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)

**resistance**is defined as its equation

### Equations

Q = I t

V = W/Q

R = V/I

## 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 lawthis 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

### 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

**Advantages of superconductors**

reduces energy dissipation

high-speed operation

high sensitivity

can generate large magnetic fields

**Disadvantages of superconductors**

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

brittle

expensive

# 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)

**resistance**is defined as its equation

### Equations

Q = I t

V = W/Q

R = V/I

## 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 lawthis 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

### 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

**Advantages of superconductors**

reduces energy dissipation

high-speed operation

high sensitivity

can generate large magnetic fields

**Disadvantages of superconductors**

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

brittle

expensive