Current, Charge and Voltage
Charge (symbol Q)
Charge takes the unit Coulomb (C)
Electrons have negative charge, protons have positive charge.
The charge on an electron is 1.6 x 10-19 C
Current (symbol I)
Current is the rate of flow of charge ( how much charge is transferred
per second )
It takes the unit Ampere (A)
It is given by the equation I =Q/t (t is time in seconds)
Current is the same across components in series and is conserved at a
junction.
o Kirchoff’s first law is that ΣI=0 (which just means that the total
current entering a junction is the same as the total current
leaving the junction)
The current through a conductor is dependent on the charge carrier
density, the cross-sectional area of the conductor, the “drift velocity”
and the charge, with the relationship I=nAvq
o The charge carrier density is usually the number of electrons per
cubic metre, but can be ions instead of electrons if the question
specifies
o The cross-sectional area is like a road- the more lanes/space, the
more charge carriers can travel through in the same time.
o The drift velocity is another way of saying the total velocity-
electrons move randomly, but there will be an overall net
movement and therefore a net velocity (which is the drift
velocity), which is typically quite slow.
o The charge mentioned here is the charge on a single charge
carrier, which is usually the charge on an electron (1.6 x 10-19),
but may be different if the question states that the charge carrier
is an ion.
Current is measured by an Ammeter (with very little resistance) , which is
placed in series with the component being measured.
Voltage (potential difference and e.m.f, both with symbol V)
Voltage is the energy per unit charge.
It takes the unit Volt (V)
It is given by the equation V=E/Q
Voltage is shared across components in series, but is the same for
components in parallel.
There are 2 “types” of voltage (please don’t use the word “types” like
this in an exam)
Charge (symbol Q)
Charge takes the unit Coulomb (C)
Electrons have negative charge, protons have positive charge.
The charge on an electron is 1.6 x 10-19 C
Current (symbol I)
Current is the rate of flow of charge ( how much charge is transferred
per second )
It takes the unit Ampere (A)
It is given by the equation I =Q/t (t is time in seconds)
Current is the same across components in series and is conserved at a
junction.
o Kirchoff’s first law is that ΣI=0 (which just means that the total
current entering a junction is the same as the total current
leaving the junction)
The current through a conductor is dependent on the charge carrier
density, the cross-sectional area of the conductor, the “drift velocity”
and the charge, with the relationship I=nAvq
o The charge carrier density is usually the number of electrons per
cubic metre, but can be ions instead of electrons if the question
specifies
o The cross-sectional area is like a road- the more lanes/space, the
more charge carriers can travel through in the same time.
o The drift velocity is another way of saying the total velocity-
electrons move randomly, but there will be an overall net
movement and therefore a net velocity (which is the drift
velocity), which is typically quite slow.
o The charge mentioned here is the charge on a single charge
carrier, which is usually the charge on an electron (1.6 x 10-19),
but may be different if the question states that the charge carrier
is an ion.
Current is measured by an Ammeter (with very little resistance) , which is
placed in series with the component being measured.
Voltage (potential difference and e.m.f, both with symbol V)
Voltage is the energy per unit charge.
It takes the unit Volt (V)
It is given by the equation V=E/Q
Voltage is shared across components in series, but is the same for
components in parallel.
There are 2 “types” of voltage (please don’t use the word “types” like
this in an exam)
