Electric current
3.31 - Electric current
Electric current (I) is the rate of flow of charged particles, or the flow of charge per unit time.
Potential difference
Potential difference (V) is the energy transferred per unit charge between two points in a circuit.
Resistance
Resistance (R) is a measure of how difficult it is for charge carriers to pass through a component.
It is measured by dividing the potential difference across a component by the current flowing
through it.
Where V is the potential difference and I is the current.
Ohm’s law
Ohm’s law states that for an ohmic conductor, current is directly proportional to the potential
difference across it, given that physical conditions (e.g temperature) are kept constant.
Charge conservation
The principle of charge conservation states that the total electric charge in a closed system
does not change.
An application of the principal of charge conservation is Kirchoff’s first law, which states:
. The total current flowing into a junction is equal to the current
flowing out of that junction.
In series current rule
The current is the same everywhere in the circuit.
In a parallel circuit rule
The sum of the currents in each parallel set of branches is
equal to the total current.
Energy conservation
The principle of conservation of energy states that energy cannot be
created or destroyed, but can be transferred from one form to another. Therefore, the total energy
in a closed system stays constant.
The sum of all the voltages in a series circuit
The sum of all the voltages in a series circuit is equal to the
battery voltage or the sum of all the voltages in a loop is zero.
In a series circuit voltage rule
The battery p.d is shared across all elements in the circuit,
therefore the total sum of the voltages across all
elements is equal to the supply p.d.
In a parallel circuit rule
The potential difference across each branch is the same.
This is easy to see with the series circuit as it is a direct application of
Kirchoff’s second law (as described above) but takes a little more thought with the parallel circuit:
Combining resistances In series circuit
There are two rules for adding the resistances of resistors in circuits, which are used depending on
whether the resistors are in series or in parallel.
In a series circuit -
RT = R + R2 + R3 + …
Combining resistances In a parallel circuit
1/R1 + 1/R2 + 1/R3 = 1/RT
Power (P)
Power (P) is the energy transferred over time or rate of transfer of energy. It can be calculated by
multiplying the voltage across a component by the current flowing through it:
P =VI
Energy
transferred (W).
W= Pt
W = V It
Ohmic conductor
Ohmic conductor - this component follows Ohm’s law therefore its current-voltage graph
will look like a straight line through the origin. (This is provided physical conditions are kept
constant).
Semiconductor diode
when looking at the current-voltage graph of this component you
must consider its forward and reverse bias. The forward bias of a diode is the direction in
which it will allow current to flow easily past the threshold voltage, which is the smallest
voltage needed to allow current to flow. In the direction of the reverse bias, the resistance
of the diode is extremely high meaning that only a very small current can flow.
Filament bulb
This component contains a length of metal wire, which heats up as current
increases, therefore the resistance of this component increases as current increases. At
low currents the metal wire will not heat up significantly, therefore for very low currents,
Ohm’s law is obeyed. However, as the current increases (in either direction), the graph
begins to curve due to the increasing resistance.
(Negative Temperature Coefficient) Thermistor
This component acts in the opposite way
to a filament bulb because as it heats up (due to an increase in current), the resistance
across it will decrease. This is because increasing the temperature of a thermistor causes
electrons to be emitted from atoms, therefore the number of charge carriers increases and
so current increases causing resistance to decrease. Similarly to a filament bulb, at low
Resistivity
Resistivity (p) is a measure of how easily a material conducts electricity, it is defined as the
product of resistance and cross-sectional area, divided by the length of the material. Resistivity will
give the value of resistance through a material of length 1 m and cross-sectional area 1 m2 which
is useful when you need to compare materials even though they may not be the same size,
however resistivity is also dependent on environmental factors, such as temperature.
Potential along a uniform current-carrying wire
Consider a uniform current-carrying wire, which has constant resistivity and cross-sectional area.
Therefore, as the length of a wire increases, its resistance will increase uniformly. Using Ohm’s law
(V = IR), you can see that as resistance increases, potential will also increase.
This means that the potential along a uniform current-carrying wire increases uniformly with
the distance along it.
Potential divider circuits
A potential divider is a circuit with several resistors in series connected across a voltage source,
used to produce a required fraction of the source potential difference, which remains constant.
You can also make a potential divider supply a variable potential difference by using a variable
resistor as one of the resistors in series, therefore by varying the resistance across it, you can
vary the potential difference output. For example, if the resistance across R, increases, the output
p.d will decrease as circuit current has decreased and V=IR.
Potential divider circuits with thermistors and LDRs
You could replace the variable resistor in the circuit above with a thermistor or light dependent
resistor (LDR), in order to form a temperature or light sensor.
A light dependent resistor’s resistance decreases as light intensity increases.
LDRS
These types of sensors can be used to trigger certain events, for example in the circuit above, a
light dependent resistor is used. If the light intensity falls, resistance across R, will increase. This
will cause the total circuit resistance to increase and so the circuit current will decrease. Using
Ohm’s law (V = IR), you can see that this means that the voltage across R2 decreases, so the p.d
out decreases. If you want this effect to be reversed, you can switch the position of the LDR
and resistor, meaning that the p.d out would increase as light intensity decreases and so this
circuit could be used to cause a light bulb to be switched on, once a certain threshold voltage has
been met.
