Electric Circuits Basics

Circuit Symbols

• There are 12 circuit symbols to learn
• The variable resistor has an arrow through it
• The thermister does not have an arrow
• The fuse has a straight line through it
• Arrows within circuit symbols represent light

Current

• Current is defined as the flow of electric charge
• The equation for current is Q=It where:
  • Q = Charge (Coulombs)
  • I = Current (Amps)
  • t = Time (Seconds)
• Current flows through wires via electrons
• Imagine wires as roads, electrons as cars, and charge as passengers

Potential Difference/Voltage

• Voltage drives electrons through a circuit
• Imagine voltage as the engine of a car

Resistance

• The equation for resistance is V=IR where:
  • V = Potential Difference (Volts)
  • I = Current (Amps)
  • R = Resistance (Ohms)
• Resistance can be affected by:
  • Strength of components (like resistors)
  • Filament lamps
  • Length and diameter of wires
• Imagine resistance as traffic on a road

Light Dependent Resistor (LDR)

• An LDR's resistance is dependent on light intensity
• Higher light intensity = lower resistance
• Remember the graph of the relationship between light intensity and resistance to remember this rule

Thermister

• A thermister's resistance is dependent on temperature
• Higher temperature = lower resistance
• Remember the graph of the relationship between temperature and resistance to remember this rule

Series Circuits

• Components are connected in a single loop
• Total potential difference is the sum of all PDs in the circuit
• Current is the same everywhere in the circuit
• Total resistance is the sum of all resistances in the circuit

Parallel Circuits

• Components are connected in branches
• Potential difference is the same everywhere in the circuit
• Total current is the sum of all currents in the circuit
• Current splits at junctions
• In two resistors wired in parallel, total resistance is lower than the smallest resistor

Electricity in the Home

• Red Wire (Live): Carries the current
• Green and Yellow Wire (Earth): Grounds the electricity for safety
• Blue Wire (Neutral): Completes the circuit
• Power equation: P=IV where:
  • P = Power (Watts)
  • I = Current (Amps)
  • V = Potential Difference (Volts)

The National Grid

• Power stations produce electricity
• The National Grid aims to deliver this electricity to homes
• Step-up transformers increase voltage in wires to reduce current:
  • This reduces resistance
  • Prevents overheating
  • Reduces energy loss through thermal energy
• Step-down transformers reduce voltage to a safer level
• Electricity is delivered to homes at 230 volts on average.