A-Level Physics: Alternating Current and the National Grid
Almost all the electrical energy we use arrives as alternating current, and A level Physics expects you to handle it quantitatively: to move between peak, peak-to-peak and root-mean-square values, to read an oscilloscope trace, to find the mean power in a resistor, and to explain with numbers why electricity is transmitted at very high voltages. This material covers those skills; the induction physics of transformers themselves is treated in the magnetic fields and induction material of this category.
The quiz (12 questions) begins with values of an alternating p.d.: the peak and peak-to-peak voltages of the 230 V rms mains supply, the relationships between peak, rms and peak-to-peak values, and the frequency and period of the mains. Oscilloscope questions describe traces in words so that you practise the reading: a peak-to-peak height and a Y-gain setting give the peak voltage, peaks a given number of divisions from the centre line give the rms value, and a cycle length with a time-base setting gives the frequency. Power questions check that mean power uses rms values and work out the mean power when a sinusoidal supply of known peak p.d. is connected across a resistor. The transmission questions show why losses fall so sharply at high voltage: comparing I²R losses at 25 kV and 400 kV for the same power, and finding the percentage of power lost on a 132 kV line. The set ends with the causes of energy loss in a real transformer and the role of step-up transformers.
The flashcards (12 cards) collect the rms formula, mean power, UK mains values, power loss in cables, period and frequency, peak-to-peak voltage, transformer efficiency, eddy currents, hysteresis and the time-base and Y-gain controls.
The written work (8 questions) asks you to explain the meaning of rms and why the mean current is zero, analyse a complete oscilloscope trace, carry out the calculations for a kettle on the mains, compare transmission losses at 11 kV and 400 kV, describe the stages of the transmission network in Great Britain with typical voltages, explain how transformer design reduces each type of loss, calculate the primary current of a step-down transformer from its efficiency, and compare an a.c. voltmeter with an oscilloscope.
The content is based on the alternating currents and transmission content found in A level Physics specifications in England (for example AQA 3.7.5.5 alternating currents and 3.7.5.6 the operation of a transformer, including transmission losses).
- Convert between peak, peak-to-peak and rms values of a sinusoidal p.d. or current
- Read peak voltage, rms voltage, period and frequency from oscilloscope settings
- Calculate mean power in a resistor supplied with a.c.
- Calculate I²R transmission losses and explain why the grid uses high voltages
- Explain the causes of energy loss in real transformers and how they are reduced
Practice material written by Zestly, based on the DfE GCE AS and A level subject content for physics (DFE-00356-2014): alternating currents, the oscilloscope, transformers and electricity transmission.
Sample question
The UK mains supply is $230\ \text{V}$ rms. What are its peak and peak-to-peak voltages?
- Peak $163\ \text{V}$, peak-to-peak $325\ \text{V}$
- Peak $325\ \text{V}$, peak-to-peak $460\ \text{V}$
- Peak $230\ \text{V}$, peak-to-peak $460\ \text{V}$
- Peak $325\ \text{V}$, peak-to-peak $650\ \text{V}$
See the answer
Peak $325\ \text{V}$, peak-to-peak $650\ \text{V}$
$V_0 = \sqrt{2}\,V_{\text{rms}} = 1.414 \times 230 = 325\ \text{V}$. The peak-to-peak value is from the positive peak to the negative peak: $2V_0 = 650\ \text{V}$. Dividing by $\sqrt{2}$ instead of multiplying gives $163\ \text{V}$.