Electrical Fundamentals
Classified in Physics
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EQUATIONS
EMF and Potential Difference
emf = E/Q
V = emf -Ir
For negligible r, V = emf
Resistance
R = pL/A
CURRENT
Rate of flow of charge.
Measured using an ammeter.
POTENTIAL DIFFERENCE (P.D.)
Work done in moving a unit charge between two points.
(Work is done when a charge flows through a component, transferring energy to it.)
RESISTANCE
When a potential difference is applied across a component, a current will flow. The amount of current for a particular potential difference depends on the resistance.
OHMIC CONDUCTORS
An ohmic conductor obeys Ohm's Law.
Ohm's Law: Provided constant physical conditions, the current through an ohmic conductor is directly proportional to the potential difference across it.
IV Graph: A straight line indicates constant resistance.
IV CHARACTERISTICS
An IV characteristic is a graph that shows how the current (I) flowing through a component changes as the potential difference (P.D.) across it is increased. A shallower slope indicates less resistance.
Ideal Voltmeter: Infinite resistance, no current flows through it.
Ideal Ammeter: Zero resistance, no potential difference across it.
RESISTIVITY
Resistivity of a material describes how difficult it is for current to flow through it.
SEMICONDUCTORS AND SUPERCONDUCTORS
Semiconductors
Semiconductors are a group of materials that are not as good at conducting electricity as metals due to having fewer charge carriers. However, if energy is supplied, they release more charge carriers, and their resistivity decreases. This property makes them suitable for sensors like thermistors, diodes, and LDRs.
Superconductors
Cooling a material generally reduces its resistance. If cooled to a specific critical temperature (transitional temperature), the resistivity of a superconductor disappears. This means no electrical energy is transformed into heat, resulting in no energy loss. However, most transitional temperatures are below 10 Kelvin, making them challenging to achieve practically.
POWER
Power is the rate of energy transfer (W), where 1W = 1J/s.
E.M.F. AND INTERNAL RESISTANCE
Internal resistance (Ir) arises from electrons colliding with atoms within a battery, causing it to warm up.
R represents load resistance (external resistance), while r represents internal resistance.
Electromotive force (emf) is the amount of electrical energy a battery produces and transfers to each coulomb of charge, measured in volts (V).
Terminal potential difference is the potential difference across the load resistance. Without internal resistance, the terminal potential difference would equal the emf. However, energy is always lost due to internal resistance. The energy wasted per coulomb due to internal resistance is referred to as lost volts.
On a graph of V against I, the y-intercept represents the emf, and the gradient represents -r (internal resistance).
CONSERVATION LAWS
Kirchhoff's First Law (Conservation of Charge)
The total current entering a junction equals the total current leaving it.
Kirchhoff's Second Law (Conservation of Energy)
The total emf around a series circuit equals the sum of the potential differences across each component.
Series Circuits
In a series circuit, the current is the same throughout, and the voltage is shared across components.
Parallel Circuits
In a parallel circuit, the current splits between branches, and the voltage is the same across each branch.