Electricity and magnetism：練習題

What is the SI unit for electrical resistance?

A student uses an ammeter to measure the current in a circuit. If a total charge of \( 15\text{ C} \) passes through the ammeter in a time of \( 2.0\text{ minutes} \), what is the reading on the ammeter?

A student connects a 100% efficient transformer to a high-voltage transmission line. The primary coil has \( 8000 \) turns and is connected to a \( 400\text{ kV} \) supply. The secondary coil has \( 400 \) turns.

If the power transmitted to the primary coil is \( 2.0\text{ MW} \), what is the current flowing in the secondary coil?

Which term is used to describe a region in which a magnetic pole experiences a force?

Two resistors, \(R_1 = 4\Omega\) and \(R_2 = 8\Omega\), are connected in series to a \(24\text{ V}\) battery.
What is the current flowing through the \(8\Omega\) resistor?

An electric lamp is connected to a supply of \( 240\text{ V} \). If the current flowing through the lamp is \( 0.25\text{ A} \), calculate the power dissipated by the lamp.

A step-up transformer has \(100\) turns on its primary coil and \(500\) turns on its secondary coil. If the primary voltage is \(12\text{ V}\) a.c., calculate the output voltage from the secondary coil.

In the circuit diagram provided, a battery of \( 6\text{ V} \) is connected in series with a \( 4\text{ }\Omega \) resistor and an \( 8\text{ }\Omega \) resistor. Calculate the potential difference across the \( 8\text{ }\Omega \) resistor.

A student conducts an experiment on electromagnetic induction using a bar magnet and a coil of wire connected to a sensitive center-zero galvanometer.

(a) Describe what is observed on the galvanometer when the north pole of the bar magnet is pushed quickly into the coil.

(b) State what happens to the galvanometer needle when the magnet is held completely stationary inside the coil.

(c) State two ways by which the magnitude of the induced electromotive force (e.m.f.) can be increased.

A step-up transformer at a power station is used to transmit electrical power through long-distance cables to a city.

The transformer increases the potential difference from \(25\text{ kV}\) to \(400\text{ kV}\). The power transmitted into the primary coil is \(200\text{ MW}\) (\(2.0 \times 10^8\text{ W}\)). Assume the transformer is \(100\%\) efficient.

(a) Calculate the current in the transmission cables (the secondary circuit).

(b) The total electrical resistance of the transmission cables is \(8.0\text{ }Ω\). Calculate the power lost as thermal energy in the transmission cables.

(c) Explain why transmitting electricity at very high voltages reduces power losses across long distances.