Download P4 a-d Model Answers

Model exam answers
P4a Sparks
Explain how objects become charged. (6 marks)
Two insulators are rubbed against one another. The friction
between the materials causes electrons to be transferred
from one material to the other. The material that loses
electrons becomes positively charged. The material that
gains electrons becomes negatively charged.
Explain the process of electrostatic induction. (5 marks)
When a charged object is brought near a neutral object it
sets up an electrostatic imbalance in the object. Like charges
are repelled by the charged object and opposite charges are
attracted to the object. This leaves one side positive and the
other side negative causing the neutral object to be attracted
to the charged object.
Explain how ions are formed. (4 marks)
Ions are formed when an atom either loses electrons and
becomes a positive ion or gains electrons and becomes a
negative ion.
Explain the dangers of electrical sparks. (5 marks)
Sparks can be dangerous if they pass through the body
because they disrupt the electrical impulses in the heart
preventing the heart from contracting properly.
Sparks can also be dangerous around flammable materials. In
the refuelling of planes sparks are caused by the friction of
fuel flowing down the pipe. If this sparks it can cause fires
and explosions.
Friction caused by fine particles such as flour can also ignite
and cause explosions in factories.
How can the dangers of sparks be reduced? (4 marks)
Sparks can be reduced by the correct earthing of electrical
appliances, and in places where static can build up. Insulating
mats can be used to protect people by increasing their
resistance, as can thick insulating soles.
P4b Danger of electrostatics
Explain how an electrostatic dust precipitator works (5
marks)
Smoke containing dust/ash is pumped through a chimney. It
passes a high voltage negatively charged grid where the dust
picks up a negative charge. The dust is then attracted to
positively charged plates either side of the chimney and
clump together. The plates can be knocked to cause the dust
to fall into the collecting plates. Clean gases then exit the
chimney at the top.
Explain how electrostatic paint spraying works. (6 marks)
Paint is charged as it leaves the nozzle of the paint gun. The
paint droplets repel each other as they all have the same
charge. This produces a fine spray. The object to be painted
has the opposite charge to the paint. This cause the paint to
be attracted to it meaning there is a greater coverage and
less is wasted due to less paint covering the floor.
Explain how a defibrillator can be used safely. (5 marks)
The paddles are charged using a high voltage supply. They
are placed on the patients chest. Gel is used to increase the
electrical contact. Charge is passed through the heart
causing it to contract. The pads have insulated handles to
protect the user from shocks. Care should be taken not to
touch the patient
P4c Safe electricals
Explain how a fuse works. (4 marks)
A fuse only allows a certain amount of current to flow
through it. If too much current flows then the fuse melts
and breaks the circuit. This is due to the friction caused by
electrons passing along the wire generating heat.
Explain the structure of a plug. (6 marks)
A plug is composed of three wires. The Earth wire which is
green and yellow this prevents the appliance from
becoming live by conducting any charge to the Earth. The
Live wire is brown, this carries a high voltage and is
connected to the fuse. The neutral wire is blue and this
completes the circuit.
Compare and contrast longitudinal and transverse waves.
(5 marks)
Longitudinal waves need particles to travel through e.g
sound or ultrasound. Transverse waves do not e.g light or
radiowaves. The disturbance in a longitudinal waves is
parallel to the direction of the wave, in transverse waves it
is at right angles.
In both the type of energy transferred is kinetic.
P4d Ultrasound
Explain what ultrasound is. (3 marks)
Ultrasound is a longitudinal wave. It is high frequency sound
above 20000Hz. It cannot be heard by human as it is above
the human threshold of hearing.
Describe the medical uses for ultrasound. (3 marks)
Ultrasound can be used to produce foetal scans, it can be
used to measure blood flow and it can be used to break up
kidney stones.
Explain how ultrasound is used to produce a foetal scan. (5
marks)
Ultrasound waves are sent in pulses into the body. The
ultrasound waves are reflected off different layers of body
tissue. The time taken for the reflection is measured, longer
times mean greater depths. There must be no air gap
between the transmitter and the patient or no image will be
formed due to the reflection off the patients skin. Gel is
used to prevent this from happening.
Explain why ultrasound is used for foetal scans and not Xrays. (4 marks)
X-rays are ionising which means they cause damage to living
cells/tissue therefore harming the foetus. X-rays cannot
produce images of soft tissue and cannot produce moving
images, or vice versa.
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