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P5 Electricity in the Home (D.C / A.C (Alternating Current (supplied from…
P5 Electricity in the Home
D.C / A.C
Direct Current
supplied by cells and batteries
current passes round circuit in 1 direction only
Alternating Current
supplied from mains electricity
live wire alternates between +ve and -ve potential differences
peaks at +325V and -325V
average potential difference is 230V
equivalent to direct current power supply
neutral wire is earthed at local electricity substation
stays at 0 volts
current repeatedly reverses its direction
current fluctuates and travels in 2 directions in repeated cycles
T
=
1
÷
f
Time Period (for 1 cycle)
=
1
÷
Frequency
the UK mains supply frequency is
50Hz
current reverses direction 50 times per second
each cycle takes 1/50 = 0.02 seconds
National Grid
nationwide network of cables and transformers
links power stations to consumers
transformers only work with a.c.
Step-up Transformers
transfer electricity to the National Grid
over long distance power lines
increases the potential difference from power stations
from 25 000V to 132 000V
steps down the current because P=VI and power doesn't change
less resistance heating / friction in wires
less energy lost so more efficient
current causes wires to heat
Step-down Transformers
decreases the potential differences for use
home and office 230V
factories 100kV or 33kV
Cables and Plugs
Plugs
Pins
Pins are made of
brass
hard
will not rust/ oxidise
good conductor
longest Earth pin contacts socket first
automatically earths metal cases
outer casing is a
stiff plastic
material
good insulator
interior shaped so wires can't touch
Fuse
a thin wire between live wire and live pin in the circuit
if too much current flows, the fuse melts
cuts current off the live wire
a fuse's
current rating
is slightly higher than the current through appliance
P = IV used to work out current through appliance
p.d. of mains supply is 230V
power rating
of electrical device
Wires
neutral
blue wire
connected to neutral pin
live
brown wire
connected to live pin
earth
green and yellow striped wire
connected to earth pin
only in 3-core cables (not 2-core cables)
attached to
metal case
of some appliances
earths metal case if it touches live wire
earthed at ground outside home (0V)
only carries current when faulty
plastic cases
are not earthed
they are said to be
Double Insulated
Cables
three (or two) insulated copper wires in an outer layer of rubber
wires are made of
copper
good conductor
bends easily
outer coverings of cable made from
flexible plastic
material
good insulator
wires can be bent
larger currents need thicker cables
Short Circuits
if a live wire touches the neutral wire
a very big current passes through the two wires
the fuse blows and circuit breaks
Electrical Power
Power
the rate at which energy is transferred
P
=
E
÷
t
Power
=
Energy Transferred
÷
Time
P
=
IV
Power
=
Current
x
Potential Difference
P = E/t
V = E/Q
I = Q/t
Resistance Heating
P
=
I² R
Power
=
Current²
x
Resistance
P=IV
V=IR
Electrical currents and Energy Transfer
Work is done when batteries push electrons pass through the resistor
when a charge flows through a resistor, electrons collide with metal ions
metal ions gain KE and vibrate faster so resistor becomes hotter
work done by batteries = energy transferred to the resistor