Ohm’s Law

Ohm’s law is about three things:

Voltage (V)
Current (I)
Resistance (R)

Just remember:

 V = I x R

This equation is what Ohm’s Law says. It is a very simple bit of physics.


This is sometimes called potential difference or PD. It is measured in Volts. If you have not learnt about voltage before, you will certainly have seen something like 1.5v on the side of the batteries which you put in your walkman. The “V” in the formula given above stands for voltage. Here are some typical values:

70mV the voltage across the inside and outside of a human nerve
1.5v the voltage of a walkman battery
6v the voltage of a moped battery
12v the voltage of a car or motorcycle battery
24v the voltage of a 50 seater coach battery
110v mains voltage in the USA & some continental countries
240v nominal mains voltage in the UK
Thousands of volts voltages in amateurs’ antennas whilst transmitting
Tens of thousands of volts voltages in overhead power cables

As you can see, there is a lot of difference between the voltages in our nerves and muscles and the voltages in the mains power supply. It does not make a lot of sense to put your fingers in the mains power sockets! So be careful not to hang onto my long wire antenna when I am transmitting. You can charge yourself up with a van de Graaf generator to a few thousand volts, but the currents involved are quite small, it is not the same with the mains which could give you a lethal shock. I have tried it once and it was not a pleasant experience.

1.5 volts might be enough to light up a small tent with a torch, but not enough to light up your living room.

1mv (one millivolt) is 1 thousandth of a volt.

1Mv (one Megavolt) is 1 million volts.

Batteries, the mains, dynamos and electrical generators provide the energy to force electrons around electric circuits. The bigger the voltage is, the greater the “force” making electrons go round a circuit. You can think of it as being like a hill: if you fall down the hill you could roll to the bottom. The steeper the hill is the quicker you will roll down it.


Current is measured in amperes or amps for short. We use the symbol “I” in the formula to represent current. (The reason for using “I” rather than “C“, is that “C” is already used for something else.) The kinds of current flowing in our nerves and muscles is only a few microamps: the currents flowing in the mains might be as much as 13 amps. The currents flowing in my antenna could be very much higher! Please don’t touch my antenna whilst I am transmitting.


Resistance is to do with how easy it is for the electric current to flow through a material, e.g. a piece of copper wire. Although your physics teacher will tell you that copper is a very good conductor of electricity, it does have a measurable resistance. Some materials have virtually no resistance when they are cooled down to absolute zero, they are called super conductors. Mercury will do this.

Materials like plastic, wood, polythene, ceramics and rubber have very high resistances so that it is almost impossible for electric currents to flow through them. These materials are called insulators. They may not be perfect.

Materials like copper, silver, and gold have very low resistances. In fact all metals will conduct electricity. They are called conductors. Even so, they do have some resistance to the flow of electrons through them. A perfect conductor is called a superconductor, it has zero resistance; very cold mercury acts as a superconductor.

Resistance is measured in Ohms.

Ohm’s Law:

Ohm’s Law says that there is a relationship between these three factors. So if you know two of the values you can easily work out the third one.

V = I x R V
I = V / R
R = V / I I R

The “V over I and R triangle” should help you to remember these three equations. If you know the current and resistance and want to calculate the voltage, you use the first equation. If you know the voltage and resistance and want to calculate the current, you use the second equation. Lastly, if you know the voltage and current and want to calculate the resistance, you use the third equation.

Note: these pages were first written for students of the Radio  Amateurs Exam

I have written a page about powerwhich is measured in watts. It is quite easy to calculate the power dissipated  in a circuit using the equation W = V x I (some people use P = V x  I). It is important for us to know about power because our licences limit  how much power we are allowed to use. In the UK the maximum power allowed is  400w, though we are limited to smaller power levels on some bands. If you cannot  calculate and measure power levels you will end up by breaching the terms and  conditions of your licence and losing it.


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