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# E004B Topic 1

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### E004B Topic 1

1. 1. Ohm’s Law V= I x R<br />V = voltage of the circuit<br />I = Current flowing in the circuit<br />R = Resistance of the circuit<br />Power formula P = V x I<br />P = power in watts<br />Revision02<br />1<br />Revision<br />
2. 2. Resistance is the opposition to current flow.<br />Resistance is measured in OHMS<br />Symbol for Ohms is Ω<br />We would say…<br />For the resistance of the heater element …..<br /> R = 27Ω<br />Revision02<br />2<br />Resistance<br />
3. 3. Length of conductor ()<br />CSA cross-sectional area (A)<br />Resistivity of the material (ρ)<br />The temperature also affects resistance.<br />R = ρ x <br /> A<br />Revision02<br />3<br />Factors affecting resistance<br />
4. 4. The temperature coefficient of resistance of a material is the change in resistance per ohm per degree Celsius.<br />The resistance of most metallic materials increases as heat increases.<br />This is positive temperature coefficient.<br />We have α0 at the resistivity at 00<br />And α20 at the resistivity at 200<br />Revision02<br />4<br />Resistance<br />
5. 5. Revision02<br />5<br />Resistance and temperature<br />
6. 6. All conductors have some resistance.<br />The CSA is important to determine the current carrying capacity of the conductor.<br />Small conductors have smaller current carrying capacity.<br />Large conductors have a larger current carrying capacity. <br />More resistance means more voltage drop.<br /> V = I x R<br />Revision02<br />6<br />Resistance<br />
7. 7. Depending on the material…some resistance will go up while others will go down.<br />With copper, the resistance increases.<br />This can affect the voltage drop of the cable.<br />Example 2.14 page 40p<br />Revision02<br />7<br />The effects of temperature on cable.<br />
8. 8. In general terms, resistors oppose current flow and as such we have a drop in voltage across the resistor.<br />Resistors get warm due to the opposition to current flow.<br />This can be likened to friction.<br />Revision02<br />8<br />Types of resistors<br />
9. 9. It is important to remember that resistors must be able to dissipate the heat which is caused.<br />Heat is a loss which is an inefficiency.<br />All resistors have a set POWER rating so we know how hot they can get before they are damaged.<br />Some large industrial resistors need a fan to aid in cooling.<br />Revision02<br />9<br />Resistors cont.<br />
10. 10. Commercial resistors are given a value of resistance which is known as a preferred value.<br />These values have a value and a tolerance.<br />Revision02<br />10<br />Resistor colour code.<br />
11. 11. Revision02<br />11<br />NTCs<br />
12. 12. Revision02<br />12<br />Temperature dependent resistor<br />
13. 13. Revision02<br />13<br />Potentiometer<br />
14. 14. Revision02<br />14<br />Light dependent resistor<br />
15. 15. Revision02<br />15<br />Resistor colour code<br />