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Energized and De-Energized Charges
Energized and De-Energized Charges
Units for Potential Difference
Potential Difference
V = E / Q Q = E / V E = V x Q
- The units for potential energy per unit of charge are joules per coulomb (J/C)
- This combination of units has a special name - the volt (V)
- If one coulomb of charge has one more joule of potential energy at one point in a circuit, compared to another point, the potential difference between those 2 points is one volt
- You can write this relationship in mathematical form using V for potential difference, E for energy and Q for amount of charge
V = E / Q potential difference = energy / charge
- All forms of energy are measured in joules (J)
- However, when describing energy of electrons in a circuit, we describe the potential energy of each coulomb of charge, instead of the total energy of all the charges
- We also always compare one point in a circuit to another
- Potential difference means the difference in potential energy per coulomb of charge at one point in the circuit compared to the potential energy per coulomb of charge at another point in the circuit
Example
Batteries and Separated Charges
In a battery, 45 J of chemical energy are converted into electrical energy by separating negative and positive charges. This energy places 15 C of negative charge at the negative terminal, leaving a deficit at the positive terminal. What is the potential difference between the negative and the postive terminals in the battery?
Given: E = 45 J Q = 15 C
Need: Potential difference, V, in volts (V)
Analysis: To find potential difference, use V = E / Q
Solution: V = E / Q V = 45 / 15 V = 3.0 J/C or 3.0 V
Paraphrase: A battery that uses 45 J of chemical energy to separate 15 C of charge has a potential difference of 3.0 V between the terminals of the battery
- In a circuit, movement of electrons is continuous and controlled
- However, the reason that electrons move in a circuit is much the same as the reason they move in a lightning bolt
- An excess of electrons accumulate at one terminal of a battery, making it negatively charged
- This causes electrons to be drawn from the other terminal, leaving it positively charged
- The energized electrons now have the ability to do work on something else
- The electrical energy stored is called electric potential energy, but cannot do work until the battery is connected to a load and the circuit is closed
Lightning and Separated Charges
- The energy for pushing electrons through the air comes from separated positive and negative charges
- An excess of electrons accumulates in the lower region of the clouds, making the layer negatively charged
- A lack of electrons causes the ground to be positively charged
- The positively charged ground pulls on the electrons, while the negative charges in the clouds repel them
- For a brief moment, charges race through the air and a bolt of lightning appears