CURRENT AND VOLTAGE
Current
 Number of coulombs per second
(by the way, do you see now why a coulomb
was defined to be such a ‘large’ amount of
charge?)
Voltage
 Number of joules per coulomb (i.e. the energy
carried by each coulomb of electrons)
We’ve got all this
energy! Let’s go
and give it to a
bulb, or a motor,
or something!
1V
1J
e
e e
e
e
The Coulomb
Gang
2V
1J 1J
e
e e
e
e
This battery gives
us twice as much!
Cells – Fixed Voltage
 A cell is a fixed-voltage device. It has its voltage
written on it.
 The cell will always give the same number of
joules to each coulomb coming through. A 5 V
battery will always give 5 J to every coulomb of
electrons.
 Somehow it is always able to keep up. Even if
the coulombs are coming through thick and fast
(high current), it will still manage to give 5 J to
every coulomb passing through.
electrons all start moving at
once when switch is closed
 We always refer to the voltage across a
component (not through it).
 The voltmeter tells you the difference between
the joules per coulomb going into the component
and the joules per coulomb coming out.
Conservation of Energy
 All the joules given by the battery to the
electrons are exactly used up by the time the
electrons get back to the battery.
 Question: You’ve already seen that you can
get different currents with the same battery,
depending on what you connect to it. What
difference will the size of the current make to
the lifetime of the battery?
2V
The Coulomb
Gang – all out
of energy
e
e e
e
1 J1 J
e
e
e e
e
e
V
This battery is giving 2 J
to every coulomb of
electrons passing
through.
The voltage is (+)2 V.
1 J1 J
e
e e
e
1J
e
e
e e
e
e
V
This bulb is taking 1 J
from every coulomb of
electrons passing
through.
The voltage is (-)1 V.