Page 79 - Physics_Form_2

Current electricity

(ii) The current from the
battery.
S S 2 (d) (i) Show that there is a p.d of
1
12.0 Ω 11.8 V across the headlamp
when switch S is closed
1
12 V Starter motor and switch S is open.
FOR ONLINE READING ONLY
2
48 W
r 0.05 Ω 0.12Ω (ii) Calculate a p.d across
starter motor when both
switches S and S are
2
1
Figure 2.40 now closed.
(a) State what is meant by “the battery emf of Electrical installations
12.0 V”.
(b) What is the resistance of the headlamp Electricity is a highly convenient and
when used at its rated voltage? clean source of power. Electricity is
(c) Assuming that the resistance of the supplied in houses by low-resistance
headlamp does not change, and all switches
are closed, calculate: wires (copper or aluminium)
(i) The effective resistance of the insulated with rubber. The cables
circuit. are rated according to the maximum
current they can carry.

Live, Neutral and Earth

Domestic electricity is supplied through two cables: the live cable (L), coloured brown
or red, and the neutral cable (N), usually coloured blue or black. For a single-phase
system, the live cable is at a potential of 240 V relative to the neutral line. The current
in the cable alternates 60 times a second (60 Hz). The neutral cable is earthed; it is
connected to the ground at the power station. This ensures that even though current
flows through this cable, it remains at zero potential. In this situation, it cannot give an
electric shock when touched. To provide extra safety, especially in electrical appliances,
a third cable called earth (E), coloured yellow or green, is also provided and is earthed
by connecting it to the ground via a thick copper rod. This cable connects the metal
body of an electrical appliance to the ground through a three-pin plug. A three-pin plug
and electrical wiring cables are connected as shown in Figure 2.41.

Earth
Live

Neutral

Fuse

Figure 2.41: A three-pin plug

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Physics Form 2 Final.indd 73 25/10/2025 10:26