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Physics Electricity1
Physics Autumn Y12
| Question | Answer |
|---|---|
| What is the definition of current? | Rate of flow of charge (based on equation I = Q/t) |
| What carries the charge in metals? | Electrons |
| What can carry the charge in fluids? | Ions |
| Diode how to draw | Circle isn't necessary, but if it's there, line must extend to the diode. Make sure it points in the direction of conventional current (+ to -) |
| Charge of an electron (in data booklet, but will know by the end) | -1.6 x 10^-19 C (make sure you write -19 not 19) The electric charge on all particles is quantised (discrete) and a multiple of elementary charge (-e) |
| What is the base unit for energy and why? | E = 0.5 m v^2 (0.5 not a unit so ignored) J = kg m^2 s^-2 |
| What is the base unit for force and why? | F = ma N = kg m s^-2 |
| When you're defining something in terms of SI base units | Remember to combine indices of repeated SI units - check |
| What does 1 Joule equal in relation to force and displacement? | 1J = 1Nm because Work = Force x Displacement |
| K________'s first law spelling, definition, and usage | Kirchhoff's First Law The sum of the currents entering any point in a circuit is equal to the sum of the currents leaving at the same point Most useful for parallel circuits where the circuit branches |
| What is Kirchhoff's First Law an expression of? | The 'Conservation of Charge' |
| Electron drift background explanation | Electrons have very high speed, colliding with metal ions in the wire constantly even when p.d. is 0. When p.d. is on, they still do this, but gradually drift |
| Drift velocity definition | The average displacement per unit time (don't repeat the 'velocity' in question) of electrons along the wire If asked to describe, more detail: move slowly through metal lattice, colliding constantly with ions |
| What is n? | The 'number density of free electrons' - very specific for each metal - amount of electrons per m3 |
| What is the electric charge of all particles a multiple of? | The elementary charge (-e). Electric charge on all particles is quantised (discrete values - only integer multiples of -e) |
| When working out current flow direction in a complex circuit with multiple supplies | Don't try to work out the direction from each supply. Increasing current hugely on one supply will flip relative current directions. |
| How to demonstrate Kirchhoff's first law on complex circuit | Choose point. Wire up ammeters on all wires leading to it, with the same polarity facing the ammeter for each. 'Determine value and direction of current'. Current in = current out, so ammeters always sum 0. Repeat with several values for power supplies |
| In experiment, current of 3.5A being passed through electrolyte in container with X dimensions. Contains equal numbers of both ions which have +2 and -2 electron charge units. 6 x 10^26 of each in solution. Work out mean speed | Remember + and - 2, not 1. Remember that there are 6x10^26 of EACH |
| Which have a greater number density of charge carriers: conductors or semi-conductors? | Conductors have MUCH higher number density than semiconductors |
| If A is proportional to B^2, are A and B directly proportional? | A is directly proportional to B^2, not B |
| P.d./EMF equation | V = W/Q |
| What is the symbol for EMF? | E but curly and overlapping center (ε) |
| What is the EMF of a supply? | The *energy gained per unit charge* by charges passing through the supply, when a form of *energy is transferred to electrical energy* carried by the charges (electrons gaining energy) |
| What is potential difference across a component | The *energy transferred per unit charge by the charges* passing through component (electrons losing energy) |
| Calculate velocity of an electron accelerated through a p.d. of 12V (we know electron mass) | EMF = W/Q EQ = W 12 x e = 0.5 x (electron mass) x v2 |
| What is resistance? | The ratio of pd across component to current through it |
| What is an IV characteristic? | A plot of (conventional) current on y axis and p.d. on x axis. It tells us how current changes with p.d. |
| Can you use gradient to find resistance on IV characteristic? | NO. Cannot use inverse of gradient. Must do basic way. |
| Why filament bulb have that IV curve? | For a filament bulb, as p.d. increases, temp increases. This increases the Ke of the lattice ions, which vibrate with greater amplitude. This increases the frequency of collisions with electrons and therefore the resistance. |
| What is p.d. | Energy per unit charge |
| Why diode have that IV curve? | For a semiconductor diode, the resistance is infinite in the reverse direction but in the forward direction resistance decreases as p.d. increases. This is because semiconductors release more charge carriers as more energy transferred |
| What is a thermistor and how does it work? | Circuit component whose resistance decreases as temp increases. Semi-conductor. As the temp rises, more electrons are released which causes the number density to increase. |
| What is an LDR and how does it work? | Works in exactly the same way - resistance decreases as light intensity increases because extra electrons released (by photons colliding with them) |
| What is resistivity? | Fundamental property (like density) of a material. Allows us to compare resistance of different materials without a dependence on size or shape. |
| e.g. 100kW generator at 500V. power lost in 800m cables is 6% of power produced. What is the diameter? Resistivity of copper = 1.6 x 10^-8 | Calculate current as normal (doesn't change due to power loss). When working out R (which you have seen the need for via the resistivity equation), must use P = (I^2)R. Power here is 94kW |
| Why does power loss use P = I^2 x R | Power loss always uses (I^2)R because V would refer to the energy lost per unit charge, which we're never given |
| When we cost for power, what units? | kW for power and hours for time. To find cost, multiply energy in kWh by cost per kWh |
| Diode graph (positive x only). Describe significant features of graph in terms of V, I and R. Make clear how features of graph relate to action of LED | Data. No current until 1.5V. *no light from LED*. After 1.5V the resistance decreases and current increases. Current not proportional to V. Not an ohmic component. After 1.5V brightness of LED increases. |
| How to calculate % error | (True - measured)/True x 100 |
| Describe why resistivity of copper lower than graphite | Much higher number density - must say MUCH |
| Semiconductor why lower resistance when temp increases | More free electrons released, so number density increases, increasing conductivity and reducing resistance |
| State and explain whether heater obeys ohms law | State the law (generally a good idea). Since temperature is not constant, Ohm's law doesn't apply You can change the temperature of an Ohmic component, but it's still ohmic although Ohm's law doesn't apply (heater is probably ohmic but heats up) |
| Ohms law | V proportional to I at constant temp |
| Electron gun used to accelerate electrons from rest through voltage V. Electrons emerge with speed u. Voltage in gun halved to V/2. At what speed do electrons emerge in terms of u? | u/sqrt2 |
| Circuit with fixed resistors in one parallel and thermistor in other. When temp of thermistor decreases, what happens to current of whole circuit and non-thrmistor branch | Stays the same current in parallel branch - resistors constant R. Overall circuit decreases current. |
| Define resistivity | Resistivity is a property that describes the extent to which a material opposes the flow of electric current through it. |
| For proportionality questions | Look for all the variables that are the same so you can cancel stuff/ e.g. same metal is same number density and resistivity. Parallel means same V. Look for mentions in Q to e.g. drift velocity. Find necessary equations and try to cancel stuff |
| When trying to improve resolution of instrument | Don't just say digital - say higher res too |
| How to measure A | Measure *current* WITH AMMETER |
| What is 1 amp? | 1 coulomb per second |
| What does a higher resolution of an instrument give? | Higher accuracy of the data |
| Possible exam question to do | Properly working out area vs length resistance of wire proportionality |
| If I and V mentioned | Probably have to mention R |
| When identifying component | Draw IV curve. Can also mention why others excluded. |
| Diode IV curve | Resistance doesn't stay constant after curvy part of diode - I is increasing faster than V (why gradient isn't enough). When justifying using infinite resistance, show calculating it: 2/0 = infinity Remember stays on 0V for a bit |
| Multiple parts of question | Must reference data from previous parts |
| 6 markers | Say how to draw the graph. Rearrange into y = mx + c and show it. Then say how you use m or c to find other thing: generally not happy just to sub in data. NEED ANALYSIS |
| E.g. 6 marker analysis | Determine R from R = V/I Graph of R against theta is a straight line Say how to get correct interpretation of gradient m to find k (rearranging equation to show what to do with m) Can mention extrapolation |
| Accuracy and precision of data | Not instruments |
| Water bath | Always mention stirring |
| Why ionic solutions can transfer current | Cations will be attracted to the cathode, and the anions will be attracted to the anode. This produces an electrical current. |
| When the value of n is lower | the electrons must travel faster to carry the same current. |
| (probably not needed) How to calc I = AneV | I = dQ/dt = neV/dt = Anev |
| Why something would be considered non-ohmic | Electrons moving through them vibrate metal ions and so it heats up, more freq collisions with electrons, higher resistance Resistors made so constant even with environ changes |
| Resistivity | Varies with temperature |
| Finding internal resistance | Ammeter and voltmeter on cell. Variable resistor changed, and terminal p.d. for each current recorded. Negative gradient of IV graph shows internal r, and point where current = 0 shows emf. |
| Potential divider equation | Vout = R2/(R1 + R2) * Vin Vin is emf of circuit Vout is measured across R2 |
| Check if resistor actually in parallel, or is instead in same branch as the power supply | That means its in series so total R of circuit, as well as split of p.d.s is different |
| 'Dry cell' | Normal cell |
| Circuit diagram for finding internal r | Draw the resistor with a box |
Created by:
Pyrogearos2