Download AQA Physics I–V characteristics Specification references PS 3.1 3.5

12.4 Components and their
characteristics
Follow up sheet
AQA Physics
I–V characteristics
Specification references
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PS 3.1
3.5.1.1 Basics of electricity
3.5.1.2 Current–voltage characteristics
Learning objectives
After completing the worksheet you should be able to:
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apply your understanding of the heating effect of a current to a range of
situations
link the heating effect to electrical resistance and power.
Information
The heating effect of a current in a wire can be used in filament lamps, electrical
heaters, cookers, and some cutting devices. These questions will test your ability to
apply your understanding of the heating effect in a wide range of situations. You will
also need to apply your knowledge of electrical power and resistance to answer the
questions fully.
Questions
This first set of questions checks your ability to apply your understanding of the
heating effect of a current to a range of scenarios.
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Figure 1 shows the current–voltage (I–V) characteristics for a filament lamp.
a State the value of the current in the lamp when it stops obeying Ohm’s law.
b Explain why the lamp stops obeying Ohm’s law as the current increases
beyond this point.
c Calculate the resistance of the lamp when the current is 0.2 A.
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(1 mark)
(1 mark)
(2 marks)
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12.4 Components and their
characteristics
Follow up sheet
AQA Physics
Figure 1 Current–voltage characteristics for a filament lamp
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3
A car is fitted with an automatic window defrosting device, consisting of several
very thin metal filaments running through the front windscreen in parallel. These
elements are activated remotely by the owner several minutes before returning
to the car to ensure that the windows are defrosted.
a When first activated, the system operates with a current of 1.20 A, and a
potential difference of 12.0 V across it. Calculate the electrical power of
the device.
b After the frost on the windscreen melts, the current into the device reduces
to 1.00 A. Explain this effect.
c A chip in the windscreen breaks one of the filaments. Explain what effect,
if any, this will have on the current into the device.
Polystyrene foam can be cut and shaped using a hot metal wire. As the wire
is moved through the foam, the polystyrene melts. Figure 2 shows how the
temperature of a cutting wire varies with the current in it. The melting point
of polystyrene is 220 °C.
a What is the minimum current in the wire that will allow it to be used to cut
polystyrene?
b The wire has a potential difference of 12 V and a current of 0.25 A in it during
normal cutting.
i What is the temperature of the wire during normal operations?
ii What is the resistance of the cutting wire during normal operations?
iii What power is dissipated by the wire?
c Whilst being used, the wire snaps and is replaced by a wire made from the
same material and of the same length but with twice the diameter of the
original wire. The potential difference across the wire remains at 12 V.
Discuss whether the wire will still be able to cut the polystyrene foam.
© Oxford University Press 2015
(2 marks)
(3 marks)
(2 marks)
(1 mark)
(1 mark)
(1 mark)
(1 mark)
(3 marks)
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12.4 Components and their
characteristics
Follow up sheet
AQA Physics
Figure 2 Effect of current on the temperature of a cutting wire
Research questions
These final two questions allow you to research the heating effects of a current in
more detail.
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Electrical faults are one of the most common causes of fire in the home. Many
of these fires are preventable as they are caused by overloading sockets or
using an incorrect fuse.
Explain why overloading a plug socket so that the current from it increases from
13 A to 15 A is much more likely to cause an electrical fire than an increase from
11 A to 13 A. Your explanation should refer to the I–V characteristics of a wire
and to electrical power.
(4 marks)
Many tall buildings are protected from lightning strikes by a lightning protection
system. Summarise the development of these systems and explain why such a
large current does not melt the conductors used in them. You should also
discuss how you could be protected from a lightning strike.
Suggested references:
 http://en.wikipedia.org/wiki/Lightning_rod
 http://europe.tnb.com/pub/en/furse_lightning-protection
 http://stormhighway.com/protection.php
 http://en.wikipedia.org/wiki/Faraday_cage
 Search the following on YouTube:
o Electrified David Blaine
o Richard Hammond struck by lightning in a car
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websites. The content of these websites may have changed since publication.
© Oxford University Press 2015
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This resource sheet may have been changed from the original
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