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
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
live wire alternates between +ve and -ve potential differences
each cycle takes 1/50 = 0.02 seconds
neutral wire is earthed at local electricity substation
peaks at +325V and -325V
average potential difference is 230V
equivalent to direct current power supply
National Grid
nationwide network of cables and transformers
links power stations to consumers
Step-up Transformers
transfer electricity to the National Grid
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
transformers only work with a.c.
Cables and Plugs
Plugs
Pins
Pins are made of brass
hard
will not rust/ oxidise
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
Wires
neutral
Cables
three (or two) insulated copper wires in an outer layer of rubber
wires are made of copper
outer coverings of cable made from flexible plastic material
good insulator
wires can be bent
good conductor
larger currents need thicker cables
bends easily
Short Circuits
if a live wire touches the neutral wire
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
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
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
a very big current passes through the two wires
the fuse blows and circuit breaks
good conductor
longest Earth pin contacts socket first
live
brown wire
blue wire
earth
green and yellow striped wire
only in 3-core cables (not 2-core cables)
connected to live pin
connected to neutral pin
connected to earth pin
automatically earths metal cases
stays at 0 volts
attached to metal case of some appliances
earthed at ground outside home (0V)
only carries current when faulty
earths metal case if it touches live wire
plastic cases are not earthed
they are said to be Double Insulated
over long distance power lines