topic 2 Flashcards
what is the national grid
a giant system of cables and transformers that covers the UK and connects power stations to consumers
what does the national grid transfer
electrical power from power stations anywhere on the grid (the supply)to anywhere else on the grid where it’s needed (the demand)
to transmit the huge amount of power all over the uk what needs to be done
you need either a high potential difference or a high current
what is the problem with high current
is that you lose loads of energy as the wires heat up and energy is transferred to the thermal energy store of the surroundings
what is done in the N.G to keep costs low
boosting up the p.d up really high (40,000v) and keeping the current as low as possible
for a given power, what happens when you increase the p.d
you decrease the current, which decreases the energy lost by heating the wires and the surroundings
why is energy always lost in the power cables
because of the resistance of the wires
the bigger the difference between the power stations and the homes…
the greater energy loss
what can be used to reduce the energy loss
use transformers
what happens first, what transformer does the electricity pass through
the step-up transformer which increase the p.d to several hundred thousand volts
after passing through the step-up transformer, where does the electricity pass through
the high voltage cables
why does electricity pass through the step-up transformer
because less energy is lost in the power cables when the p.d is very high
from the step-up transformer to the high voltage cables, where does electricity pass through next
step-down transformer
why does electricity pass through step-down transformers
because the potential difference is too high and too dangerous to pass through to homes
what does the step-down transformer do
the electricity passes through here which reduce the potential difference to about 230 V
how is energy transferred between stores
electrically by electrical appliances
what are the 2 types of electricity supply
alternating and direct current
what happens to the current in alternating supplies
it is constantly changing direction
how are alternating currents produced
by alternating voltages in which the positive and negative ends keep keep alternating
what type of electricity supply is the uk mains supply
an alternating current at around 230 V
what is the frequency of the ac mains supply
50 Hz
what type of electricity do batteries and cells supply
direct current
what is direct current
s current that is always flowing in the same direction, it’s created by a direct voltage
most cables have how many wires
3
give the names of the 3 types of wires
1) live wire
2) neutral wire
3) earth wire
how are most electrical appliances connected to the mains supply and what does it mean
by 3 core cables, it means that they have 3 wires inside them, each with a core of copper and a coloured plastic coating
the 3 core cables have a core of what
copper
what colour is the live wire
brown
what current does the live wire provide
an alternating current at about 230 V from the mains supply
what colour is the neutral wire
blue
what does the neutral wire do
completes the circuit and carries away current
what voltage is the neutral wire at
0V
what colour is the earth wire
yellow and green
what does the earth wire do
it protects the wiring for safety- it stops the appliance casing from becoming live
what voltage is the earth wire at
0V
what happens when a human touches a live wire
our bodies are at 0V and if we touch the live wire a large P.D is produced across your body and a current flows through you
why is the live wire dangerous for humans
it can give you an electrical shock, injure or even kill you
why is any connection between the earth and live wire dangerous
if the link creates a low resistance path to earth, a huge current will flow, which could result in a fire
what is an electrical current
a flow of electrical charge around a circuit
if we close a switch in a series circuit what happens to the electrons
electrons flow out of the cell and move around the circuit
what are the flow of electrons that move out of the cell called
an electric current
in a series circuit how does the electric current flow
from the negative end of the cell to the positive end
in a series current what happens to the potential difference
it is shared between the various components, so the potential difference round a series circuit always adds up to equal the source p.d
in a series circuit what happens to the current
the same current flows through all the components
how is the size of the current determined by in a series circuit
by the total p.d of the cells and the total resistance of the circuit i.e. I + V/R
how do you work out the total resistance in a series circuit
it is the sum of the resistances in the series circuit
the bigger a component’s resistance in a series circuit …
the bigger its share of the total p.d
what happens to the p.d in PARALLEL circuits
all of the circuits get the full source p.d so the p.d is the same across all components
so if you connect a bulb in a parallel circuit what happens to the light of the bulbs
they will both be the same
what is the total current in parallel circuits
the total current flowing around the circuit is equal to the total of all the currents through the separate components
what happens to the current in parallel circuits when at a junction
it splits or rejoins
what happens if you have 2 resistors in parallel
their total resistance is less than the resistance of the smallest of the 2 resistors
how will electrical charge flow round a complete circuit
if there is a p.d
what is the unit of current
ampere, A
describe the current in a single, closed loop
the current has the same value everywhere in the circuit
what is potential difference
(voltage) the driving force that pushes the charge around
what is the unit for p.d
V
what is resistance
anything that slow the flow of p.d down
the greater the resistance across a component, ….
the smaller the current that flows
what is size of current
the rate of flow of charge
what happens when a current flows through an ohmic conductor at a CONSTANT TEMP
it is directly proportional to the p.d across it
what happens when an electrical charge flows through a filament lamp
it transfers some energy to the thermal energy store of the filament, which is designed to heat up
what happens to the resistance in a filament lamp
resistance increases with temperature, so as the current increases, the filament lamp heats up more and the resistance increases
what does resistance in a diode depend on
the direction of the current
what is an LDR
a resistor that is dependant on the intensity of light
in a bright light, what happens to the resistance
it falls
in darkness, what happens to the resistance
it is at it’s highest
what is a thermistor
a temp dependant resistor
what happens to the resistance in hot conditions
it drops
what happens to the resistance in cool conditions
it goes up
what do thermistors make that are useful
temp detectors
