alexandria said:
I just started learning about electricity and magnetism, and I am not really understanding what potential difference means, its making questions like these seem really confusing. Correct me if I am wrong, but from what i read, potential difference is either the amount of energy required to move a coulomb of charge from one point to another in a circuit, but then my lesson also says that potential difference is the decrease in electrical potential energy for each coulomb in a circuit. I am a bit confused, can someone explain to me (in english) what potential difference is??
You are correct in your definition: Electrical Potential difference between two points is the work required to move a unit charge from one point to the other.
Now, work (seen from the charge) can be positive (external actor does the work on the charge, i.e. gives the energy) or negative (the charge does the work, i.e. gives the energy to the surroundings).
In a circuit, the power supply is the source of energy and charge (a chemical reaction in a battery for example). It gives the charges energy by placing them all in one location (every charges feel the (repulsive) electric fields of their neighbours, that is why the battery needs to work on the charges to place them there). the difference of potential between one side of the battery (the +), where the positive charges having this energy are located, and the reference (the -) is a positive potential difference. energy is given to the charge: this positive potential difference is called emf.
In a resistor made of a lattice of atoms, the charges give the energy to the lattice by interacting with it (the lattice gains KE, that translate in T increases vs the surrounding, so heat towards outside the circuit (transfer of energy). Here the potential energy of the charges decreases. The potential energy per unit charge (the electrical potential) will be lower after the resistor than at its entrance. The potential difference is negative: it is called a potential drop.
Conclusion: both cases are potential differences, it is their sign that differs expressing if the circuit receives the energy (+) or loses it (-). You can imagine a circuit like a box: when the PD is positive it's an entrance to energy, when the PD is negative, it's an exit. The magnitude of the difference shows how much energy moves through the door per unit charge. If you take this quantity per second, you get power delivered (when the PD is +) or power dissipated (when the PD is -).
