Powering Earth

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Question 25
Medium

This question is about how electricity is transferred in power networks.

a.

State:

  1. What a step-down transformer does to the potential difference.
  2. The primary reason for using a step-down transformer before electricity enters homes.
[2]
b.

The table below shows information for a transmission-line transformer:

QuantityValue
Potential difference across primary coil (VpV_pVp​)275 000 V275\,000\text{ V}275000 V
Current in secondary coil (IsI_sIs​)1500 A1500\text{ A}1500 A
Potential difference across secondary coil (VsV_sVs​)33 000 V33\,000\text{ V}33000 V

Assuming the transformer is 100%100\%100% efficient, calculate the current in the primary coil (IpI_pIp​) of the transformer.

[3]
c.

An oscilloscope is used to view the potential difference across the secondary coil of a transformer connected to the domestic mains supply. The screen shows a sinusoidal wave that constantly changes polarity between positive and negative peaks.

  1. Identify the type of current supplied by the mains.
  2. Explain how this type of current is indicated by the oscilloscope trace.
[2]

Powering Earth Questions

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