1

2 Georg Simon Ohm Georg Simon Ohm, , German physicist. Born: 16 March 1789 in Erlangen, Bavaria (now Germany) Died: 6 July 1854 in Munich, Bavaria, Germany Ohm's most important discovery (of 1826) now bears his name:.Ohm's importance was not recognized through most of his lifetime, but in 1852 he became professor of physics at the University of Munich.

3 The unit of resistance is called Ohm in honor of a German scientice by the name of Georg Simon Ohm, who discovered that when a conductor has a resistance of 1 ohm than an emf of 1 volt will cause a current of 1 amp to flow through a conductor. This is a very simple bit of physics. 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.

4 This equation is what Ohm's Law says. V = I x R I = V /R R = V / I

5 An easy way to remember the formulas is by using this diagram. To determine a missing value, cover it with your finger. The horizontal line in the middle means to divide the two remaining values. The "X" in the bottom section of the circle means to multiply the remaining values. If you are calculating voltage, cover it and you have I X R left (V= I times R). If you are calculating amperage, cover it, and you have V divided by R left (I=V/R). If you are calculating resistance, cover it, and you have V divide by I left (R=V/I). Note: The letter E is sometimes used instead of V for voltage.

6 In this simple circuit there is a current of 12 amps (12A) and a resistive load of 1 Ohm (1W). Using the first formula from above we determine the Voltage: V = 12 x 1 : V = 12 Volts (12V) If we knew the battery was suppling 12 volt of pressure (voltage), and there was a resistive load of 1 Ohm placed in series, the current would be: I = 12 / 1 : I = 12 Amps (12A) If we knew the battery was suppling 12V and the current being generated was 12A, then the Resistance would be: R = 12/12 : R = 1W

7 Voltage 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

8 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. Voltage

9 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. Voltage

10 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.

11 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.

12 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.

13 AH/AS TypeMH/MV TypeGF/MF Type QH Type CH Type CF/QF TypeCR/QR Type CD Type CE Type