Ohms Law

The three parameters involved with Ohms law are voltage, current and resistance. Here we will give a brief introduction to these three parameters and give a basic explanation as to how each one works not forgetting we are keeping this as simple as possible.

Voltage - referred to as 'V' (also referred to as 'E' but we will stick to 'V')- this is the pressure behind the flow of current in an electrical circuit. Voltage 'pushes' the current around the circuit, you can have voltage without current but you cannot have current without voltage. Current also needs a complete circuit (including a resistance) in order to flow.
Try to think of voltage as the cause and current as the effect, in other words when we turn the switch to on, voltage causes the current to flow which in turn lights the lamp and gives an effect (the lamp being the resistance)

The circuit to the left is a complete circuit. The circle with a cross in it is the symbol for a light. Without going too far and confusing things the correct term for voltage is 'potential difference'. What is meant by that is the difference of potential at any two points of an electrical circuit. Again looking at the circuit to the left you can see in red that this circuit is fed by a 240 volt power source, so if we put an instrument which is capable of reading voltage (i.e. a voltmeter) one probe on the + and the other probe on the - the potential difference (reading on the voltmeter) would be 240.
With this circuit being complete and having a power source, and also a resistance (the light) current can now flow.

Just to show a incomplete circuit, which would be referred to as an open circuit, take a look at the circuit to the right, we have inserted a switch (in red) and the switch is open (in the off position) which means voltage can flow upto the switch but then has 'no where to go'. Because the voltage has not come up against a resistance at this point current will not flow.

Current - referred to as 'I' - to try to simplify current if we think of voltage as the 'energy carried by the charge' then current is the 'rate of flow' of that charge, or the 'rate' of movement of the charge. Possibly a good way of thinking is to compare it to the amount of water which flows through a hosepipe at a certain point. Current is the amount of charge which is flowing at a certain point on an electrical circuit. Current is measured in amperes (a)

Resistance - referred to as 'R' and has the greek letter omega (Ω) as its symbol - Resistance is quite simply what it sounds like, it is the amount of resistance an object, or material, resists or opposes to the flow of current.

When using basic ohms law or theory we will take all the above parameters as being constant, by that we mean there are other variables which can be taken into consideration such as the type of conductor or the temparature of the conductor, but for basic ohms law theory we will ignore these and move onto that later.

To put hopefully put in an easier way to understand, what Ohm found was that if we put a voltage of (V) volts across a resistance of (R) then a current of (I) amps will flow and the relationship is V=IR (volts = current x (times) resistance).
With these values being proportional to each other, if we know any two of these values we can calculate the remaining value. The three versions of Ohms law can be remembered by using the 'VIR' triangle, which you can see just above to the right

• To calculate voltage, (V) put your finger over V on the triangle and this leaves you with I R, so the equation is V = I × R (volts = current x (times) resistance)
• To calculate current, (I) put your finger over I on the triangle and this leaves you with V over R, so the equation is I = V/R (amps = voltage / (divided by) resistance
• To calculate resistance, (R) put your finger over R on the triangle and this leaves you with V over I, so the equation is R = V/I (resistance = voltage / (divided by) amps)

And that is all there is to basic Ohms law theory, quite simple really isn't it ?
Keep reading and below you will find some simple Ohms law circuits and a good video to introduce Ohms law.

Just to let you know the squiggly line in the following circuits represents a resistance.

• V = I x R or 100 = 10 x 10
• I = V / R or 10 = 100 / 10
• R = V / I or 10 = 100 / 10

• V = I x R or 200 = 20 x 10
• I = V / R or 20 = 200 / 10
• R = V / I or 10 = 200 / 20

Can you see the similarities with the above two circuits. Using the Ohms law triangle you can see that all equations run true and hopefully the way the answers are proportional to each other.

Now your turn
You'll find the answers below (under the guestbook)

Answer A = 20 Ohms
Answer B = 40 Ohms
Answer C = 15 amps
Answer D = 4 amps