AQA AS Physics A : Electric Current

Key points from chapter 4 from Unit 1

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  • Created on: 27-05-10 10:08
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Current and Charge
Current is the rate of flow of charge, and is present due to the passage of charged particles.
The convention for the direction of current is from +ve to -ve. However, because the charge carriers
inside metals are conduction electrons, the electricity flows from ­ve to +ve.
Charge is measured in Coulombs. Q=I t (where Q = charge; I = current; t = time)
DEF COULOMB: One coulomb is defined as the amount of charge that passes in 1 second
when the current is 1 Amp.
Insulator - each electron is attached to an atom and cannot move away
from the atom. When a voltage is applied across an
insulator, no current passes through, because no electrons
can move through it.
Conductor - most electrons are attached to atoms, but some are not,
and these become the charge carriers. When voltage is
applied, the conduction electrons are attracted towards the
positive terminal of the metal.
Semiconductor - number of charge carriers increases with an increase in
temperature. Thus, the resistance of a semiconductor
decreases as its temperature is raised. Sometimes these are
referred to as intrinsic due to the electrons that break free
from atoms of the semiconductor. (Used in sensors for
detecting changes in the environment ­ thermistors, LDRs and
Potential difference and Power
In a current, energy transfer is due to electrons. The battery supplies each
electron with electrical potential energy. When the battery is present, each electron passing through a
component does work to pass through it and so uses some or all of its e.p.e. The work done by an
electron is equal to its loss of p.e. The work done per unit charge is defined as potential
difference/voltage across a component.

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DEFN P.D: work done / energy transferred per unit charge.
DEFN VOLT: Potential difference across a component is 1 volt when you convert 1 Joule of
energy moving 1 Coulomb of charge through a component.
DEFN EMF: the emf () of a source of electricity is defined as the electrical energy produced
per unit charge passing through the source.…read more

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NB: Area of a conductor with a circular cross-section is d2 or r2
For a conductor of length L and uniform cross-sectional area A, its resistance R is:
proportional to L
inversely proportional to A
Therefore, Resistance = constant for the material (resistivity) x length
Thus, = R x A
A superconductor is a wire or device made of material that has zero resistivity at and below a critical
temperature that depends on the material.…read more

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The resistance of an LDR depends on light intensity. The greater the light intensity shining on an LDR,
the lower its resistance. This works in exactly the same way as the thermistor, but instead of the
temperature, it's the light intensity.
Diodes only let current flow in one direction (LEDs are diodes too).
1. Forward bias is the direction in which the current is allowed to flow
2. Most diodes require a threshold voltage of about 0.…read more


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