TOPIC 5 ELECTRICITY Discharging vehicles before re-fuelling
Ships and planes experience huge friction when travelling. This is caused by sea or air resistance. Before they can be re-fuelled, they need to be neutralised. Copper strips in the rubber tyres ensure aircraft discharge as they land.
Even aircraft re-fuelling in the air need to be aware of static discharge to avoid explosions (Fig. 20.18). If you look closely at the re-fuelling rod from the tanker plane, you will see an ‘earth’ rod or cage that touches the inlet pipe before the fuel is pumped. This discharges the exterior of the plane to allow safe re-fuelling.
Fine dust in industry
Flour mills can be just as explosive as oil refineries. Given the density of airborne dust, a static discharge can ignite the material in the air. To lower the explosion or fire hazard in high-density dust environments, all electrical equipment is earthed and no naked flames are allowed. Good ventilation is also required to keep the equipment free from clogging and unnecessary friction.
QUESTIONS AND ANSWERS H
3. If a 5 μC charge experiences a 50 N force when placed in an electric field, calculate the electric field strength at that point.
Solution E = F
__ Q = 50
__________ 5 × 10−6 = 1 × 107 N C −1
4 . A charge of +2 μC is placed at X, as in Fig. 20.19. Calculate the force on the +2 μC charge and its resultant direction, when all charges are arranged linearly in air. (Permittivity of air, ε0 = 8.9 × 10−12 F m−1)
+3 μC 0.4 m FIG. 20.19 X 70 cm +5 μC
Solution Calculate the force from the +3 μC charge and +5 μC charge individually and then combine for the resultant force.
Force from the +3 μC charge: F1 = 1
______ 4π ε0
F1 = F1 =
( (
_______ d2
Q1 Q2
____________________ 4π (8.9 × 10−12)
1
__________________ 1.118 × 10−10
1 ) ( F2 = 1 Q1 Q2
______ 4π ε0
_______ d2
Continued 236 FUSION
_________________________ (40 × 10−2)2
(3 × 10−6)(2 × 10−6)
) ( 6 × 10−12 ____________
0.16 )
F1 = 0.335 N (Away from +3 μC) Force from the +5 μC charge:
)
FIG. 20.18 F-18 Hornet re-fuelling an Ea-6B Prowler
