FACTORS AFFECTING THE RESISTANCE OF A UNIFORM WIRE.pptx

FACTORS AFFECTING THE RESISTANCE OF A UNIFORM WIRE

1. FACTORS AFFECTING THE RESISTANCE OF UNIFORM WIRE GENERAL PHYSICS 2

2. OBJECTIVES Apply the relationship of the proportionality between resistance and the length and cross-sectional area of a wire to solve problems

3. 4 FACTORS AFFECTING THE RESISTANCE OF A UNIFORM WIRE: Cross Sectional Area Length Kind of Material Temperature

4. CROSS-SECTIONAL AREA Resistance varies inversely from the cross-sectional area (A) of the wire As the area increases, the resistance decreases R 1 A 1 =R 2 A 2

5. LENGTH The resistance of a wire is directly proportional to its length A longer wire has a greater resistance than a shorter wire of the same material and cross sectional area.  

6. MATERIAL OF THE WIRE The effect of the kind of material on the resistance of the wire is determined by its resistivity Resistivity ρ is the reciprocal of conductivity.

7. The Effect of A, L, and material on Resistance  

8. TEMPERATURE Resistance and resistivity vary with temperature. As temperature increases, resistance and resistivity increase for conductors and decrease for insulators and semiconductors The T dependence of R and ρ is nearly linear if the change in T is not too great.

9. TEMPERATURE An expression similar to linear thermal expression approximates the effect of T on R and ρ The linear relationship is given by: ρ T = ρ o + ρ o α (T-T o ) R T =R o + R o α (T-T o )

10. The linear relationship is given by: ρ T = ρ o + ρ o α (T-T o ) R T =R o + R o α (T-T o ) Where: ρ T - resistivity @ temperature T Ρ o - resistivity @ reference temperature α – temperature coefficient of resistance R T – resistance @ T R o – resistance @ reference temperature

11. SAMPLE PROBLEM # 1 RESISTANCE The ratio of the resistance of two wires made of the same material and having the same length is 1:4. Find the ratio of their areas and diameters.

12. SAMPLE PROBLEM #2 LENGTH A wire is 100 cm long has a resistance of 25 Ω . This wire is cut into five equal parts. What is the resistance of each piece of wire? If the five parts were bundled together, what is the resistance of the resulting wire?

13. SAMPLE PROBLEM #3 L, A, and material An insulated extension cord is made of a 1.5 m long copper rod with a diameter of 2.3 mm. What is the resistance of the extension cord? If an aluminum wire of the same length will be used instead of copper, what should be the diameter of the aluminum wire to have the same resistance?

14. SAMPLE PROBLEM #4 Temperature The resistance of a platinum wire at 30°C is 15 Ω . Find its resistance at 45°C. Note that α =0.0039/°C at 20°C.

15. OHMIC AND NONOHMIC RESISTANCE Resistance is also defined as the ratio of voltage to the resulting current. In symbols, R=constant= This is known as Ohm’s Law who investigated the relationship between current and voltage.  

16. SAMPLE PROBLEM: An electric flat iron draws a current of 15 A when connected to a 110 V line Assuming that the heating element of the flat iron obeys Ohm’s Law, what is its resistance? What is the resistivity of the heating element if it is a wire that is 2.5 cm long with cross-sectional area of 2x10 -4 m 2

17. EMF AND TERMINAL VOLTAGE Cell also offer small resistance to electric current this is called internal resistance (r) Because of its internal resistance, the potential difference across the terminals of a cell when current is flowing through is less than when there is no current.

18. EMF AND TERMINAL VOLTAGE The PD across the terminals when no current is flowing is called electromotive force ( ε ) EMF is the voltage indicated in a battery The PD across the terminals of a cell when current (I) flows through it is called terminal voltage (TV) This is the working voltage of the battery- the voltage for an external load, the EMF and TV are related: TV=  

19. SAMPLE PROBLEM The electromotive force of a cell is 1.5 V. When connected to an external load of 3 Ω , the voltage drops to 1.35 V. Find the Current Internal resistance of the cell

20. ELECTRICAL POWER AND ENERGY The power expended in an appliance is the rate at which it consumes electrical energy. Electrical power is obtained by multiplying the voltage by the current. In symbols, P = VI

21. ELECTRICAL POWER AND ENERGY If the voltage V is in volts and the current I is in amperes, then the power P is in watts. Power may also be expressed in terms of resistance. From Ohms Law, V=IR P=VI=I 2 R=  

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