IB Physics SL B 5 5 Electrical Power Questions

Practise IB Physics SL B.5.5 by calculating power in circuits, motors, lamps and transmission cables.

Exam points

State and apply the two postulates of special relativity: relativity principle and invariant light speed.
Use the postulates to reason about observations in inertial frames with complete mark-scheme evidence.

[Maximum number: 2]

A cable consisting of many copper wires is used to transfer electrical energy from a generator to an electrical load. The copper wires are protected by an insulator.

Figure for Question IB Physics SL B 5 5 Electrical Power Questions question 1 — IB Physics SL

The cable consists of 32 copper wires each of length 35 km . Each wire has a resistance of $64 \Omega$ . The resistivity of copper is $1.7 \times 10^{-8} \Omega \mathrm{~m}$ .

There is a current of 730 A in the cable. Show that the power loss in 1 m of the cable is about 30 W .

$R=2\,\Omega$ $P=I^2R=(730\,\mathrm{A})^2\times2\,\Omega=1.07\times10^6\,\mathrm{W}=1.07\,\mathrm{MW}$ $\text{power loss per metre}=\frac{1.07\times10^6\,\mathrm{W}}{35\times10^3\,\mathrm{m}}=30.6\,\mathrm{W\,m^{-1}}$

Allow [2] for a solution using the resistivity and the answer to (b)(i), giving a resistance per unit length of $5.7\times10^{-5}\,\Omega\,\mathrm{m^{-1}}$ .

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IB Physics SL B 5 5 Electrical Power Questions question 1