Potential Difference, EMF and Resistance

Energy per charge and what opposes the flow.

Potential difference is the energy transferred per unit charge between two points, so one volt is one joule per coulomb. The EMF of a cell is the energy it gives to each coulomb around the whole circuit. Under load the voltage at its terminals is lower because the cell has internal resistance.

Resistance opposes current. For a uniform wire it grows with length and shrinks with cross-section: a long thin wire resists more than a short thick one of the same material.

• Terminal voltage = EMF − I × internal resistance.
• Resistivity depends on the material, resistance also on size.
• Metals usually resist more when they get hotter.

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