SI Units and OHMs Law

SI UNITS

The system of units used in Technology and Science is the Systeme Internationale d’unites (International system of units). Usually abbreviated to SI units and is based on the metric system. This was introduced in 1960 and is now adopted by the majority of countries as the official system of measurement. The basic units in the SI system are listed in the table to the right with their symbols.

PREFIX	NAME	MEANING
T	tera	multiply by 1 000 000 000 000 (i.e. X 10¹²)
G	giga	multiply by 1 000 000 000 (i.e. X 10⁹)
M	mega	multiply by 1000 000 (i.e. X 10⁶⁾
k	kilo	multiply by 1 000 (i.e. X 10³)
m	milli	divide by 1000 (i.e. X 10^-3)
µ	micro	divide by 1 000 000 (i.e. X 10^-6)
n	nano	divide by 1 000 000 000 (i.e. X10^-9)
p	pico	divide by 1 000 000 000 000 (i.e. X 10^-12)

Ohm’s Law

Provides us with a very important formula for working out current, resistance and voltage (Potential Difference). In order to use this formula properly you must understand SI Units.

Voltage = V Resistance = R Current = I

Problem 1. If the current through a resistor is 0.8A and the voltage is 20v - what is the resistance ?

Problem 2. Determine the p.d. (voltage) which must be applied to a 2kW resistor in order that a current of 10mA may flow.

Resistance R = 2kW= 2X 10³ = 2000 W.

Current I = 10mA = 10 X 10^-3 A or 0.01 A.

From Ohm’s law, potential difference,

V = IR = (0.01)(2000) = 20V

Problem 3. A coil has a current of 50 mA flowing through it when the voltage is 12 V. What is the resistance of the coil?

Have a go at answering the following questions:

Problem 4. A 100 V battery is connected across a resistor and causes a current of 5mA to flow. Determine the resistance of the resistor. If the voltage is reduced to 25 V, what will be the new value of the current flowing?

Problem 5. What is the resistance of a coil which draws a current of (a) 50 mA and (b) 200mA from a 120 V supply.

What is an INSULATOR ?

An insulator is a material that does not allow current to flow through it. Materials such as rubber and plastic are good insulators and this is why they are used in the electrical industry to insulate parts.
Consider electric drills. Why do they have a plastic case ? The plastic insulates the person using the drill from all the electrical parts so that they are protected from electrocution.

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