P5 Electrcitity At Home Flashcards
DC series circuit
-A direct current is defined as a current that is steady,constantly flowing in the same direction in a circuit, from positive to negative
-the potential difference across a cell in d.c circuit is one direction only
- power supply has a fixed positive and negative terminal
-electric cells or batteries produce direct current
Alternating current
An alternating current is defined as a current that continuously changes its direction, going back and forth in a circuit
-on an oscilloscope, direct current and alternating current are represented by a straight line and s wavy line
-an AC power supply has two identical terminals
-the potential difference between terminals varies as a sine curve
-the frequency of an alternating current is the number of times the current changes direction back and forth each second
-in the UK, mains electricity is an alternating current with a frequency of 50HZ and a potential difference of around 230V
-Direct current is produced by electricity generators and has 2 identical terminald
-alternating current has a graph pf a curve oscillating from positive to negative
Difference between AC and DC current
Direct current:
-comes from battery
-with a fixed positive and a fixed negative terminal
-current flows in the same direction from positive to negative
-has a graph of a hozironztal line
Alternating current:
-comes from a generator
-with two identical terminals switching from Posotive to negative
-current flows continuously changing direction
-has a graph of a curve oscillating from positive ti negative
Mains electricity
-mains electricity is the electricity generated by power stations and transported around the country through national grid
-everyone connects to mains when plugging in an appliance
-mains electricity in UK is an alternating current supply
-in Uk, domestic supply has frequency of 50Hz and a potential difference of 230v
-main electricity, being a.c, does not have positive to negative sides to the power source
The three wires-live wire, neutral wire and earth wire
Live wire:
-carries the alternating potential difference from the mains supply to a circuit
-it is most dangerous of 3 wires
-if it touches the appliance without earth wire it can cause electrocution
Neutral wire:
-forms the opposite end of circuit to the live wire to complete the circuiy
-because of its lower voltage, it is much less dangerous than live wire
Earth wire:
-acts as a safety wire to stop appliance from becoming live
-this prevents electric shocks from occurring of the appliance malfunctions or the live wire breaks off and touches the case of plug
Colours for each wire
The insulation covering each wire js colour coded for easy identification:
Live wire-brown
Neutral wire-blue
Earth wire-green and yellow stripes
Plugs,sockets and cables
Plugs have cases made off plastic materials, the live pin, the neutral pin and earth pin stick out through the plug case
-the pins are made of brass as brass is a good conductor and dosen’t rust or oxidise
-the caee material is an electrical insulator, the inside case is shaped so that the wires and pin ca’t touch each other when plug is sealed
-the plug contains a fuse between live pin and live wire. If too much current passes through the wire, it melts snd cuts the live wire off
-the brown wire is connected to live pin
-the blue wire is connected to the neutral pin
-the green and yellow striped wire is connected to earth pin.
Plug features:
-the metal parts are made of copper because it is a good electrical conductor and bends easily
-the cade,cable grip and cable insulation are made from plastic as it is a good insulator, so if anyone touched the cable, it stops them from getting an electric shock
Potential difference lf live,neutral and earth wires
Live wire-230v
Neutral wire-voltage close to 0
Earth wire-voltage of 0
Dnagers of main electricity
many electrical appliances have metal cases that pose a safety hazard:
-if a live wire came into contact with the case, the case would become electrified
-therefore if anyone touched it, they would be risk of electrocution
The earth wire is an additional safety wire that can reduce risk:
-provides a low resistance path to earth
-this causes a surge of current in the earth wire and hence also in live wire
-the high current through the fuses cause it to melt and break
-this cuts of supply of electricity to appliance, making it safe to touch
-because of the large potential difference between live and earth, if the two are connected together, a very large current can be created
-providing a potentially lethal shock
Electrical power and 2 formulas
Power is defined as the rate of energy transfer or the amount of energy transferred per second
-the power device depends on voltage and current of device
Equation:
Power=current x potential difference
P=VI
Power=W
Current= A
Voltage=V
2nd equation:
Power= energy/time
P=E/T
Power=w
Energy=J
Time=S
Power and resistnace formula
-the voltage across and electric device depends upon the current and resistant of that device
- using equation P=VI with V=IR:
Power= current*2 x resistance
Diffrence fuses
3A
5A
13A
Energy and power
-everyday appliances transfer energy electrically from the mains to energy stores within the appliance. E.g in a heater, energy will be transferred to the thermal store of the heating element, and then to thermal store to surroundings
-the amount of energy transferred to and from Ann appliance depends on:
-the time appliance is switched on fir
-the power of the appliance
Energy transfers in appliances and motors
as charge flows around a circuit, energy is transferred from the power source to the various components:
-as the electrons pass through the power supply, energy is transferred to electrons
-as electrons pass through each component they transfer energy to components
Some domestic appliances, such as remote control,transfer energy electrically from the chemical store lf the DC cells and batteries
-most larger household appliances transfer energy electrically from the AC mains
-this energy can be transferred to kinetic energy store of an electric motor
Motors are used in:
-vacuum cleaners-to create suction to suck in dust and dirt of carpets
-washing machines-to rotate the drum to wash clothes
-refrigerators-to compress the refrigerant chemical into a liquid to reduce the temperature
How to calculate energy transferred
Work id done when charge flows through circuit so is equal to energy transferred
Amount of energy transferred by electrical work in component depend on:
-current
-potential difference
-time
E=P x t
Where:
E=energy (J)
P= power (W)
T=time (s)
P=IV can also be written as:
E=IVT
Where:
I=current (A)
V= potential difference (V)
Energy transferred electrically also depends in charge and potential difference:
E=QV
Where:
Q=charge (C)
V= potential difference (V)
Calculating charge
Charge= current x time
Q=IT
Charge= C
Current= A
Time= S
Power ratings and formulas needed
-the power lf an appliance is the amount of energy it transfers by electrical working every second
-every electrical appliance has a power rating which tells you how much energy it needs to work
The power rating for a domestic appliance is normally given on an label. Will include:
-the potential difference required to make the device
-the frequency of supply
-power rating
Formulas needed:
Energy= Power x time
E=PT
Power= current x potential difference
P=IV
Efficiency= output power/input power x 100
National frid
-consists of a system of cables and transformers linking power stations to consumers
-electrical power is transferred from power stations to consumer using national grid:
Transformer includes:
-step-up transformers which increase voltage and reduces the current through wires
-step-down transformers which decrease the voltage and increase the current through the wires
Benefits of national grid
-national grid is an efficient way to transfer energy due to use kf step-up snd step-doen transformers
-when electricity is transmitted over large distances, the resistance in wires causes heating, which resukts in wasted energy transfer
-by increasing potential difference at which current is transmitted, the same amount of power can be transferred using much smaller current which results in less heating and less wasted energy
Therefore:
-high potential difference means low current (less energy dissipated) for same power
-low potential difference means high current (more energy dissipated) for same power
Use of transformers
-type of current produced in power stations is alternating current which is transferred to home via national grid
-transformers are used to increase snd decrease potential difference of the current before and after transmission across national grid
Made up of 2 coils of wires, called primary and secondary coils, around a magnetic iron core
-step up transformer has more turns on secondary coil than primary
-step down transformers has more tuns on primary coil than secondary
