P2: Electricity

when two electrically insulating materials are rubbed togethr, electrons are rubbed off one and deposited onto the other. 

• gains E = negative charge
• loses E = positive charge

opposite electric charges attract

like, repel

bigger distance between the objects, the weaker the force between them. 

charge flows from positive terminal on cell, around ciruit, then to negative terminal

positive terminal is the longer line, negative the shorter

(think: 'I' can make + because longer than 'i' which is only -)

DIAGRAMS:

Two things are important for a circuit to work:

• There must be a complete circuit
• There must be no short circuits

To check for a complete circuit, follow a wire coming out of the battery with your finger. You should be able to go out of the battery, through the lamp, and back to the battery.

To check for a short circuit, see if you can find a way past the lamp without going through any other component. If you can, there is a short circuit and the lamp will not light.

current through a component - AMMETER (connected in SERIES with component)

ampere, A

potential difference across a component - VOLTMETER  (placed in PARALLEL with component)

volt, V

work done = J

charge = C (coulombs)

RESISTANCE is the opposition to current flow, OHM

An electric current flows when electrons move through a conductor, such as a metal wire. The moving electrons can collide with the ions in the metal. This makes it more difficult for the current to flow, and causes resistance.

The resistance of a long wire is greater than the resistance of a short wire because electrons collide with ions more often.

The resistance of a thin wire is greater than the resistance of a thick wire because a thin wire has fewer electrons to carry the current.

current-pd graphs are used to show how the current through a compnent varies with the pd across it

= the current through a resistor at a constant temperature is directly proportional to the potential difference across it

- straight line through origin of current-pd graph if obeys.  ohmic conductor

FILAMENT BULB: line on graph is a curve - resistance increases as temp increases.  (increased current means increased resistance, as increased current makes the temperature increase - makes the lattice of ions of the filament VIBRATE more.  this leads to more collisions between the lattice ions and the charge-carrying electrons, so it is harder for the electrons to pass through > increased resistance

DIODE: current flows in one direction only through a diode.  'forward' direction = very low resistance, 'reverse' direction = very high resistance (approx infinity)

THERMISTOR: resistance decreases as heat increases

LDR: resistance decreases as light intensity increases

steeper gradient = higher resistance

series:components connected one after another - so, if a break anywhere in the circuit, charge stops flowing

• SAME current
• SHARE voltage - ADD UP to voltage of cell
• ADD UP resistances for total resistance

bigger resistance = bigger share of supply pd

parallel: each compnent is connected ACROSS the supply - if a break, currnet can still flow in the other parts

• ADD UP current to get total current
• SAME pd - same as supply
• bigger resistance of a component = smaller current flows through it

cells and batteries supply d.c current - DIRECT CURRENT that passes round the circuit in only ONE DIRECTION

mains supply has a.c current - ALTERNATING CURRENT that repeatedly reverses its direction

frequency of the mains supply = 50Hz (changes direction 50 times per second)

voltage = 230V

the live wire of mains supply alternates between a peak volatge of +325V and -325V with repsect to the NEUTRAL WIRE WHICH REMAINS AT ZERO VOLTS

the alternating between peak voltages is the equivalent to a direct pd of 250V in terms of electrical power

Why a.c.?

Alternating current can easily be converted to higher and lower voltages by a transformer. Transporting the electrical energy from a power station at a high voltage and low current is more efficient, because if a high current and low voltage was to be used most of the energy would be wasted as heat due to the resistance in the power lines. Converting d.c. to the high voltage low current form is very difficult and cannot be done efficiently.

Mains electricity generated from power stations is supplied to UK households via the National Grid.

Mains electricity (electricity from the power station) enters the house via the Live wire. The live wire carries the incoming electricity and is therefore at 230V and so very dangerous. Mains voltage is more than enough to kill somebody.

The neutral wire is also supplied from the power station and is used to complete the circuit. it is earthed back to the power station. Therefore once the electricity from the live wire has given its energy to the appliances in the household the current travels back out of the house via the neutral wire – hence the neutral wire has a lower voltage than the live wire.

The earth wire is used for safety purposes and carries the current away when there is a fault.

A complete cycle is peak to peak

FREQUENCY of mains supply: 1/ time for one complete cycle (seconds) (use the time information given in the question)

most electrical appliances are connected to the sockets of the mains supply using CABLE and a THREE PIN PLUG

• outer cover of three pin plug = plastic of rubber > very good electrical insulators
• pins of plug = brass > good electrical conductor, hard, does not rust/ oxidise
• earth wire is connected to the longest pin
• cable grip must be fastened tightly over the cable
• should be no bare wires showing inside the plug
• the correct cable must be connected firmly to the terminal of the correct pin

• the brown wire = connected to the live pin (BROWN WIRE: This is connected to a fuse on the live pin)
• blue = neutral
• yellow and green = earth

two-core cable doesn't have an earth wire - only a three-core does

appliances with metal casings must be earthed - the case is attached to the earth wire in the cable

this stops the case becoming live if a fault develops and the live wire touched the case > you could be elctrocuted

a FUSE is always fitted in series with the live wire - this cuts the appliance off from the live wire if the fuse blows.

if a fault develops in an earthed appliance, a large current flows to earth and melts the fuse, disconnecting the supply

the rating of a fuse should always be slightly higher than the normal working current of the appliance

appliances with plastic cases do not need to be earthed because plastic is an insulator and cannot become live.  these appliances are said to be double insulated and are connected to the supply with two-core cable which contains no earth wire - just a neutral wire and a live wire

cables of different thicknesses are used for different purposes - the more current to be carried, the thicker the cable needs to be

a circuit breaker can be used instead of a fuse.  an ELECTROMAGNETIC SWITCH that opens and cuts off the supply if the current is bigger than a certain value

• safer than fuse:
• can't be 'meddled' with
• determined when it's installed
• works faster and can be reset faster

a residual current circuit breaker cuts off the current in the live wire if it is different to the current in the neutral wire

• works faster than a fuse AND a normal circuit breaker

a fuse contains a thin wire that heats up and melts if too much current passes through it - this cuts off the current - cuts the appliance off from the live wire.  fitted in series with the live wire. 

an electrical appliance transfers electrical energy into other forms of energy

the rate at which it transfers electrical energy into other forms of energy = its POWER

the power supplied to a device is the energy that is transferred to it EACH SECOND

• power = watts
• energy transferrred = joules
• time = seconds

an electrical CURRENT is the RATE OF FLOW OF CHARGE

charge = coulombs, C

current = amperes, A

time = s

when charge flows through an appliance, electrical energy is transferred to other forms - in a RESISTOR, electrical energy is transferred to the resistor so that the resistor becomes HOTTER when charge flows through it

electrical faults may occur as a result of damage to:

• sockets
• plugs
• cables
• appliances

they are dangerous because they can cause electric shocks and fires

• check electrical equipment regularly for wear
• ELECTRICIAN should replace or repair worn or damaged items
• avoid overloading sockets - can cause overheating then risk FIRE
• handle electrical applainces safely - never use in a bathroom or with wet hands
• cable should always be appropriate for intended use
• filament and halogen bulbs are much less efficient than low-energy bulbs, and they do not last as long

Earth wire: Electrical appliances with metal cases usually contain an earth wire. If there is a fault in the appliance and the live wire makes contact with the metal casing, a large current flows from the live wire to the earth wire. This larger current melts the fuse in the live wire breaking the circuit and isolating the appliance from the live wire.

The earth wire and the fuse together protect the user and the appliance.

Circuit breakers: Fuses work on the principal of a large current melting the fuse wire. In some cases faults can occur in which the current may not be large enough to melt the fuse but enough to seriously harm the user of the electrical appliance. For protection against faults of this nature a circuit-breaker is used in place of a fuse.

Circuit-breakers offer the following advantages compared to fuses.

• Circuit-breakers respond quicker than fuses.
• Circuit-breakers are more reliable.
• Circuit-breakers are more sensitive.
• Unlike fuses which only operate once and need to be replaced a circuit-breaker can be reset.