy efficient (less power loss) to send high voltages over a distance than small voltages *it is easier to decrease/increase AC voltages than DC.* *worldwide the standard is 220-240 V but in north america it is common to use 120 V*

The reason for the decrease in output voltage for depleted and/or overloaded batteries is that all voltage sources have two fundamental parts

a source of electrical energy and an internal resistance

voltage sources have two fundamental part

a source of electrical energy and an internal resistance

sodium potassium pump is a great example of what sort of diffusive concept

active transport across a cell

silver

best conductor of all elements for electricity since its p value is lowest at 1.59×10^−8 Copper and then gold follows.

myelin

coats axons, consisting of fat-containing cells.

Electric current

concept: rate at which charge flows maths: I = ΔQ/Δt where Q is charge passing through area unit: Ampere (A) coulombs per second

Voltmeters

connected in parallel with whatever device's voltage is to be measured.A parallel connection is used because objects in parallel experience the same potential difference.

Ammeters

connected in series with whatever device's current is to be measured. A series connection is used because objects in series have the same current passing through them.

amperes

coulombs per second (charges flowing through area in wire per second)

Resistors in series

current flows through sequentially R = R1 + R2 + R3 ... Rn since current must flow each resistor device sequentially. The same current flows through each resistor in series Individual resistors in series do not get the total source voltage, but divide it

R = ρL/A, the resistivity for a cylinder

definition: resistance equation variables: ρ is the constant for a specific materials resistiveness, L is length of medium current flows through, A is the cross section size

the total resistance of a combination of resistors (component that limits the flow of charge in a circuit)

depends on both their individual values and how they are connected (Parallel? Or series).

resistors in parallel

each resistor is connected directly to the voltage source by connecting wires with negligible resistance. Each resistor thus has full voltage of the source applied to it. Since each resistor in the circuit has the same voltage, the currents flowing through the individual resistors is V/R1, V/R2, V/Rn, etc and the summation of the n resistors is the total current. Total resistance R_p of a parallel connection is related to the individual resistances by 1/R_p = 1/R1 + 1/R2 + ... 1/Rn..... this results in a total resistance R_p that is LESS THAN THE SMALLEST OF THE INDIVIDUAL RESISTANCES When resistors are connected in parallel, more current flows from the source than would flow for any of them individually, and so the total resistance is lower. Parallel resistors do not each get the total current, they divide it

E = Pt

energy transferred equation used to predict heating of a substance, energy burned, electricity used, even the radiation dose of an x-ray image is related to power * time exposed

direct current (DC)

flow of electric charge in one direction only constant voltage source, such as a battery steady state of constant voltage

voltage between the terminals fluctuates in an AC circuit

given by equation V = V_0sin2πft v_0 peak voltage f is frequency in hertz the AC current is I = I_0sin2πft I_0 is peak current

The action potential is a voltage pulse at one location on a cell membrane. How does it get transmitted along the cell membrane, and in particular down an axon, as a nerve impulse?

impulse? The answer is that the changing voltage and electric fields affect the permeability of the adjacent cell membrane, so that the same process takes place there. The adjacent membrane depolarizes, affecting the membrane further down. Thus the action potential stimulated at one location triggers a nerve impulse that moves slowly (about 1 m/s) along the cell membrane.

Factors other than current that affect the severity of a shock

its path, duration, and AC frequency.

how to get a smaller terminal voltage

larger the current, the smaller the terminal voltage smaller the internal resistance r, the smaller the terminal voltage Larger currents are inefficient as the result of heating effects and resistivity. The power dissipated as heat (P) is given by P = I^2 * R.

sinusoidally

like a sine wave

voltage drop (series)

loss of electrical power In series: voltage drop across R1 is V1 = IR1, across R2 is V=IR2... sum of these voltages equals the voltage output of the source; that is V = V1 + V2 ... Electrical potential energy in a circuit can be described by PE = qV. energy supplied by the source is qV, while that dissipated by the resistors is qV1 + qV2 + qVn V = IR_s implies that the total or equivalent series resistance R_s of n resistors is R1 + R2 + Rn

Null measurements

methods of measuring current and voltage more accurately by balancing the circuit so that no current flows through the measurement device

unmyelinated gaps

nodes of Ranvier

voltaic cell

potential difference between two different metals called electrons/terminals. Place wires between opposite electrodes. 1+ voltaic cells combined = voltages add up = creates a battery.

R=R_0(1+αΔT)

resistivity is temperature dependent R_0 is the original resistance R is the resistance after a temperature change ΔT

alternating current (AC)

sinuisoidal movement current that periodically reverses direction commercial and residential places utilize this The voltage and current are sinusoidal and are in phase for a simple resistance circuit. The frequencies and peak voltages of AC sources differ greatly.

Kirchhoff's Voltage Law (loop, 2nd)

states that the sum of the voltages around any closed loop in a circuit is zero. This law is based on the principle of conservation of energy and indicates that the total voltage rise in a loop is equal to the total voltage drop.

rub things together

static electricity can be generated by

as a capacitor charges, why does the current get progressively slower?

the build up of charges increasing on the capcitor plates

Thermometers measure

the effect of temperature on resistance. common: "thermistor" semiconductor crystal with a strong temperature dependence, the resistance of which is measured to obtain its temperature

internal resistance

the inherent resistance to the flow of current within the source itself. a 12-V truck battery will deliver more current than a 12-V motorcycle battery with similar properties but different in size (remember: cross section and size influence resistance)> The truck battery would have lower internal resistance, r. Interesting characteristics of a source's internal resistance (r): r increases as a battery is depleted. r may depend on magnitude and direction of the current through a voltage source, its temperature, and its history. The internal resistance of rechargeable nickel-cadmium cells, for example, depends on how many times and how deeply they have been depleted.

the cumulative effects of a load resistance R_load connected to a voltage source in a series circuit... with respect to the total resistance in a system

the resistance from a load PLUS the internal resistance of the voltage source contributes to the total resistance of the circuit, R_load + r THUS the current is given by Ohm's law to be I = emf / R_load + r (Smaller internal resistance will let current go through).

V=IR

the voltage drop across a resistor produced by the flow of current I (The phrase IR drop is often used).

what does the waves represent on an AC graph?

the voltage goes up and down, as does the current. the current repeatedly goes to 0 as it goes through 0, and this happens 120 times per second (for example). In AC, the voltage and current are to be in phase but their peaks are different. Voltage drives current anyway.

when using a voltmeter how can you negligibly affect the circuit

the voltmeter, which is always placed in parallel with the device being measured. Very little current flows through the voltmeter if its resistance is a few orders of magnitude greater than the device, and so the circuit is not appreciably affected. one.) If, however, the voltmeter's resistance is comparable to that of the device being measured, then the two in parallel have a smaller resistance, appreciably affecting the circuit.

α for semiconductors

their values are negative due to the unique characteristic for semi conductors to actually decrease R when temperature increases.

when there is a potential difference between point A and B the voltage gives electrons the ability

to flow (high to low voltage, + to -)

Kirchoff's Current Law (junction, 1st law)

total current entering a junction (or node) in a circuit is equal to the total current leaving that junction. In other words, the algebraic sum of currents at any junction in a circuit is zero, based on the principle of conservation of charge.

nerve conduction

transfer of impulses along a nerve by the movement of sodium and potassium ions branch of bioelectricity

T or F: whatever energy is supplied by emf must be transferred into other forms by devices in the loop Reminder: emf is the potential difference of a source when no current is flowing

true; since there are no other ways in which energy can be transferred into or out of the circuit.

thermal hazard

undesired heating effects in which electric energy is converted into thermal energy at a rate much faster than safely dissipating

where would wire resistance be relatively large?

word or long extension cord If a large current is drawn, the IR drop in the wires can be significant Scenario: when rummaging in the fridge and the motor comes on, the refrigerator light dims momentarily. (look at image)

power

work over time rate of energy use or energy conversion

α values for metals is...

α value increases with temperature for metals always Some alloys have been developed specifically to have a small temperature dependence. Manganin for example Manganin Constantan Nichrome

material resistivity

ρ

Over relatively small temperature changes (about 100ºC100ºC or less), resistivity ρ varies with temperature change ΔT as expressed in the following equation

ρ=ρ_0(1+αΔT) ρ0_ is the original resistivity and α is the temperature coefficient of resistivity