Transformers

Four identical magnets slide down four ramps P, Q, R and S of the same length and width. The slope of each ramp is the same.

The table shows the resistivity and thickness of each ramp.

Friction is the same for each ramp. The time t for each magnet to slide down the entire length of its ramp is measured.

Which row shows the ramp with the smallest t and the ramp with the largest t ?

A transformer is used to connect an ac power supply to a 100 W filament lamp.

When the lamp is operating at its rated power, the power supply has a peak potential difference of 50 V and supplies a peak current of 5.0 A.

What is the efficiency of the transformer?

Which change will increase the efficiency of a transformer?

A transmission cable consists of many strands of wire. Electrical energy is transmitted along the cable at a frequency of 50 Hz.

Which change gives the largest increase in the efficiency of the electrical energy transfer along the cable?

A transformer for use in a 230 V ac supply is 90% efficient. The transformer provides a current of 3.00 A at 12.0 V.

What is the current in the primary coil?

Figure 7 shows a transformer.

Explain the functions of the core and the secondary coil.

Figure 8 shows a cross-section through the transformer core. Thin iron sheets are separated by material M.

Explain how the efficiency of the transformer is increased by constructing the core in this way.

Figure 9 shows a schematic diagram of a power transmission system.

Voltages between 33 kV and 400 kV are used for long-distance transmission.

Suggest why engineers have chosen 132 kV for this system.

The industrial consumers use 72 MW of power. Transformers 1 and 2 each have an efficiency of 98% and the transmission line has an efficiency of 94%.

Calculate the current in the 25 kV line from the power station.

The primary winding of a transformer has 200 turns and the secondary winding has 1600 turns. A root mean square (rms) alternating voltage of 25 V is applied to the primary winding causing a primary rms current of 4.0 A. The transformer is 90% efficient.

What are the rms values of the secondary voltage and the secondary current?

A transformer has an efficiency of 80% It has 7000 turns on its primary coil and 175 turns on its secondary coil. When the primary of the transformer is connected to a 240 V ac supply, the secondary current is 8.0 A

What are the primary current and secondary voltage?

What conditions for the transmission voltage and the transmission current give the most efficient transfer of energy through the National Grid?