Flashcards
Electrons
Flashcards do conjunto (65)
- 1What are the three things a circuit needs to work?An energy source (a source of emf which may be a battery or power pack)
A closed path (electrons will not flow in an open circuit)
Electrical components (These can transfer electricity, turn the circuit on and off etc) - 2This isn't a flash card per say just make sure you know the componants on page 152-153 of the text bookOnes you have struggled with include: terminals, variable resistor, fuse, heater, thermistor, LED, LDR
- 3Which way does conventional current flow? Which way do electrons flow?Positive to negative
Negative to positive - 4What are the two forms of charge? What is the overall charge of electrons?Positive and Negative
Neutral! - 5How can atoms become charged?Lose or gain electrons to become positive and negative ions
- 6What is the flow of charge made up in metals and how does it work?Electrons
Metals are made up of tightly packed rows ions in a crystal lattice structure with lots of delocalized electrons that are free to flow - 7What is an electrolyte and how do they conduct electricity?Solutions that conduct an electric current are called electrolytes. They are formed when ionic compounds are dissolved in water and the ions are free to flow as charge carriers.
- 8What is Kirchoff's first law?Because charge is always conserved, the amount of charge entering a node must be equal to current leaving it. Because Q=IT this must be true for current also.
- 9What is the elementry charge? What is the signifigance of this to any other charged particle?1.6x10^-19
The charge of anything will always be a multiple of this - 10What is the charge of an 'up' quark? What is the charge of a 'down' quark? What are protons composed of? What are Neutrons conposed of?+2/3e
-1/3e
Protons are mad up of two 'up' quarks and a 'down' quark
Neutrons are made up of two 'down' quarks and an 'up' quark - 11What equation relates charge and current? If we know the elementry charge and the number of particles of past a point each second and the time elapsed, what other equations can we use?Q=IT
Q=net
and therefore
I=ne - 12when there is no potential difference in a circuit, what is the movement of electrons like?completely random
- 13Why do electrons drift so slowly?The don't move very far before colliding with metal atoms
- 14What do the letters mean in I=nAve? (tip- on page 158 of the text book it says how to derive this equation, you may want to look this over before the exam)I = current
n = number density of electrons (number of free electrons per unit volume)
A = cross sectional area of the wire
v = mean drift velocity of electrons
e = elementary charge - 15Why are metals good conductors? what is the value of 'n' for many insulators? what about for semi-conductors?They have a large number density of delocalized electrons
close to zero
somewhere between conductors and insulators - 16define the emf of a supplyemf of a supply is the amount of energy gained per unit charge by charges passing through a supply, when a form of energy (normally chemical is transferred to electrical energy carried by the charges (Volts or Joules per Coulomb)
- 17Define the potential difference measured across a componantEnergy transferred per unit charge by the charges passing through a component (Volts or Joules per Coulomb)
- 18What are the equations for emf and potential difference?Emf= energy transferred / charge
P.d= work done / charge
(basically both the same just remember W=VQ) - 19When electrons are passed through a pd, kinetic energy is gained, how do we calculate this?Ke= 1/2 mv^2
where m is the mass of an electron - 20What is Ohms law?The current through a conductor is directly proportional to potential difference across it, provided that physical conditions remain constant
- 21What do you need to use to repeat Ohms experiment?Use a circuit with a variable number of cells, connected to a length of thin wire and a switch. Include an ammeter and a voltmeter. Keep the current low so the heating affect is negligible.
- 22What will a graph of an ohmic conductor look like (the IV characteristic)?directly proportional and through the origin
- 23What is the equation for resistance, current and voltage? Therefore how would you define resistance?V=IR
volts per ampere - 24What are some of the factors that affect resistance?Material of wire
Length of wire
Temperature of wire
Cross sectional area of wire - 25How does the length of a wire affect resistance?In a longer wire the resistance is higher because the electrons experience a smaller potential gradient. The voltage per meter is smaller which causes lower drift velocity and lower current. Theres also a higher chance of collisions with metal ions.
- 26How does the cross sectional area of a wire affect resistance?With a thicker wire there is no increase in potential gradient, but there is a higher volume of electrons able to flow at the same drift velocity. CSA is inversely proportional to resistance.
- 27How does the temperature of a wire affect the resistance?As the temperature increases, the vibrational kinetic energy of the metal ions increases. This means the CSA of the ions increases because they move around more around more, and it's harder for the electrons to get through. This means the likelyhood of electron-ion collisions is higher and therefore the resistance is higher.
This is true for regular conductors, but semiconductors like thermistors behave in the opposite way. - 28What is resistance of a material a measure of?It's opposition to the flow of current through it
- 29What is the I-V characteristic of an ohmic resistor?
- 30What does the I-V characteristic of a filiment lamp look like?
- 31What makes a diode different from other resistors?A diode will only allow current to flow through it in a forward direction, from positive to negative (as of conventional current). When current tries to flow through in the opposite direction, the diode can be said to have almost infinite resistance.
- 32what does the I-V characteristic of a diode look like?
- 33What are the benefits of using LEDs?Switch on instantly
Robust
Versatile
Operate with low potentials
Long working life - 34What does the graph of resistance against temperature for a thermistor look like?
- 35What does the graph of of resistance against light intensity look like for an LDR?
- 36What factors do NOT affect resistivity?The physical dimensions of the material like length and CSA
- 37What is the equation for resistivity?
- 38Why can you figure out the resistance of a material at a certain temperature if you know it's resisitivity at that temperature?The resistance of a wire is proportional to it's length and inversely proportional to it's cross sectional area.
- 39I wasn't sure how to turn this into questions, but take a look at the investigation for resistivity on page 174, I'll briefly describe it on the underside of this card.We can find resistivity by finding the current and potential for varying lengths of wire.
If we arrange the equation for resistivity to get resistance, the equation can be compared to y=mx+c
If we plot a graph of resistance against length, then we get the gradient R/l.
If we multiply the gradient by the CSA, we obtain a value for resistivity.
When measuring the diameter of a wire with a micrometer it's important to take values from various points on the wire. - 40What is resisitivity affected by? How is this relationship shown?Affected by changes in temperature.
R= resistivity of material at given temperature.
Alpha= the temperature coefficient.
T= the temperature of the material
T0= reference temperature at which the resistivity of the material is quoted
- 41Define power.The rate at which energy is transferred from one form to another (measured in watts).
If a material has power of 1 watt, it will transfer 1 Joule or energy in 1 second - 42What equations do we use to calculate the amount of energy transferred or the amount of work done?Energy or work done (J) =
Power (W) x time (s) - 43What are the three equations genrally used for electrical power?P=IV
P=I^2R
P=V^2/R - 44Define one kilowatt hour1000 watts of energy in 3600 seconds, therefore it is 3600000 Joules!
Don't panic, it's just based off the equation
Energy = power x time - 45How do you convert Joules to kWh?divide by 3600000
- 46THIS IS NOT A QUESTION BUT REMEMBER KWH IS A UNIT OF ENERGY, NOT POWERKWH (power x time) which is energy not power
- 47What is Kirchoff's first law?The sum of currents entering a node is equal to the sum of currents leaving it. (Current is conserved)
- 48What is Kirchoff's second law?In any closed loop circuit, the sum of the emf is equal to to sum of the products of current and resistance. (the conservation of energy)
emf= IR
therefore emf= V - 49What do you need to check when completeing questions on kirchoff's laws with two sources of emf?That current flowing in one direction is taken as positive and current flowing in the other direction is negative
- 50Some advice that on page 189 the worked example looks pretty important. It's a lot of words and I'm very tired and hungry so don't want to read them but look at it I recommend it.apparently you need to put something on the other side of the card
- 51What are the three factors true for a series circuit?The current has the same value at any point in the circuit
The emf is equal to the sum of the potential differences across the resistors
The total resistance is equal to the sum of the individual resistors. - 52What is the equation for total resistance in a series circuit?Rtot = R1 + R2 + R3 +.....
- 53How are ammeters and volmeters connected in a circuit and what are their resistances like?Ammeters are always connected in series and have close to zero resistance.
Voltmeters are always connected in parallel and have close to infinite resistance. - 54What is the rule about voltage for parallell circuits?Vtot = V1 = v2 = v3 =....
- 55What is the rule for current in parallel circuits?Itot = I1 + 12 + I3 ..... + In
- 56What is the rule for resistance in parallel circuits?1/Rtot = 1/R1 + 1/R2 + ... + 1/Rn
- 57Define a potential divider circuitA circuit which uses 2 resistors in series to split or divide the voltage of a supply in a chosen ratio so that a chosen voltage can be supplied to another supply.
- 58What is the potential divider equation?
- 59In a potential divider circuit how can we ensure the potential differece can be easily adjusted?Make one of the resistors a variable resistor
- 60What is the terminal Potential Difference?The potential difference measured across the terminals of a cell.
- 61What is the 'lost volts'?The difference between emf and terminal p.d. when a charge flows through the cell.
- 62Why do we get 'lost volts'?Internal resistance
- 63What is the internal resistance of a source of emf?The resistance of the materials inside the circuit (chemicals, wires, components).
- 64What are the three equations for internal resistance?emf = IR + Ir
emf = I(R+r)
emf= V + IR - 65Theres a piece on the use of simultaneous equations on page 199 I woud reccomend looking at itBasically emf and internal resistance are constant so there's a possibility to have different currents and resistances which create simultaneous equations (theres worked examples I don't really get it either)