A1: Voltage is the amount of energy per unit of charge needed in moving from a lower electric potential to a higher electric potential. Alternatively, voltage is the amount of energy per unit charge released in moving from a higher to lower electrical potential.
Q2: What are the units for voltage?
A2: Voltage is measured in joules per coulomb (following from the definition given in A1). The volt (V) is defined as one joule per coulomb.
Q3: What is the voltage at a point?
A3: Recall from definition A1 that voltage is a relative physical quantity, concerned with differences in electric potential. Since there is no difference in electric potential at a point with respect to itself (at a particular instance in time), there is zero voltage.
Q4: So the voltage at any point with respect to itself is zero. But why do I see circuit diagrams with points labeled with nonzero voltages?
A4: It is true that the voltage at any point with respect to itself is zero. And it is perfectly valid to label points or nodes in a circuit with nonzero voltages. The solution to this apparent paradix: not all voltages are measured at a node with respect to that same node. That wouldn't be very useful - you'd have a mess of zeroes cluttering your diagram. Instead, one node is usually picked as a reference node, the node to which the voltages at all other nodes are measured with respect to. You can designate this reference node arbitrarily, but some selections may be intuitive than others. Because this reference node is widely used, we give it a name ground.
Q4: Why should we keep track of voltage with respect to time?
A4: Changes in voltage indicate that something interesting is happening in the circuit.
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