Module 4 Flashcards
What is current?
The rate of flow of charge.
What is current measured in?
Amperes
What is an ampere?
One coulomb of charge passing a given point per second.
What is a coulomb?
The electrical charge flowing past a point in one second when there is an electrical current of one ampere.
What is the elementary charge?
The charge on one proton. It is equal to 1.6x10^-19 C
How do you calculate the net charge on an object?
Q=+-ne, where Q is the charge in coulombs, n is the number of electrons added or removed, and e is the elementary charge.
What can result in a larger current in a metal wire?
- a greater number of electrons moving past a given point each second (a wire with a greater CSA)
- the same number of electrons moving faster through the metal.
What is the direction of conventional current?
From the positive terminal to the negative terminal.
What are electrolytes?
Liquids that can carry an electrical current.
What ions are attracted towards the cathode?
Cations (positive ions).
What ions are attracted towards the anode?
Anions (negative ions).
How do you place an ammeter in a circuit?
In series at the point where you want to measure the current.
What is the conservation of charge?
Electrical charge can neither be created nor destroyed. The total amount of electrical charge in the universe is constant.
What is Kirchhoff’s first law?
For any point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point.
What is Kirchhoff’s first law based on?
Conservation of charge.
What is number density?
The number of free electrons per cubic metre of material. The higher the number density, the better the electrical conductor.
How fast do charge carriers move?
Most move slowly. Free electrons repeatedly collide with positive metal ions as they drift through the wire towards the positive terminal. The reason that lights turn on so quickly is that all the free electrons in the wire start moving almost at once.
How should you draw circuit diagrams?
- Only use correct circuit symbols
- Do not leave any gaps in between the wires
- When possible use straight lines drawn with a pencil and ruler
What is a battery?
Two or more cells connected end-to-end, or in parallel.
How do you tell which end of a cell is the positive terminal?
The longer terminal on a circuit diagram.
What is the circuit symbol for a diode?
A triangle with a vertical line at the point. The wire can be seen through the triangle.
What is the circuit symbol for a resistor/variable resistor?
A rectangle which does not have the wire running through it. A variable resistor has an arrow pointing up and to the right through the rectangle.
What is the circuit symbol for a fuse?
A rectangle which has the wire running through it.
What is the circuit symbol for a thermistor?
A resistor with a “hockey stick” through it.
What is the circuit symbol for an LDR?
A resistor with two arrows pointing towards it.
What is the circuit symbol for an LED?
A diode with two arrows pointing away from it.
What is the circuit symbol for a capacitor?
Two parallel lines that break the wire.
What is potential difference?
The transfer of energy by charge carriers.
What is a volt?
The p.d. across a component when 1 J of energy is transferred per unit charge passing through the component.
How do you place a voltmeter to measure the p.d. of a component?
In parallel. An ideal voltmeter should have infinite resistance so that when connected, no current passes through the voltmeter itself.
What is electromotive force?
Work done on the charge carriers. It is the energy transferred from chemical energy to electrical energy per unit charge.
How do electron guns work?
- small metal filament is heated by an electrical current
- the electrons in this piece of wire gain kinetic energy
- some of them gain enough kinetic energy to escape the surface of the metal
- if the heated filament is placed in a vacuum and a high p.d. is applied between the filament and an anode, the filament acts as a cathode, and the freed electrons accelerate towards the anode
- if the anode has a small hole in it, then electrons in line with this hole can pass through it, creating a beam of electrons with a specific kinetic energy.
How can you calculate the speed of electrons fired from an electron gun?
eV=1/2mv^2. This assumes the electrons have negligible kinetic energy at the cathode.
How do you calculate the resistance of a component?
R = p.d. across component/current in component
What is an Ohm?
The resistance of a component when a p.d. of 1 V is produced per ampere of current.
What is Ohm’s law?
For a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the p.d. across its ends.
Why does resistance increase with temperature?
When the temperature of the wire increases the positive ions inside the wire have more internal energy and vibrate with greater amplitude about their mean positions. The frequency of collisions between the charge carriers and the positive ions increases, and so the charge carriers do more work, transferring more energy as they travel through the wire.
What are the I-V characteristics for resistors?
- the potential difference across the resistor is directly proportional to the current in the resistor. As a result: a resistor obeys Ohm’s law, so is an ohmic conductor, the resistance of the resistor is constant.
- the resistor behaves the same way regardless of polarity.
What are the I-V characteristics for filament lamps?
- the potential difference across a filament lamp is not directly proportional to the current through the resistor. This means that: a filament lamp does not obey Ohm’s law making it a non-ohmic component, the resistance of a filament lamp is not constant.
- the filament lamp behaves in the same way regardless of polarity.
What are the I-V characteristics for diodes?
- the potential difference across a diode is not directly proportional to the current through it. This means: the diode does not obey Ohm’s law making it a non-ohmic component, the resistance of a diode is not constant
- the diode’s behaviour depends on the polarity.
What is the threshold p.d. for a diode?
The value for which the resistance drops sharply and above this point the diode has very little resistance. Different LEDs have different values for their threshold p.d.s related to the colour of light they emit.
What factors effect resistance?
- the material of the wire
- the length of the wire
- the cross-sectional area of the wire
What is resistivity?
The resistivity of a material at a given temperature is the product of the resistance of a component made of the material and its cross-sectional area, divided by its length. As the material gets hotter, the resistivity increases.
How is resistance related to length?
Resistance is directly proportional to length.
How is resistance related to cross-sectional area?
Resistance is inversely proportional to cross-sectional area.
What is the unit for resistivity?
Ohm-metre
What is a negative temperature coefficient?
The resistance decreases as the temperature increases. This is because in some semiconductors, as the temperature increases the number density also increases.
What are thermistors?
An electrical component made from a semiconductor with a negative temperature coefficient. As the temperature increases, its resistance drops.
What are the uses of thermistors?
- thermometers
- thermostats to control heating and AC units
- monitor the temperature of components inside electrical devices so they can power down before overheating
- monitor engine temperatures
How can you investigate how the resistance of a thermistor changes with temperature?
Use a water bath to control the temperature of a thermistor and an ohmmeter to record the resistance of the thermistor.
What are the I-V characteristics for thermistors?
- non ohmic
- like a filament lamp, but it curves the other way
- looks like a tan graph
What are LDRs made from?
Semiconductors in which the number density changes depending on the intensity oof incident light. When light shines on the LDR, the number density increases dramatically, leading to a rapid decreases in the resistance of the component.
What are the equations for electrical power?
P=IV
P=I^2R
P=V^2/R
What is the kilowatt-hour?
The energy transferred by a device with a power of 1 kW operating for a time of 1 hour.
What is Kirchhoff’s second law?
In any circuit, the sum of the electromotive forces is equal to the sum of the p.d.s around a closed loop.
What is Kirchhoff’s second law based on?
Conservation of energy.
How do you add resistors in series?
R=R1+R2+R3+…
How do you add resistors in parallel?
1/R=1/R1+1/R2+1/R3+…
What are lost volts?
The difference between the e.m.f. and the terminal p.d.
What is the relationship between the terminal p.d. and the e.m.f.
electromotive force = terminal p.d. + lost volts
epsilon = V+Ir
epsilon = IR+Ir
epsilon = I(R+r)
What are potential divider circuits?
Circuits that can vary the p.d. across an output when connected to a fixed input.
For a potential divider circuit, the current is the same in each resistor, meaning V is directly proportional to R. This means the ratio of p.d.s is the same as the ratio of resistances.
How do you produce a varying V out in a potential divider circuit?
Replace one of the fixed resistors with a variable resistor.
What is a progressive wave?
An oscillation that travels through matter or a vacuum. They transfer energy from one place to another, but not matter.
What is a transverse wave?
The oscillations are perpendicular to the direction of energy transfer. They have peaks and troughs where the oscillating particles are at maximum displacement from their equilibrium position.
What are some examples of transverse waves?
- waves on the surface of water
- electromagnetic waves
- waves on stretched strings
- S-waves produced in earthquakes
What are longitudinal waves?
The oscillations are parallel to the direction of energy transfer. When they travel through a medium they create a series of compressions and rarefactions.
What are some examples of longitudinal waves?
- sound waves
- P-waves produced in earthquakes
What is the displacement of a wave?
The distance from the equilibrium position in a particular direction.
What is the amplitude of a wave?
The maximum displacement from the equilibrium position.
What is wavelength?
The minimum distance between two points in phase on adjacent waves.
What is the period of oscillation?
The time taken for one oscillation or time taken for a wave to move one whole wavelength past a given point.
What is the frequency of a wave?
The number of wavelengths passing a given point per unit time.
What is wave speed?
The distance travelled by the wave per unit time.
What is the wave equation?
v=f(lambda)
What is phase difference?
The difference between the displacements of particles along a wave, or the difference between the displacements of particles on different waves.
What is reflection?
When a wave changes direction at a boundary between two different media, remaining in the original medium.
What is the law of reflection?
The angle of incidence is equal to the angle of reflection.
What is refraction?
A wave changes direction as it changes speed when it passes from one medium to another.
If a wave slows down, how will it refract?
Towards the normal. Sound waves normally speed up when they enter a denser medium, whereas electromagnetic waves usually slow down.
Does reflection change the frequency or wavelength?
No.
Does refraction change the frequency or wavelength?
The frequency does not change, but the wavelength does. If a wave slows down, its wavelength must decrease to keep a constant frequency.
What is diffraction?
When waves pass through a gap or travel around an obstacle, they spread out.
Does wave speed, frequency, and wavelength change when a wave is diffracted?
No.
When will the most diffraction occur?
When the size of gap or obstacle is about the same as the wavelength of the wave.
What is polarisation?
The particles oscillate along one direction only, which means the wave is confined to a single plane. The wave is said to be plane polarised.
Why can longitudinal waves never be plane polarised?
The oscillations are always parallel to the direction of energy transfer, so they are already limited to one plane.
What does partially polarised mean?
There are more waves oscillating in one particular plane, but the wave is not completely plane polarised. For example, light reflected off the surface of water becomes mostly horizontally polarised.
What is the intensity of a progressive wave?
The radiant power passing through a surface per unit area.
I=P/A
What is the relationship between intensity and distance?
Inverse square.
I=P/(4 pi r^2)
What is the relationship between intensity and amplitude?
Intensity is directly proportional to amplitude squared.
What are electromagnetic waves?
Electric and magnetic fields oscillating at right angles to each other.
What are the wavelengths of the EM spectrum?
Radio waves, Microwaves, Infrared, Visible, Ultraviolet, X-rays, Gamma rays
10^-1,10^-3, 7x10^-7, 4x10^-7, 10^-8, 10^-10/10^-13
X-rays are emitted by fast moving electrons, whereas Gamma rays come from unstable atomic nuclei.
How do you calculate refractive index?
n=c/v
where n is the refractive index, c is the speed of light in a vacuum, and v is the speed of light in the material.
What are the two conditions required for total internal reflection?
- the light must be travelling through a medium with a higher refractive index as it strikes the boundary with a medium with a lower refractive index
- the angle at which the light strikes the boundary must be above the critical angle.
What is the principle of superposition?
When two waves meet at a point the resultant displacement at that point is equal to the sum of the displacements of the individual waves.
What is coherence?
Waves emitted from two sources that have a constant phase difference.
What is path difference?
The difference in the distance travelled by waves.
What path difference do waves in phase have?
n lambda where n is an integer.
This results in constructive interference at this point.
What path difference do waves in antiphase have?
(n+1/2) lambda where n is an integer.
This results in destructive interference at this point.
What is the path and phase difference at the central maxima?
Zero
What did the Young double slit experiment demonstrate?
The wave nature of light.
How are stationary waves formed?
Two progressive waves with the same frequency travelling in opposite directions are superposed.
What is a node?
A point on a stationary wave where the displacement is always zero.
What is an antinode?
The point of greatest amplitude on a stationary wave.
What part of a stationary wave has all points in phase?
Between adjacent nodes, all points reach their maximum positive amplitude at the same time.
What part of a stationary wave has all points in antiphase?
On different sides of a node, the particles on one side of the node reach their maximum displacement as the other side reaches minimum.
What is the fundamental frequency?
The minimum frequency of a stationary wave. It occurs when the string is in its fundamental mode of vibration.
What is the relationship between f0 and the harmonic?
The harmonic has a frequency equal to the corresponding multiple of f0 (second harmonic has frequency 2f0).
Where are the nodes and antinodes in a tube closed at one end?
Must be a node at the closed end and an antinode at the open end. This makes the even harmonics impossible.
Where are the nodes and antinodes in open tubes?
Must have an antinode at each end in order to form a stationary wave. All harmonics are possible.
What are photons?
A quantum of electromagnetic energy.
What is an electronvolt?
The energy transferred to or from an electron when it moves through a potential difference of 1 V.
How can you determine the plank constant using LEDs?
At the LEDs threshold p.d., the work done is the same as the energy of the emitted photon. Use a voltmeter to determine the p.d. required to turn on the LED. Therefore, threshold p.d. x charge on electron =hc/lambda
The wavelength of light emitted by the LED will be known so you can find the frequency, and then plot a graph V against 1/lambda/
What is the photoelectric effect?
Electrons (photoelectrons) are emitted from the surface of a metal upon the absorption of electromagnetic radiation.
How can you demonstrate the photoelectric effect?
Using a gold leaf electroscope, touch the top plate with the negative electrode from a high voltage power supply. Excess electrons are deposited onto the plate and stem of the electroscope. As the stem and gold leaf have the same charge, they repel each other, causing the gold leaf to lift away. If zinc is placed on top of a negatively charged electroscope and UV radiation shines on the zinc surface, then the gold leaf falls back towards the stem.
What were the three key observations from the photoelectric effect?
- photoelectrons were emitted only if the incident radiation was above a certain frequency, called the threshold frequency for each metal. This was not effected by the intensity of incident radiation.
- if the incident radiation was above the threshold frequency, emission of photoelectrons was instantaneous.
- if the incident radiation was above the threshold frequency, increasing the intensity of the radiation did not increase the maximum kinetic energy of the photoelectrons. Instead more electrons were emitted.
How does the photoelectric effect provide evidence for the particulate nature of light?
- there is no time delay in releasing electrons, as the electrons cannot accumulate energy from multiple photons. One to one interactions are possible between photons and electrons?
- increasing the intensity of the radiation means more photons hit the metal surface, as each photon has a one to one interaction, more photons are emitted.
What is the work function?
The minimum energy required to free an electron from the surface of a metal.
What is the threshold frequency?
The minimum frequency of light needed to cause electrons to be emitted.
What is Einstein’s photoelectric effect equation?
Energy of a single photon = work function + KE max
All energies should be consistently in Joules or electronvolts.
What is wave-particle duality?
A model used to describe how all matter has both wave and particle properties.
How can you make an electron diffract?
If an electron gun fires electron at a thin piece of polycrystalline graphite, the electrons pass between the individual carbon atoms in the graphite. The gap between the atoms is so small that it is similar to the de Broglie wavelength of the electrons, so the electrons diffract and form a diffraction pattern on the end of the tube.
What is the de Broglie equation?
wavelength = plank constant/momentum
