fairylights...principles and definition of ohms law

Application of Health and Safety and Electrical principles

Principles and Definition of Ohms

Law

Aims and objectives State what resistance is and what affects it. Understand Ohm’s law and how to apply it.

Resistance and

Ohm’s law

The very important equationV = I x R

is an expression of Ohm's Law.

Resistance is the opposition to the current flow

http://www.gcsescience.com/pe5.htm


How Electricity Works. Ohm's Law Clearly Explained

Available from: http://www.sengpielaudio.com/calculator-ohmslaw.htm

http://www.sengpielaudio.com/calculator-ohmslaw.htm

Electric CircuitAn electric circuit requires three things...

A source A load A means of transmission

Current can be simplified as a

‘flow of electrons’

The Source Of SupplyThe Source Of Supply provides an electromotive force (emf) to the circuit.The emf is the drive or push that is applied to the circuit to force the current around the circuit.

In this simple circuit we have a Source Of Supply (the cell), a load (what we want to use), and

transmission wires that link the source to the load.

Symbol is emfUnit is Volt (V)

+ -

Potential Difference

.

In the simple circuit the emf is the same as the voltage that we would measure across the load.This would not always be the case, because the load would be the potential difference or voltage, that is dropped across the load.

Potential Difference

The emf provides the force that drives the current around the circuit.

The circuit may have several loads connected to it, then the emf would be made up of all of the voltages dropped.

Volt DropThe voltages dropped across each resistor are

different to the emf , but combined they add up to the same value of the source emf

Although emf and volt drop (potential difference) are measured in volts the symbol is different.

Symbol: U or V Unit: Volt (V)

∆U = voltage dropV or U for the dimension 'voltage'

In lots of mathematical and similar contexts, capital ∆ is used to denote a change in something

It means a change in voltage — in this instance, between one point in the

circuit and another.

Ohm’s Law

In 1827, George Simon Ohm discovered that if you have metal, maintained at a constant temperature for a constant length and area, and then if you then increase the voltage the current increases in the same proportion. He discovered that resistance of the conductor linked the voltage to the current.

Ohm’ law triangle

Voltage forms a relationship with current

U UU IR I RR I

= = =

Resistance Resistance measured in Ohms (unit Ω)

When any material has an electrical supply connected to it, it has a particular

resistance. This resistance

is called the resistivity of the material

or its specific resistance

Specific resistance of materialParticular materials have a specific resistance

If the length of the material increases the resistance will increase in proportion.If the same material changes in size or area it will change its resistance.

Increasing the area reduces the resistance.

(rho) is the symbol for resistivityR = l

AWhere R = resistance ()

= resistivity (m)l = length(m)A = area (m2)

Ohm's Law is the linear proportionality between current and voltage that occurs for

most conductors of electricity. A graph of voltage against current is a

straight line. The gradient is the resistance.

Principles and Definition of Ohms Law

Resistance

Resistance is measured in Ohms (symbol )

Resistance is a measure of how much the

current is slowed down.

The bigger the resistance,

the smaller the current.



Test Circuit for a Component.Anything in an electric circuit

(e.g., lamp, resistor, motor, diode) is called a component.

Each component has its own circuit symbol.


A test circuit is used to find a range of voltages and currents for a component.



Components which obey Ohm's Law are Wires and Resistors.A component will only obey Ohm's Law at constant temperature(meaning that the temperature must not change).




If the resistance of a component is constant (stays the same) For different values of V and I,

then a plot (graph) of V against I will be a straight line.

The gradient (slope) of the line shows how big the resistance is.



In reality, an increase in current through a component will change

its temperature (the temperature usually goes up)

and so Ohm's Law is only an approximation but it works quite

well for many components.


The thin wire (filament) inside the light bulbgets very hot when a current flows through it and it glows brightly.This rise in temperature causes an increase in resistance of the filament, and so the gradient (slope) of the plot is seen to increase.


Components which don't obey

Ohm's Law

Calculate the resistance of a component.(A component can convert electrical energy into heat)

Firstly, we need to measure the current flowing through the component, and the voltage across the component.The circuit below shows where to place an ammeter and a voltmeter.If the ammeter reads 2 A, and the voltmeter reads 6 V,

then V = I x R R = V divided by I = 6 / 2 = 3 Ohms

The same method can be used to calculate the resistance of any component.



http://www.gcsescience.com/pen2-heat.htm









A thermistor is a special type of resistorwhich has been deliberately manufactured so

that its resistance decreases as its temperature rises.


Components which don't obey

Ohm's Law

Current in a Series CircuitThe current in a series circuit is the same everywhere. An ammeter placed anywhere in a series circuit always gives the same reading.

In the circuit above, A1 = A2 = A3 = A4.



If an identical cell (battery) is placed in series with the original cellthe current doubles because the total voltage of the circuit doubles.However, two cells together provide electricity for only the same amount of time as one cell before they both run out.

Current in a Series CircuitPrinciples and Definition of Ohms Law



Switches and Lamps in Series Circuits

An open switch in a series circuit will turn everything off, because the circuit will be disconnected from the cell.


When lamps are connected in series,the more lamps in the circuit the dimmer they get, because the voltage is divided between them. If one lamp in a series circuit breaks or fails,all the others will go out with it.

For this reason, lamps are always connected in Parallel (except Christmas Tree Lights or Fairy Lights,where the large mains voltage is conveniently divided between the lamps).

Switches and Lamps in Series Circuits




Current in a Parallel Circuit.1. The current in a parallel circuit depends on the resistance of the branch.2. The total current flowing in to the branches is equal to the total current flowing out of the branches.A1 = A5




The current at A2 flowing through the 2 Ohm resistor can be found using V = I x RIf the supply voltage is 12 Volts…

I = V divided by R= 12 / 2

= 6 Amps

Current in a Parallel Circuit.Principles and Definition of Ohms Law



You would get the same answer for the 2 Ohm resistor, whether or not the other resistors are connected in the circuit.For parallel circuits, each component behavesas if it is connected independently to the cell, and is unaware of the other components



If an identical cell (battery) is placed in parallel with the original cell the current stays the same because the total voltage of the circuit is the same.Two cells together provide electricity for twice as longbefore they both run out.




A switch at S1 or S5 will switch all the lamps off and on together(assume that all the other switches are "closed" which means "on")

Switches and Lamps in a Parallel Circuit


The switch at S2 will only light the lamp at L1.This is very useful because it means that we can switch the lamp on and off without affecting the other lamps.



Similarly, the switch at S3 will only light the lamp at L2.The switch at S4 will only light the lamp at L3.



The brightness of the lamp does not change as other lamps in parallel are switched on or off.For this reason lamps are always connected in parallel (except Fairy Lights).




fairylights...principles and definition of ohms law

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