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Revision notes for A-Level AQA Physics about topic 5, Electricity
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ElectricityCurrent
☆ Current is defined as the rate of flow of charge in a circuit.
☆ It is measured in Amperes.
☆ You should be aware of two different ways of describing current
flow:
1. Electron flow is from negative to positive and is the
direction that electrons actually flow in a circuit,
2. Conventional current flow is from positive to negative and
is more commonly used to describe direction.
☆ As an equation, current is defined as:
∆𝑄
I = ∆𝑡
☆ 1 Ampere is the current flow when 1 Coulomb of charge passes a
point each second.
Potential Difference
☆ Potential difference is equal to the work done per coulomb of
charge travelling in a circuit.
☆ As an equation this is defined as:
𝑊
V = 𝑄
☆ It is measured in Volts, where 1 Volt is equal to 1 Joule of work
being done by each Coulomb of charge.
☆ To measure the potential difference over a specific component, a
voltmeter can be used. It should be connected in parallel over
the component.
☆ An ideal voltmeter has infinite resistance, so that no current is
drawn from the circuit.
Resistance
☆ Resistance is a measure of how difficult
it is for current to flow in a circuit.
☆ It is directly proportional to the
potential difference in a circuit, and
inversely proportional to the current
flow.
☆ As an equation it is defined as:
𝑉
R = 𝐼
☆ It is measured in Ohms.
, Electricity
☆ An ohmic conductor is a conductor through which the current flow
is directly proportional to potential difference, when held at a
constant temperature.
Resistivity
☆ The resistance of a material depends on several different
factors.
☆ They are:
1. Cross sectional area
The greater the area, the easier the passage of
electrons,
2. Length
The longer the length, the harder it is to send a
current through it,
3. Resistivity
A property of the material that is assumed under
standard conditions.
☆ Combining these factors allows an equation for resistivity to be
formed:
𝑅𝐴
ρ = 𝐿
☆ The unit for resistivity is Ωm.
Superconductors and Semiconductors
☆ Superconductors are materials which have zero resistivity at and
below a critical temperature.
☆ This temperature differs for each material, but currently are too
low to be usefully used.
☆ If materials with high enough critical temperatures can be
developed, they could be used in electricity transmission to
reduce energy loss as well as in the production of strong
magnetic fields.
☆ Semiconductors are components for which the resistance changes
depending on external conditions.
☆ The two main types you need to know about are:
1. Light Dependent Resistors are light sensitive components -
when light intensity increases, their resistance decreases,
2. Thermistors are temperature sensitive components - when the
temperature increases, their resistance decreases.
Power
☆ Power is defined as the rate of energy transfer.
☆ It can be calculated using:
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