p2 - electricity

179 - current and circuit symbols

• current is essentially a measure of speed!

• its the rate of charge in a circuit, how fast charged particles (electrons and ions) travel through a circuit

• current is measured in A (amps)

• an ampmeter is used to measure current - it must be placed in series with whatever you’re investigating

• mA to A = divide by 1000

• in a series circuit current is the same everywhere, as it can only travel one way, so can’t split up (be faster in some parts than others)

• in a paralell circuit current is shared between branches (it will split off - e.g if the first Ammeter shows 3A, on one branch it may be 2a and on the other it may be 1A

• how fast the current flows depends on resistance, which slows down the flow (measured in ohms)

• current can only flow if potential difference is supplied

• potential difference also means voltage, it’s the driving force that makes the current travel (measured in volts/V)

• voltmeters measure voltage - they must be placed paralel (over the top in one of those seperate square thingies) with whatever you’re investigating

• charge flow can be measured using the equation - Q = I x T

• Q - charge flow (coulombs/c) , I = charge (amps/A) x T = time (seconds/s)

180 - resistance and V = I x R

• V = I x R is also called ohms law

• resistance is measured in ohms

• when a resistors are in a series circuit, you can add together their resistance to get the total resistance

• if you have 2 resistors in paralell circuit on the same branch, the sum of their resistance will be less than the total of the smallest resistor

• resistance slows current down, but in a series circuit, the current has more than one direction to go in, this increases the total current that can flow around a circuit

• when current increases, resistance decreases 9it slows the flow less, because there’s too much flow!)

• overall, putting resistors in series increases the total resistance of the circuit (and decreases the flow) and putting resistors in paralell decreases the total resistance, as the current can flow in more directions, the more you put in, the more resistance will decrease whereas the more you put in series, the more the resistance will increase

• use the equation V = I x R to calculate voltage/ potential difference, current or resistance

• method to test the factors affecting resistance (length of wire)

• 1 - attach a crocodile clip to the wire at 0cm then attach another at 10cm, measure the length between them with a ruler

• 2 - close the switch and measure the current and p.d across it (remember to keep the voltmeter in paralell, seperate from other bits of equipment and the ameter in series)

• 3 - open the switch and move the crocodile clip another 10cm away, measure the new length, p.d and current

• 4 - repeat this for a number of different lengths of wire, use your measurements to calculate resistance for each length of wire using V = I x R

• 5 - draw a graph, plotting length of wire and resistance against each other, draw a line of best fit to show the relationship - it should be a straight line through the origin, showing a directly proportinal relationship, if not it could be a systematic error if the first clip isnt attatched at exactly 0cm

• overall, as length of the wire increases, so does resistance

• this is because by increasing the length of the wire, there is a longer path for the current (electrons) to travel along, leading to more collisions between the electrons and the wires atoms, slowing down the current

• the wire can also get hot, temperature also effects resistance

• as temperature increases, so does resistance, this is because the metal atoms in the wire vibrate more, making it harder for electrons to travel along the wire, causing resistance and slowing the current

181 - resistance and ohmic conductors

• At constant temperature, the current through an ohmic conductor like a wire is directly proportional to the potential difference across it, because resistance stays the same. If temperature increases, resistance also increases, which changes the current and breaks that direct proportionality

• overall, inc in temp causes inc in resistance, this is why ohmic conductors are shown at a constant temp

• the resistance of an ohmic conductor such as a wire is not affected by current, so long as it stays at a constant temp!!!, the current is dp to the p.d across from it

• in some ohmic conductors, the current does affect resistance - in diodes and filament lamps

• filament lamps - current affects resistance becauase the lamp gets HOTTER!

• as the charge flows through the filament lamp, it transfers some energy to its thermal energy store, making it heat up - resistance increases with the temp, so as the current increases, the lamp heats up more, increasing the resistance

• diodes - only lets current flow in one direction, if the current tries to flow in the opposite direction, the current is met with lots of resistance and slowed down

182 - circuit devices

• LDR is short for light dependant resistor - it depends on light intenisty

• when light intensity is low (night) - resistance increases

• when light intensity is high (bright) - resistance decreases

• they have lots of applications, such as automatic night lights, outdoor lighting and burglar detectors

• the resistance of a thermistor depends on temperature

• in hot conditions, resistance drops

• in cold conditions, resistance increases

• they can be used to detect temperature - car engine temp detectors and electronic thermostats

• they can be used in sensing circuits to increase the power or turn the circuit off/on - this depends on the conditions they’re in

• for example : a thermistor can be used in the sensing circuit of a fan

• the potential difference across the fan component and the fixed resistor will be equal, as components in paralell have the same p.d throughout

• the p.d of the power supply is shared out between the thermistor and loop with the fixed resistor and fan according to their resistances, the bigger a components resistance, the more p.d it takes

• when the temp increases, the thermistor’s resistance will decrease, meaning more potential difference travels across the fixed resistor and the fan component (as it has a bigger resistance now so can take more p.d)

• as the p.d increases, the fan spins faster, which is good because the room is hotter!

183 - series circuits

• if you remove one thing from a series circuit, the whole thing will be broken, making them less useful than paralell circuits

• everything is connected in a line, end to end from the + to the - side of the cell, except from voltmeters, that are always in paralell

• potential difference is shared - the p.d coming from the cell/power supply is shared evenly between the components in the circuit, meaning their p.ds will equal the source p.d when added together

• current is the same everywhere - the same current flows in each component, the size of the current is determines by the total p.d of the cells and the total resistance of the circuit, hence the equation: I = V / R (current = p.d / resistance)

• resistance adds up - adding a resistor in series with another resistor means they have to share the p.d, this means the p.d across each resistor is lower and since the current is the same everywhere, the current is decreased and resistance is increased (as resistance slows down current)

• cells potential difference adds up - there is a higher p.d when cells are connected in series, their p.d adds up, because by adding another cell, you are adding more p.d to the circuit

184 - parallel circuits

• in parallel circuits, components can be connected in series or in branches, apart from ameters, which are always connected in series

• if you take something out of the circuit, it will hardly effect it, making them more useful

• potential difference is the same across all components - all components get the same share, this means bulbs connected in series will be at the same brightness

• current is shared between branches - current can flow in more directions in a parallel circuit than a series, so total current is decreased - if 2 indentical components are connected in series on the same branch, they’ll have the same current flowing through them

• resistance decreases when another is added in paralell - the total resistance will be always be lower than the smallest resistance in one resistor

185 - investigating resistance

• method to investigate how adding resistors in series effects total resistance of the circuit

• 1) get 4 identical resistors, set up a circuit with one resistor, an ammeter and a cell, measure the potential of the cell with a voltmeter to use in the equation - R = V / I later on

• 2) measure the current flowing through the circuit and use the equation R = V / I to find out the resistance of one resistor

• 3) add another resistor in series with the first, and repeat the earlier steps, do this until you’ve added all your resistors and calculated the resistance each time

• 4) plot a graph, number of identical resistors on the x axis and total resistance on the y axis

• 5) it should be a straight line through 0,0, a directly proportional relationship

• method to investigate how adding resistors in parallel effects the total resistance of the circuit

• 1) use the same equipment as before to ensure a fair test, build the same initial circuit with one resistor, measure the p.d in the cell and measure the current with a ammeter, use the equation R = V / I again to calculate the resistance of the first resistor

• 2) add another resistor in parallel to the first, measure the total current again and calculate the resistance the 2 resistors using R = I / V

• 3) continue this until you’ve used all your resistors, draw the graph number of identical resistors against total resistance of the circuit

• 4) the more resistors you add, the more the current decreases, the graph should show an inversely proportional relationship

186 - electricity in the home

• there are two types of current, ac (alternating current) and dc (direct current)

• ac is used in mains electricity (national grid) and dc is used in batteries

• ac is constantly changing direction - they’re produced by alternating voltages in which the positive and negative ends keep alternating (they keep switching from positive to negative)

• the mains has a supply of 230 volts, it's frequency is 50 cycles per second/Hz

• batteries supply dc current, current that only flows in one direction

• most electrical appliances are connected to the mains supply by 3 core cables, made up a copper core and plastic coating

• live wire - brown - has the alternating potential difference (230V) flowing through it

• earth wire - green and yellow - protects the wiring and for safety, stops the appliance from becomming live, has no potential difference, usually 0V unless there is a fault

• neutral wire - blue

• the live wire can give you an electrical shock, your body carries no potential difference (0V) like the earth wire, so the current from the live wire will pass through your body, which can kill you - even if the plug or switch is turned off, the switch is still open, there is still danger of a shock, as the live wire still carries the potential difference (230V)

• any connection between earth and live is dangerous, as it provides a low resistance link, so a huge current will flow, which could also result in a fire

187 - power of electrical appliances

• when charge moves, energy is transferred

• for example - a kettle transfers energy electrically from the mains ac supply to its thermal store of the heating element inside the kettle

• a fan transfers energy electrically from the battery’s dc supply to the kinetic energy store of it’s motor, which allows it to move

• the higher the current, the more energy is transferred to the thermal store of the appliance and the surroundings

• power is the amount of energy transferred over time (P = E / T)

• you can calculate energy transferred by an appliance with the equation E = P x T

• energy transferred is measured in joules / J

• power is measured in Watts / W

• an appliance that is more powerful than another doesn’t mean it transfers more energy usefully, the amount of energy an appliance transfers usefully is called efficiency

• you can calculate effciency using - useful output energy transfer / total input energy transfers

• you can calculate efficiency using power - useful power output / total power input

• appliances will have a power rating - this tells you the maximum amount of energy transferred between stores per second when the appliance is in use

• a lower power rating means the appliance is cheaper to run, as it transfers less energy per second, meaning it doesn’t require loads of energy

• the power of an appliance can be found using - P = V x I - power, voltage, current

• if you don’t know the potential difference, you can use - P = I squared x R - power, current squared and resistance

188 - more on power

• electrical charge is the property of particles like protons (positive charge) and electrons (negative charge)

• electrons are used in circuits, they carry the change, the change is the energy transferred

• when an electrical charge goes through a change in potential difference, energy is transferred

• for example : the electrons (the charge) will be supplied with energy from the cell/battery - this ‘raises it through the potential’

• to raise the charge through the potential means to bring the increase the energy level of the potential difference, low to high - this is useful as it means the charge has more energy to transfer to the components!

• to calculate energy transferred in the circuit - E = Q x V

• E = energy transfered (J), Q = charge flow ( c ) and V = voltage/potential difference (V)

189 -the national grid

• supplies electricity to the whole of the UK, from power stations to consumers

• power stations usually operate below their maximum power output, so that there’s spare capacity to deal with increased demand or if something shuts down

• may small back up power stations can be used if this happens

• you can predict when demand will be highest - on dark and cold days, in the morning when people wake up to get ready for school/work and when people come back from school/work - if there’s an important tv showing (like sports) demand will increase

• you can make something more powerful by giving it a high current or a high p.d

• power stations are efficient, they have a high voltage/p.d of 400,000V but a small current, this means that less energy is lost as thermal energy when the wires heat up

• high current flows quicker, transferring energy to the metal atoms in the wire, causing it to heat up so energy is lost

• it’s also cheaper, as less energy is lost

• for a given power (400,000V) increasing the p.d, decreases the current - this is more effiicient

• transformers are used to increase and decrease p.d

• they have 2 coils with an iron core between them - the primary and secondary coil

• step up transformers increase potential difference and decrease current - they have more turns on the secondary coil

• step down transformers decrease potential difference and increase current - they have more turns on the primary coil

• transformers are nearly 100% efficient, so power in the primary coil = power in the secondary coil