ELECTRICITY Engineering Science – National 5

Voltage, Current & Resistance On the left is a hill with rain and a river, this will be used to illustrate electricity. Rain drops from the cloud fall onto the mountain by gravity this forces the water down the mountain as a river. In an electric cable the VOLTAGE is like a force pushing electrons (very small particles) against each other. In the river the water flows fast or slow. In an electric cable the CURRENT is the rate of flow of electrons. In the river things stop or slow the flow of water such as trees & boulders. RESISTANCE in electricity slows the flow of electrons and can be used for a number of purposes.

Voltage Current Resistance Voltage is measured in Volts and can also be called Potential Difference, Potential Drop or even the Electromotive Force (emf). All batteries (power supply’s) are measure in Volts, e.g. an AA battery is 1.5V’s. Current is measured in Amperes (Amps) and given the scientific notation I. The current is usually very small such as 6 mA or Amps. Resistance is measure in Ohms and given the scientific notation Ω. The value is written like this: 200 Ω or 200 R.

Ohm’s LawV = I x R This is one of the most important formula’s for you to learn. Ohm’s Law is used to find out the value of Voltage, Current or Resistance. An easy way to remember the formula is shown below. V V V IR V = I x R V V V = I x R I = V ÷ R R = V ÷ I V V V I = R ÷ V V V ÷

Electric circuits There are three main types of circuits: series and parallel circuits and combinations of series and parallel. Circuits have a variety of purposes and components are connected in a variety of combinations depending upon the purpose. Series circuit example The LED is connected in series with a 220 R resistor to protect the LED from too much current Parallel circuit example The two rows of LED’s are connected in series to provide the same voltage to each row.

Resistance in a series circuit This symbol represents the power supply such as a 1.5V battery. 220 R330 R To calculate the Total Resistance (R T ) of the circuit shown is very straightforward. Just add the two resistor values together. R T = R1 + R2 Therefore: R T = 220R + 330R R T = 550R R1R2

Resistance in a parallel circuit To calculate the Total Resistance (R T ) of the circuit shown is more complicated with a parallel circuit than a series circuit. 1/R T = 1/R1 + 1/R2 Therefore: 220 R 330 R R1 R2 There are two ways of calculating R T (R1 x R2) (R1 + R2) R T = This formula is used for any number of components in parallel. This formula is used for only two components in parallel. (220 x 330) ( ) R T = R T = 132 R Notice the difference in answers between the series and parallel circuits 550R for the series circuit and 132R for the parallel circuit.

Comparison between series and parallel circuits SeriesParallel Voltage (V)V T = V1 + V2 + V3 + ….. Voltage is the same across each component. Current ( I ) Current is the same across each component I in = I out Depends on the values of resistance and voltage Resistance (R or Ω )R T = R1 + R2 + …… 1/R T = 1/R1 + 1/R2 + …. or (R1 x R2) (R1 + R2) R T =

Power in an electric circuit Power is the rate of dissipation of energy and is normally measured in Joules but it can also be measured in Watts. Light bulbs are measured in watts and we can use power to calculate voltage and current using the formula below: Power (Watts) = Voltage (V) x Current ( I ) or P = V x I V V P VI