Transcript File

Science Puzzle
by M. Deal
Electricity Cable
Here is an electricity extension cable.
There is nothing unusual about it but it has
a curious label and warning.
What do these warnings mean?
It shows how much power can be
provided to appliances connected
to the extension lead when it is
coiled and uncoiled.
Can it have a current of 10 A going through it when it is uncoiled or coiled?
Power (W) = Voltage (V) x Current (A)
2400W = 240V x Current
So the lead can take the greatest current (10A) when it is uncoiled.
When it is coiled what is the maximum current that can pass through the lead?
Power = Voltage x Current
So
Current = Power ÷ Voltage
Current = 960W ÷ 240V = 4A
It is the same lead whether it is coiled or uncoiled so why is there a difference in
the current that can be carried?
Hint
Solution
End presentation
Hints
Here are a few hints with increasing helpfulness. You can jump
back to the previous slide at any point by using the ‘return button’.
Hint 1: On a shorter 5m extension lead it carries the same sort of warning.
The lead can carry a 4A current when coiled and a 5A current when uncoiled.
Length of cable is a factor. Why should this be?
Hint 2: Can you think of any other wires in which the current is kept low?
What was the reason for this?
This is the last hint. Please select from the three buttons on this screen in
order to move onto the next slide.
Solution
End presentation
Solution
Electric currents have a heating effect and the larger the current, the larger
this heating effect is. Appliances that get hot (kettles, electric fires, toasters,
etc) need a large current and therefore have a high power rating.
When the lead is unwound the heat can escape from all the
large surface area.
When the cable is used but is wound up then the cable
heats itself up and is also encased so heat builds up and
cannot easily escape. This heat can be enough to melt the
insulation, possibly starting a fire.
In what way would you
expect the warning labels
to differ for shorter or
longer extension leads?
25m
lead
10m
lead
The longer the lead, the greater the possible heating effect and therefore the
greater the difference between the currents allowed when the lead is coiled and
uncoiled.
This 10 metre extension lead can deliver:
 960W of power (at 240V) when coiled
= 4A
 1200W of power (at 240V) when uncoiled = 5A
What is the maximum current that it can take each time?
This 25 metre extension lead can deliver:
 720W of power (at 240V) when coiled
= 3A
 3120W of power (at 240V) when uncoiled = 13A
What is the maximum current that it can take each time?
There is a difference between the two leads for the current
they can take when unwound because the leads are of different
thicknesses. Leads have to be thicker to carry larger currents.
End presentation
Extra information
It is not always so easy to work out the power being drawn
through an extension cable.
If more than one socket is used then the power being drawn
through each socket must be added to find the total power.
A 60W light bulb, a 30W Video player and a 100W television are all connected
into an extension lead. If mains electricity is 230V find the:
• power drawn through the extension lead
60W + 30W + 100W = 190W
• current flowing through the extension lead
I = P ÷ V = 190W ÷ 230V = 0.83A
Some extension leads and power packs are fitted
with a thermal cut out. The button ‘clicks out’
and the circuit is broken if a high current flows.
These can use the heating effect of too large a
current to bend a bimetallic strip and break the
circuit.
End presentation
End of presentation