2) Electricity Flashcards
Current, charge, energy, resistance, time, voltage, power units
Current: Ampere (A)
Charge: Coulomb (C)
Energy: Joule (J)
Resistance: ohm (Ω)
time: second (s)
voltage or potential difference: volt (V)
Power: watt (W)
Mains electricity
The source of electricity in our houses is called mains electricity.
-alternating current - changes directions
-easy to use a transformer to increase/ decrease voltage
Direct current
-current only travels in one direction
-always from a cell - battery
Live wire
The wire that carries current to the appliance at high voltage
Neutral wire
Completes circuit, carries current away at low voltage
Insulation
Some appliances are cased with insulators like plastic rather than metal to prevent user from receiving shock.
Double insulation
-designed with a plastic casing so the live wire cannot come loose, touch the casing
-casing cannot give an electric shock even if wires inside came loose
Earthing
-you would get an electric shock if the live wire came loose and touched the metal casing
-earth wire connected to casing so current goes through there instead
-strong current surges through earth wire as it has low resistance, breaks fuse
Fuse
-contains a piece of wire made from a metal that has a low melting point
-If too large a current flows in the circuit the fuse wire becomes very hot and blows, shutting the circuit off.
-fuse should be rated at slightly higher current than device needs (3A, 5A, 13A)
Circuit breakers
-similar to fuses
-if too large a current flows, a switch opens, making circuit incomplete
-once fault in circuit is corrected, the switch is reset (manually/ reset button)
Current in a resistor –> increase in temperature
-normal wiring has low resistance, current passes through them easily
-heating elements like nichrome wire have high resistance
-current flows through them, cannot pass
-energy transferred to heat energy and element heats up
-heating effect of current: electric kettle, iron, lamps, hair dryers
-when an electron collides with a metal ion, it causes it to slow down, resistance increases
-when hotter, metal ions vibrate more, more electron collisions, resistance increase, current decreases
Relationship: power, current, voltage
Power (watts) = current (amps) x voltage (volts)
P=IV
Voltage
Energy transferred per unit charge passed
-volt = joule per coulomb
-voltage across component = how much energy it is converting
-1v = 1J of electrical energy into different kind of energy each time 1C of charge passes through it
Conductors
Materials that allow current to pass through them
-copper, silver, aluminium
Insulators
Do not conduct electricity
-Don’t have free electrons
-plastics, rubber, wood
Relationship: energy, current, voltage, time
Energy transferred (joules) = current (amps) x voltage (volts) x time (secs)
Series circuit
-one switch can turn off the components on and off together
-if one bulb ( or other component) breaks, it causes a gap in the circuit and all of the other bulbs will go off
-the voltage supplied by the cell or mains supply is “shared” between all the components, so the more bulbs you add to a series circuit the dimmer they all
become.
-The larger the resistance of the component, the bigger its “share of voltage”
Parallel circuit
-switches can be placed in different parts of the circuit to switch each bulb on and off individually or all together
-if one bulb (or other components) breaks, only the bulbs on the same branch of the circuit will be affected
-each branch of the circuit receives the same voltage, so if more bulbs are added to a circuit in the parallel they all stay bright
Current in series vs parallel
Series: current is the same in all parts
Parallel: current varies with resistance and voltage
-current in branch A+ current in B = total current
-current at a junction: sum of currents entering a junction must be the same value when it exits
Voltage in series vs parallel
Series: voltage across bulbs add up to cell value
Parallel: voltage across each loop is the same voltage from the cell
Resistance in series vs parallel
Series: resistors in series add together
Parallel: total resistance of two resistors in parallel < resistance of smallest individual resistor
-with two resistors in parallel, there are two pathways for the current to take, so more current will flow through the circuit
Relationship: voltage, current, resistance
voltage = current x resistance
V=IR
Current
Rate of flow of charge (coulombs)
1C of charge flows along a wire every second = 1A current passing the wire
Conventional current
Positive charge out of positive terminal to negative terminal
-wrong, but what scientists thought when electricity was first discovered
Electron flow
Flow of negatively charged electrons flowing from the negative side of the battery to the positive side
Relationship: charge, current, time
Charge = current x time
Q = It
I/V - resistors and wires
-obey Ohm’s law
-Current is proportional to voltage
-graphs are straight lines which pass through the origin
I/V - metal filament lamps
-filament is a metal wire - gets very hot
-resistance of metal increases with temperature
-graph curves when the lamp reaches its working current and temperature (S shape)
I/V - diodes
-large resistance when voltage is applied in the wrong direction
-horizontal line when voltage is negative
-when voltage is in right direction: resistance drops, graph curves and becomes very steep
LDR - resistance/ light
Light Dependant Resistors
-resistance changes with intensity of light
-in the dark, LDRs contain few free electrons, have high resistance
-in the light, electrons are freed, resistance decreases
-I/V proportional through origin
-used in photographic equipment, automatic lighting controls
Thermistor
Temperature dependant resistor
-at room temp: number of free electrons in small, resistance is large
-if warmed, number of electrons increase, resistance decrease
-used in fire alarms, thermostats
Relationship: charge, current, time
Charge = current x time
Q = It
Electrostatic phenomena
-event where electricity has a special effect e.g. static shock
-electrons move from one material to another
-materials with a negative charge will look for some way to earth like clouds through lightning
Practical: investigate how insulating materials can be charged by friction
- Hold it at its centre and rub both ends with a cloth
- Suspend the rod without touching the ends, from a stand using a cradle and thread
- Take a Perspex rod and rub it with another cloth
- Without touching, bring each end of the Perspex rod up to each end of the polythene rod
- Record any observations
- Repeat for different materials
-two insulating materials rubbed together, electrons passed from one to another
-if material repelled from polythene, materials have the same charge
-if attracted, opposite charges
Production of static
When insulating materials rubbed against each other and become electrically charged by friction
-material gaining electrons has a negative charge
-material losing electrons has a positive charge
Electric forces between charges
Two charged particles exert a force on each other
-attractive (charges are opposite)
-repulsive (charges are the same)
Danger of electrostatic charges - aircraft
-as aircrafts fly through the air, they become charged with static electricity
-as charge on an aircraft increase, pd between it and earth increases
-with high potential differences, there is the possibility of charges escaping to the earth as a spark during refueling
-could cause an explosion
Solution to danger of electrostatic charges - aircraft
-earth the plane with a conductor as soon as it lands and before refueling
-fuel tankers that transport fuel on roads much be earthed before any fuels are transferred to prevent sparks causing a fire or explosion
Uses of static electricity
-photocopiers
-inkjet printers
Use of static electricity - photocopiers
- Image of document projected onto positively charged copying plate
- Plate loses charge in light areas, keeps positive charge in dark areas
- Negatively charged black toner powder applied to plate, sticks to part where there is a positive charge
- Toner transferred onto blank sheet of white paper
- Paper heated for powder to stick
Use of static electricity - inkjet printers
-similar to photocopiers
-small jet of coloured ink is negatively charged
-attracted to the correct place on the page
