Topic explanation

Standard explanation

Terminal potential difference is measured across a source's terminals.

Key points

• Terminal potential difference is measured across a source's terminals.
• A real source is modeled by emf ε in series with internal resistance r.
• For a discharging cell, V=ε−Ir.
• Ir is the internal voltage drop or lost volts.
• With external resistance R, I=εR+r.
• For an open circuit, I=0 and V=ε.
• Increasing load current increases lost volts and lowers terminal voltage.
• For a charging cell, the applied terminal voltage is V=ε+Ir.

Labelled diagram