Electrical Concepts and Safety
Alternating Current (AC) and Direct Current (DC)
Alternating Current (AC):
Defined as the current that repeatedly reverses direction within a given scale.
Occurs due to an alternating potential difference.
Current and voltage can be measured over time, resulting in fluctuating values between positive and negative.
Key Characteristics of AC:
The direction of charge flow constantly swaps back and forth.
Commonly used in the UK’s main supply electricity (plug sockets).
Specifications in the UK:
Frequency: 50 Hz
Voltage: 230 V
The voltage fluctuates from +230 V to -230 V and back 50 times per second.
Frequency:
Defined as the number of complete waves passing through a certain point per second.
- Mathematical Representation:
ext{Frequency (Hz)} = \frac{1}{\text{Period}}
The period is defined as the time taken for one wavelength to pass.
Parallel Circuits
Direct Current (DC):
Defined as the current that only moves in one direction around the circuit, due to a direct potential difference.
In cases of DC, voltage is either positive or negative but flows in the same direction.
Common sources of DC include cells and batteries.
Voltage-Time Graphs:
Constructed using oscilloscopes which display current and voltage readings on a monitor.
Cables and Plugs
Electrical appliances are commonly connected to the main supply using three-core cables, which consist of three wires, each with a copper core and colored insulating plastic coating.
Wire Color Codes and Functions:
Brown Wire (Live):
Carries alternating potential difference of approximately 230 V from the main supply.
Blue Wire (Neutral):
Completes the circuit by carrying away current.
Voltage of 0 V.
Green and Yellow Wire (Earth):
Normally does not carry a current; used only when there is a fault.
Provides safety by preventing the appliance casing from becoming live if the live wire comes loose.
Potential Dangers of the Live Wire:
Touching the live wire can cause an electric shock as your body is at 0 V.
A large voltage across your body can induce a current to flow through you, potentially resulting in injury or death.
Any connection between live and earth can create a dangerous low resistance path, resulting in a dangerous current and possible fire.
Fuses
A fuse is a safety device found in plugs designed to melt if the current exceeds a safe limit, thereby breaking the circuit.
Common fuse ratings include:
13 A
3 A
Electrical Currents and Energy Transfer
Electrical Current:
Defined as a flow of electrical charge that only flows around a closed circuit if there is a potential difference.
Energy is transferred between stores via electrical appliances.
Energy Transfer Processes:
Electrical energy is converted into other forms when current flows:
Kinetic energy in electric motors (e.g., blenders, fans).
Thermal energy due to friction in motors (e.g., irons, kettles).
Moving charges do work against the resistance of a circuit, resulting in energy transfer.
Efficiency of Energy Transfer:
Calculated using the formula:
\text{Efficiency} = \frac{\text{Useful energy}}{\text{Total energy}}
Total energy transferred is dependent on the duration of operation and power of the appliance.
Energy Equations
Various Energy Transfer Formulas:
Energy Transferred = Power x Time
E = P t
Energy Transferred = Voltage x Current x Time
E = V i t
Energy Transferred = Charge x Voltage
E = Q V
Electrical Power and Potential Difference
Power:
Defined as the rate of energy transfer per second; the more energy that travels in a given time, the higher the power.
Energy transferred can be expressed as:
E = P t
Power Ratings in Appliances:
Appliances come with power ratings indicating their maximum power during operation.
Understanding these ratings helps customers choose appliances based on energy consumption:
Lower power ratings imply less electricity usage, making them cheaper to operate.
However, higher power ratings do not always translate to efficient energy usage.
More Power Equations
Additional Power Formulas:
Power = Current x Voltage
P = I V
Power = Current² x Resistance
P = I^2 R
Power = Energy/Time
P = \frac{E}{t}
The National Grid
Definition:
The National Grid refers to a network of power stations and cabling that carries electricity across the country, transferring electrical power from power stations to consumers.
Power Stations:
Generate heat energy which is converted into electrical energy.
Operate according to demand; production must match demand which fluctuates throughout the day.
Power stations often maintain spare capacity to cope with high demands and emergencies.
Transmission Techniques:
To efficiently transmit power, high potential difference (PD) and low current are used.
The issue with high current is energy loss through thermal energy as wires heat up.
It is more economic to raise the PD and decrease current to minimize energy loss.
Transformers
Function:
Transformers change voltage levels for efficient transmission:
Step-up transformers increase potential difference & decrease current.
Step-down transformers decrease potential difference & increase current.
Essential to make voltage safe for homes; high voltages can damage appliances.
Example of Voltage Levels:
Power Stations Output:
25,000 V at step-up to 400,000 V for transmission.
Step down to 230 V for household use.
Underground Cables
Advantages:
Less hazardous to aircraft.
Insensitive to weather conditions.
Reduces hazards associated with electric shocks.
Disadvantages:
Higher repair costs.
Expensive insulation necessary.
Overhead Cables
Advantages:
Cheaper than underground due to lower insulation and conductor material costs.
Easier and cheaper to repair.
Disadvantages:
Can disturb the view.
Presents hazards to aircraft.