Voltage

1. Voltage Voltage = Energy

2. Voltage = energy • To make a light bulb light up, you need to supply energy. Where does this energy come from? • That’s right, a battery (or the house current if the lamp is plugged in). We will focus on the battery situation, though.

3. Role of a battery The role of the battery is to: 1. Supply the energy to do WORK upon the charge to move it. • 2. Pump the charge from – to + terminal within the battery. • 3. Maintain a voltage difference across the external circuit (wires, bulbs, etc.)

4. Some background… • Each electron carries kinetic energy. This energy is measured by a unit called a volt. • The number of electrons is measured by the charge, in coulombs. • A volt is the measure of the energy level anywhere in a circuit. • The battery does not supply electrons nor protons to the circuit; those are already present in the atoms of the conducting material (usually copper).

5. Electrical potential difference • There is a positive terminal of the battery and a negative terminal. The positive end is the high potential and the negative end is the low potential. Electricity, like water, moves from a place of high potential to a place of low potential. • Once the charge reaches the high potential end, it will naturally flow through the wires to the low potential terminal. • Voltage is sometimes called electrical potential difference. The potential difference of a fresh D cell battery is 1.5 volts.

6. How voltage works The little men (charges) leave the battery at the positive end. They are carrying full beakers of energy. The energy is used up to light the bulb. The charges with empty beakers return to the negative end of the battery where they climb the stairs (pumped up) and pick up more energy.

7. Water analogy • Work must be done on the water to move it from a location of low gravitational potential to a location of high gravitational potential. This work requires energy. This work is usually done by a pump. The pump is the battery. The energy is called voltage.

8. Once the water reaches a high gravitational position, it naturally flows to the low position. No pump is needed.

9. Check Your Understanding-1 1. Moving an electron within an electric field would change the ____ the electron. A. mass of     B. amount of charge on     C. potential energy of

10. CYU-1 • C. A force is required to move an electron with the electric field and thus increases its electrical potential energy. On the other hand, an electron moving opposite the direction of the electric field will decrease its electrical potential energy.

11. Check Your Understanding-2 2. If an electrical circuit were analogous to a water circuit at a water park, then the battery voltage would be comparable to _____. • the rate at which water flows through the circuit • the speed at which water flows through the circuit • the distance which water flows through the circuit • the water pressure between the top and bottom of the circuit • the hindrance caused by obstacles in the path of the moving water

12. CYU-2 D. The battery establishes an electric potential difference across the two ends of the external circuit and thus causes the charge to flow. The battery voltage is the numerical value of this electric potential difference. In an analogous manner, it is the difference in water pressure between the top of the water slide and the bottom of the water slide that the water pump creates. This difference in water pressure causes water to flow down the slide. Because of the similarity between electric potential difference in an electric circuit and water pressure in a water park, the quantity electric potential difference is sometimes referred to as electric pressure.

13. Check Your Understanding-3 3. If the electrical circuit in your iPod were analogous to a water circuit at a water park, then the battery would be comparable to ____. • the people which slide from the elevated positions to the ground • the obstacles which stand in the path of the moving water • the pump which moves water from the ground to the elevated positions • the pipes through which water flows • the distance which water flows through the circuit

14. CYU - 3 C. The battery in an electric circuit supplies the energy to pump the charge from its low energy terminal to the high energy terminal, thus providing a means by which the charge can flow. In an analogous manner, a water pump in a water park supplies the energy to pump the water from the low energy position to the high energy position. Because of the similarity between the battery in an electric circuit and a water pump in a water park, the battery is sometimes referred to as a charge pump.

15. Check Your Understanding-4 4. Which of the following is true about the electrical circuit in a flashlight? • Charge moves around the circuit at a fraction of the speed of light. • The batteries supply the charge (electrons) which move through the wires. • The batteries supply the charge (protons) which move through the wires. • The charge becomes used up as it passes through the light bulb. • The battery supplies energy which raises charge from low to high voltage. • ... nonsense! None of these are true.

16. CYU-4 E. As emphasized before, the battery supplies the energy to move the charge through the battery, thus establishing and maintaining an electric potential difference. The battery does not supply electrons nor protons to the circuit; those are already present in the atoms of the conducting material. In fact, there would be no need to even supply charge at all since charge does not get used up in an electric circuit; only energy is used up in an electric circuit.

17. Check Your Understanding-5 5. If a battery provides a relatively high voltage, it can: • do a lot of work over the course of its lifetime • do a lot of work on each charge it encounters • push a lot of charge through a circuit • last a long time

18. CYU-5 B. The electric potential difference or voltage of a battery is the potential energy difference across its terminals for every Coulomb of charge. A high voltage battery maximizes this ratio of energy/charge by doing a lot of work on each charge it encounters.