Pushing around

• Electromotive force (E.M.F.)

• Potential Difference

Electromotive Force (E.M.F.)Show on this circuit diagram the places where the electrons lose

energy and draw an arrow for the direction of the electrons.

Electrons lose energy

Electromotive Force (E.M.F.)Show on this circuit diagram the places where the electrons gain

energy and draw an arrow for the direction of the current.

Electrons gain energy

Electromotive Force (E.M.F.)A cell/battery provides the energy to the electrons to move around

the circuit (a bit like a water pump pushing the water around a house). This electrical pressure is called Electromotive Force (E.M.F.). The E.M.F. is measured in Volts (V).

Electrons gain energy

Electrons lose energy

Electrons move this way!

Direction of the current!

Potential difference (Voltage)How do the electrons lose energy inside the light bulbs?

The electrons give up their energy when passing through the thin wires in the light bulbs. That is how the filament inside the lamp gets hot and glows.

It’s a bit like a waterfall. The water has more energy at the top than at the bottom because there has been a fall in gravitational energy and the water loses energy when hitting the ground. This difference of electrical pressure across the bulbs is called Potential Difference (p.d.), or VOLTAGE

Higher gravitational energyLower gravitational energy

The water can’t get back up on its own. It has lost energy as heat and sound.

Electromotive Force (E.M.F.)Now put this circuit together and measure the potential difference

across each light bulb and the E.M.F. across the cell.

What do you notice?

The sum of all the drops in voltage must be equal to the gain in E.M.F. given by the power source (cell).

In other words, all the energy spent in the bulbs must be regained in the cell to keep the flow of electrons.