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OHM'S LAW

A brief over view of history and theoretical concept of Ohm's Law

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OHM'S LAW

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  1. Much of the modern electrical sciences discovered in 19th C. Published 4 key laws of electromagnetism 1865 Invented & developed the telephone 1970 - 1975 Developed the alternating current induction motor 1880s

  2. Ohm’s Law • Discovered in 1825 • Relates 3 key quantities in electrical circuits • Voltage (V) • Current (I) • Resistance (R) • V = I x R • Voltage = Current x Resistance • In scientific units: Volts = Amperes x Ohms • Think of the voltage as the FORCE which is DRIVING the total • electrical flow rate (current), against the resistance encountered • in a portion of an electrical circuit. Georg Simon Ohm

  3. In scientific units: Volts = Amperes x Ohms Alessandro Volta 1745 - 1827 Andre-Marie Ampere 1775 - 1836 Georg Simon Ohm 17889 - 1854 Ohm discovered the merger

  4. Voltage = (electrical) Current x (electrical) Resistance Compare to pushing or cycling a bike up a hill 1) The force is your capacity for work to push or cycle the bike (or to ‘drive’ it); that is like the Voltage in a circuit. 2) The resistance is like the friction force on the tyres, the stiffness of the bike components, and the steepness of the hill; all these factors work together to determine the rate of progress for a given force. 3) The rate of progress (up the hill) – is similar to the “current” in a circuit, which measures the total passage of electricity in a given time through a particular point. resistance current in wire e- Electromotive Force = VOLTAGE e- e- wire

  5. Ohm’s Law V = I x R Voltage = Current x Resistance e- = an electron, the basic physical unit of a current. (billions of billions pass through a mains circuit every second). resistance current in wire e- Electromotive Force = VOLTAGE e- e- wire The wire is not realllly on a slope, like the example of the bike up the hill. It is not gravitycreating the resistance to the work done: it is the material of the wire itself! Some materials – such as metals and water- are ‘electrical conductors’ which offer relatively little resistance to electrons passage through the material.

  6. Suppose a wire has twice the resistance Doubling the resistance of the circuit wire will mean twice the electromotive force (voltage) required to drive the same current through the circuit. resistance current in wire e- Electromotive Force = VOLTAGE e- e- wire The greater the electrical resistance, the greater the applied voltage V needs to be to drive the same current I

  7. Ohm’s Law in practise • A wire is a fixed material, so: • Usually the resistance of a wire is not varying • So the value of ‘R’ in the equation V = I x R is fixed in practise. • What is varied is the Voltage, V • As the Voltage is increased, the current increases Rearranging the equation to express the fact that voltage drives the change in current: V = I x R Voltage = Current x Resistance In scientific units: Volts = Amperes x Ohms Volts / Ohms = Amperes Divide both sides by the ‘constant’ resistance: V / R = I x R / R = I Voltage / Resistance = Current

  8. Ohm’s Law in practise • Consider a wire with resistance 0.2 ohms. • Increases voltage in steps of 50 volts from 50 to 500. • Current is: Voltage/Resistance • Current ranges from 50/0.2 = 250 amps(A) in steps of 250A to 500/0.2 = 2500A

  9. Ohm’s Law in practise • Download the accompanying simple spreadsheet • Adjust the circuit resistance from 0.2 ohms to other values • Make a note of how the current values on the graph are changing in response. • What visual quantity on the graph best represents the circuit resistance?

  10. Ohm’s Law in practise • Download the accompanying simple spreadsheet • Adjust the circuit resistance from 0.2 ohms to other values • [Just change the value in cell A2 from 0.2 to say 0.5, 1.0, 2.0, 10.0 and drag • it down the column to A11] – the graph will automatically update. • Notice that the higher the resistance, the lower the current resulting. • This makes sense, because if a wire has higher resistance, then fewer • electrons can flow through it for a given applied electrical force (‘voltage’). • INPUT = VOLTAGE, working AGAINST = RESISTANCE, to give OUTPUT = CURRENT

  11. Ohm’s Law in practise INPUT = VOLTAGE working AGAINST = RESISTANCE to give OUTPUT = CURRENT

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