1. Resistance Electricity Lesson 3

2. Learning Objectives To define resistance. To know what causes resistance. To know how to measure resistance.

3. Prefixes PrefixSymbolValue nano 10 6 k 10 9 μ milli

4. Prefixes PrefixSymbolValue nanon10 -9 megaM10 6 kilok10 3 gigaG10 9 microμ10 -6 millim10 -3

5. Resistance Resistance is a measure of how difficult it is for current to pass through a component. What causes it?

6. Resistance Resistance is caused by the collisions between the charge carriers in the material with each other and the fixed positive ions in the material through which the current is flowing.

7. Resistance The resistance of a component is defined as:- Or in symbols:-

8. Unit of Resistance The unit of resistance is the ohm (Ω) which is equal to 1 volt per ampere.

9. Example 1 If the current through a component is 3.4 nA when the voltage across it is 4.3 μV. Calculate:- A) The resistance at this current. B) The pd across the component when the current is 4.3 MA, assuming its resistance is unchanged.

10. Measuring Resistance A resistor is designed to have the same resistance regardless of the current flowing through it. Build a Circuit. Plot a graph! V vs I. Should get a straight line with the gradient equal to 1/R.

11. Assumptions The same current flows through the ammeter as the resistor. No current passes through the voltmeter. The resistance of the resistor remains constant as the current flowing through it changes. The gradient of the line only =1/R if the resistance remains constant.

12. Ohm’s Law Ohm’s law states that the p.d. across a metallic conductor is proportional to the current through it, provided the physical conditions do not change. If a conductor obey Ohm’s law then V is proportional to I and the gradient of the graph =1/R.