Electromagnetic effects：練習題

Which statement best describes the magnetic field lines around a straight current-carrying wire?

A step-down transformer has a primary coil with 2000 turns and a secondary coil with 500 turns. If the input voltage to the primary coil is 240 V, what is the output voltage from the secondary coil?

Electrical power is transmitted from a power station to a town through cables with a total resistance of \( 5.0\text{ }\Omega \). The power station generates \( 1.0\text{ MW} \) of power.

How much power is wasted as thermal energy (heat) in the cables if the transmission voltage is \( 200\text{ kV} \)?

Which material is most suitable for the core of a transformer, and what is the primary reason for this choice?

A straight wire is placed horizontally between the poles of a U-shaped magnet as shown. When a current flows through the wire in the direction indicated, what is the direction of the magnetic force acting on the wire?

A bar magnet is moved quickly towards a stationary solenoid. Using Lenz's law, explain why the induced current produces a magnetic field that opposes the entry of the magnet.

A step-up transformer is used at a power station to prepare electricity for long-distance transmission through cables.

(a) The primary voltage is \( 25000 \text{ V} \) and the secondary voltage is \( 400000 \text{ V} \). If the primary coil has 5000 turns, calculate the number of turns required on the secondary coil. (2)
(b) The current in the transmission wires is \( 60 \text{ A} \) and the wires have a total resistance of \( 5.0 \text{ }\Omega \). Calculate the power loss as thermal energy in these wires. (2)
(c) Use the concepts of changing magnetic fields and electromagnetic induction to explain how an alternating current in the primary coil induces an e.m.f. in the secondary coil. (3)

A high-voltage transmission line is used to transfer electrical power from a power station to a distant town. The power station generates electricity at a potential difference (p.d.) of \(25\text{ kV}\). A transformer at the station steps this up to \(400\text{ kV}\) for transmission.

(a) The primary coil of the transformer has \(800\) turns. Calculate the number of turns on the secondary coil, assuming the transformer is ideal. (2)

(b) The power transmitted is \(100\text{ MW}\). Calculate the current in the transmission cables. (2)

(c) The total resistance of the transmission cables is \(4.0\text{ }\Omega\). Calculate the power lost as thermal energy in the cables. (2)

(d) Explain, in terms of current and power loss, why electricity is transmitted at such high voltages over long distances. (2)