Electricity

• Current is the rate of flow of charge.
• In order to work out the number of electrons carrying a charge, you need to divide the charge by the charge of each electron (1.6x10^-19)
• Conventional current flows from the positive terminal to the negative terminal whereas electron flow shows the direction the electrons flow, negative to positive
• When you increase the potential difference, the current increases. On the other hand, increasing the resistance decreases the current
• Ohm's law is that the current flowing through a metallic conductor is proportional to the potential difference applied across it at a constant temperature
• Ohm's law applies when the component has a fixed resistance
• The potential difference is the word done (energy transferred) by each coulomb of charge moving between two points
• A circuit 'short circuit' if there is an available path with 0 resistance. The current tends towards infinity and the circuit heats up
• The shape of the I-V graph for a filament: as the current increases, the temperature of the filament increases. This increases lattice ion vibrations, which increases the number of collisions per second with electrons. Therefore, resistance increases
• The shape of the semi-conductor diode (in positive bias): as the potential difference increases, weakly bound electrons in the conductor gain energy. After the threshold pd, some electrons become free to carry a current. The lattice vibrations still increase but this is less significant
• If a semi-conductor diode is connected in reverse bias, no current flows until the breakdown voltage is reached (~50V) The diode breaks and all current flows through
• In a series circuit, P.D is sharded across the components (by resistance) and the current is constant throughout
• In a parallel circuit, P.D is the same for parallel branches and the current separates at junctions (according to branch resistance)
• Kirchoff's 1st law: at any junction in a circuit, the sum of the current flowing into the junction is equal to the sum of the current flowing away from it
• Kirchoff's 2nd law: in any complete "loop" of a circuit, the sum of the potential difference equals the source potential difference
• In order to combine series resistors in the same branch, you add them
• In order to combine resistors in parallel, you use the equation Rt = (1/R1 + 1/R2)^-1
• An advantage of placing resistors in parallel arrangements is that the total resistance is always less than the smallest resistance
• If the resistance of each branch is different, the current will not split equally
• If the resistance of components are different, they will not receive the same voltage
• A potential divider circuit is a circuit with 2 or more resistors connected in series with a power supply (usually one if a thermistor or LDR)
• For an NTC thermistor, as temperature increases, resistance decreases
• For an LDR, as light intensity increases, resistance decreases
• An advantage of setting up a rheostat as a variable resistor: the circuit is simpler, the current is constant throughout, but cannot get 0V across the bulb
• An advantage of setting up a rheostat as a potential divider: the bulb can receive full range of voltage, current through bulb can be reduced to 0A but the maximum current is lower
• Changing the dimensions of a piece of metal affects its resistance as increased length equals increased resistance and increasing cross-sectional area decreases resistance
• Metals with a greater cross-sectional area have a lower resistance as there are more paths for the electrons to propogate
• In order to calculate the potential difference across branches, you need to work out the P.D of each component, make a loop connecting the branches, then subtract the P.Ds of one branch from the other

  
• A superconductor is a material with 0 resistance at and below the critical temperature
• A material becomes superconducting at and below its critical temperature because the lattice ion vibrations reduce to 0 and so the electrons can pass through without collisions
• The advanatge of superconductors is that they transmit large currents with 0 resistance so negligible thermal energy losses. Used to create high power magnets (MRI machines) and high processing power circuits (supercomputer)
• The emf of a power source is the work done per unit charge on the whole circuit by the power source
• The terminal potential difference is the potential difference across the terminals when a current is flowing through
• The lost voltage in a circuit is the potential difference used up by pushing a current through the battery
• When working with a circuit involving internal resistance, treat the internal resistance as another resistor in series with the components. Then solve as a regular circuit (using ohm's law, kirchoff's laws, P=IV etc)