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# Resistance

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1. Resistance

2. What is a resistor? • A resistor is an object that slows down the flow of electrons and changes electrical energy into a different form • Imagine construction on the highway forcing all the cars into one lane • The construction is the resistor and the cars are electrons

3. The filament of a lightbulb resists the flow of electrons, slowing down the electrons and transforming the electrical energy into light energy • Wires connecting a lightbulb to a battery have very little resistance, so the electrons travelling through the wire lose almost no electrical energy

4. Voltage • Voltage is the difference in potential energy of a charge between one point and another point in the circuit. • If you increase voltage you also increase current • Consider two circuits with the same voltage. One has large resistance (A), and the other has small resistance (B). Which do you think will have the larger current?

5. B has the larger current

6. Ohm’s law • If you know the current and the voltage of a circuit you can calculate the resistance using “Ohm’s Law”: • R=V/I • (sometimes written in the form V = IR) • Resistance is measured in ohms

7. What is the resistance of a flashlight bulb if there is a current of 0.75 A through the bulb when connected to a 3.0 V battery?

8. R = V/IR = 3.0/0.75R = 4.0 Ω

9. Try the following questions • The current through a load in a circuit is 1.5A. If the potential difference across the load is 12V, what is the resistance? • The resistance of a car headlight is 15Ω. If there is a current of 0.80 A through the headlight, what is the voltage across the headlight? • A 60V potential difference is measured across a load that has a resistance of 15Ω. What is the current through this load?

10. Answers • 8.0 Ω • 12 V • 4.0 A