Electricity

Basics

Definitions

current - the rate of flow of charge

electric potential - the amount of energy per unit charge

potential difference - the work done per unit charge (1V = 1C gaining/losing 1J)

electromotive force (emf) - the energy transferred to each unit of charge as it passes through through the source (p.d. across the cell)

resistanceis defined as its equation

Equations

Q = I t

V = W/Q

R = V/I

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Current-Voltage Characteristics

At A-level, you need to:

• know the shapes of the characteristics for an ohmic component, a filament bulb and a diode (V on horizontal axis and I on vertical and vice versa)
• be able to explain why the characteristics for each component are the shape they are
• describe an experimental process that will let you collect valid data to obtain the characteristic for a component

Ohmic component

• the potential difference is directly proportional to the current (straight line through origin) - Ohm’s law
• this means there is a constant resistance

Filament bulb

• at a low current, the potential difference is directly proportional to the current
• as the current increases, the temperature increases causing the ion lattice to vibrate more making it harder for electrons to flow meaning the resistance has increased
• for a given p.d., the current is less than for an ohmic conductor

Semiconductor diode

• in the forward bias direction, the resistance rapidly decreases after roughly 0.7V
• in the negative direction, there is a very high resistance (around 0 current) until voltage breakdown at -50V where there is basically 0 resistance

Potentiometer circuit

Potentiomenter circuits are used to find the characteristics for different components. They must be used in favour of just a variable resistor, as they provide a full range of currents (particularly I = 0A)

• when using this circuit to determine the characteristic of a semiconductor diode, a protective (or ballast) resistor must be used to protect the circuit components from too high a current

Resisitivity and Superconductors

Resistivity

resistivity is defined by the equation:

resistivity = resistance x cross-sectional area / length

or ρ = (RA) / L

where resistance is measured in ohms, length is measured in meters and area is measured in metres squared, meaning the unit of resistivity is the ohm meter (Ωm)

Thermistors - UNFINISHED

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Superconductors

a superconducting material is one with no resistivity below a critical temperature (often very low)

Uses of superconductors

• MRI scans - the weak resistance of superconductors allows very strong currents to flow with no heating in the material, and hence enables to get very high field values
• particle accelerators - they can produce very strong magnetic fields (due to high current) and particle accelerators use magnetic fields to accelerate the particles
• transformers and generators - they reduce energy dissipation in the transmission of electric power and reduce the fire risk
• microchips
• Maglev trains

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Advantages of superconductors

• reduces energy dissipation
• high-speed operation
• high sensitivity
• can generate large magnetic fields

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Disadvantages of superconductors

• only useful below critical temperature (often requires expensive cryogenic technology)
• brittle
• expensive

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