Use these quick notes to help you revise each topic from the Chapter. You can also download a PDF with revision cards to print double sided and 2 slides per sheet to get small cards to play revision games Animated Science Physics AS Revision Cards Chapter 6 but the information is the same as on this post!

**AC Current and Power**

** Mains UK **

If the current constantly changes direction it is called alternating current, or ac. Mains electricity is an alternating supply. The UK mains supply is about **230V**. (RMS)

It has a frequency of 50 cycles per second or 50 Hertz, which means that it changes direction 50 times a second.

**Frequency Calculations **

If Mains frequency is **50 Hz** one cycle lasts **1/50 sec**

Hence we can say that

**0.02s = time period T**

**1/T = f = 1/ 0.02s = 50s ^{-1}**

**50s ^{-1} = 50Hz**

Easy way to think is more cycles per second is a higher frequency. Use the X-Scale for time and add it up to form a complete cycle. i.e. 1ms/div is 1 x 10^{-3}s per 1cm block.

**Peak to RMS**

When we talk about AC Voltage or Current it changes as a sinewave. This means that it is not steady and you cannot use normal V=IR equations. So we convert it from that to the “rms” or “DC” equivalent in terms of power or energy delivery. This means we can compare the two and then do normal circuit calcs.

**Mains Peak and RMS**

The AC supply goes from ±325V as a peak but we class this as only 230V D.C or RMS equivalent as there is a conversion formulae.

RMS voltage or Current = Peak voltage or Current / √2

You can think of it as being scaled down as RMS is always less than Peak.

**Circuit Calculations **

If you have the peak voltage you must convert to RMS before you use the normal equations i.e.

Peak Voltage (V_{o}) **= 5V **

so RMS Voltage = **5V/****√****2 = 3.5V**.

Then if we put 5V peak across a 4W resistor the RMS current would be

**3.5V / 4****W**** = 0.88A (**RMS Current)

**Power and AC**

We can also see from the idea that RMS voltage or Current = Peak voltage or Current / √2 so if Power = Voltage x Current…

**P _{rms} = V_{rms }x I_{rms} **

**= V _{o}/√2 x I_{o}/√2 **

**= P _{o}/2**

So the RMS power is half that of the Peak power. Which also makes sense as V and I are lower!

**Timebase**

On an oscilloscope the time base is a scale for the voltmeter. It means that when switched on you can see changes to V with time. This allows us to see a sinewave or decay of a capacitor. If turned off you only see a dot with DC or a vertical line with AC.

**Voltage Scale**

The vertical axis on an oscilloscope shows you the PD across whatever you connect it to. It is a simple way of comparing traces and can be scaled to enable you to better see a result. We can also use an oscilloscope to see peaks of sound which arrive a distance or time apart and work out their speed.