Electricity P2

Electric Current definition

• The rate of flow of electrical charge in a circuit. • It is measured in Amperes (A) using an ammeter connected in series.

Electrical Charge equation

• Charge flow = Current × Time • Symbol equation: Q = It • Charge (Q) is measured in Coulombs (C), Current (I) in Amperes (A), and Time (t) in seconds (s).

Potential Difference (PD) definition

• A measure of the energy transferred per coulomb of charge that passes between two points in a circuit (also called voltage). • Measured in Volts (V) using a voltmeter connected in parallel.

Potential Difference, Current & Resistance equation

• Potential Difference = Current × Resistance • Symbol equation: V = IR • Potential Difference (V) is measured in Volts (V), Resistance (R) in Ohms (Ω).

Ohmic Conductor I-V characteristics

• Current is directly proportional to the potential difference across it at a constant temperature. • The graph is a straight line passing directly through the origin (constant resistance).

Filament Lamp I-V characteristics

• As current increases, the temperature of the filament increases. • This causes the metal ions to vibrate more, increasing resistance and causing the I-V graph to curve, flattening out at higher voltages.

Diode I-V characteristics & function

• Allows current to flow in one direction only (forward direction). • Has a very high resistance in the reverse direction, blocking current completely. • Graph shows zero current until a specific forward voltage threshold is met.

LDR (Light Dependent Resistor) behavior

• Resistance decreases as light intensity increases. • Used in automatic night lights, outdoor security lights, and street lighting.

Thermistor behavior

• Resistance decreases as temperature increases. • Used in digital thermostats, car engine cooling systems, and fire alarms.

Series Circuit: Current rules

• Current is exactly the same at any point in a series circuit (I₁ = I₂ = I₃). • There is only one single loop or pathway for the charge to flow through.

Series Circuit: Potential Difference rules

• The total potential difference of the power supply is shared between all the components (V_total = V₁ + V₂ + V₃).

Series Circuit: Resistance rules

• The total resistance is equal to the sum of the individual resistance of each component (R_total = R₁ + R₂ + R₃). • Adding more resistors in series increases the total resistance.

Parallel Circuit: Current rules

• The total current through the whole circuit is equal to the sum of the currents in the individual branches (I_total = I₁ + I₂ + I₃).

Parallel Circuit: Potential Difference rules

• The potential difference across each parallel branch is exactly the same (V_total = V₁ = V₂ = V₃).

Parallel Circuit: Resistance rules

• The total resistance of two or more resistors in parallel is always less than the resistance of the smallest individual resistor. • Adding more resistors in parallel decreases total resistance because it adds more pathways for current.

Mains Electricity: AC vs DC

• Alternating Current (AC): Current continuously changes direction, produced by alternating voltages (e.g. mains electricity). • Direct Current (DC): Current flows in one constant direction only, produced by a direct voltage (e.g. batteries and cells).

UK Mains Electricity values

• Frequency: 50 Hertz (Hz) (changes direction 50 times per second). • Potential Difference: 230 Volts (V).

Three-Core Cable: Live Wire

• Colour: Brown. • Function: Carries the alternating potential difference from the supply into the appliance. • Potential Difference: Approximately 230 V relative to the earth.

Three-Core Cable: Neutral Wire

• Colour: Blue. • Function: Completes the circuit by carrying current away from the appliance back to the source. • Potential Difference: Close to 0 V.

Three-Core Cable: Earth Wire

• Colour: Green and Yellow stripes. • Function: A safety wire that provides a low-resistance path to the ground if a fault occurs, preventing the metal casing from becoming live. • Potential Difference: 0 V (only carries current during a fault).

Live wire danger when switch is open

• A live wire is dangerous even if the appliance switch is open because the wire between the mains supply and the switch is still sitting at a high potential difference of 230 V. • Touching it allows current to flow through your body to the ground, causing electrocution.

Live wire and Earth wire short circuit

• Providing a direct connection between the live wire and the earth wire creates a very low resistance pathway. • This causes a massive surge in current, generating intense heat that can easily cause an electrical fire.

How Earth wire and Fuse protect you

• If the live wire comes loose and touches a metal casing, current immediately flows through the low-resistance earth wire. • This huge surge in current instantly exceeds the fuse rating, melting the fuse wire and cutting off the live electricity supply safely.

Electrical Safety: Fuses and Circuit Breakers

• Fuse: Contains a thin wire that melts and breaks the circuit if the current exceeds a specific rating, cutting off the live supply. • Circuit Breaker: An electromagnetic switch that opens and breaks the circuit instantly if current gets too high, easily reset without replacement.

Electrical Safety: Double Insulation

• Appliances with plastic casings that do not conduct electricity. • They do not require an earth wire because it is impossible for the outer casing to become live and shock a user.

Power Equations: P = VI and P = I²R

• Power = Potential Difference × Current (P = VI) • Power = Current² × Resistance (P = I²R) • Power (P) is measured in Watts (W), Current (I) in Amperes (A), Resistance (R) in Ohms (Ω).

Energy Transferred Equations: E = Pt and E = QV

• Energy Transferred = Power × Time (E = Pt) • Energy Transferred = Charge Flow × Potential Difference (E = QV) • Energy (E) is measured in Joules (J), Time (t) in seconds (s).

The National Grid definition

• A nationwide system of cables and transformers linking power stations to consumers (homes, factories, and schools) across the UK.

Step-Up Transformers

• Located at the power station end. • Increases potential difference and decreases current. • Purpose: Minimizes energy loss as heat in the transmission cables, making the National Grid highly efficient.

Step-Down Transformers

• Located at the consumer end near towns and homes. • Decreases the potential difference and increases current. • Purpose: Lowers the voltage to a safe, usable value (230 V) for domestic appliances.

Static Electricity: Production (Triple Only)

• When two insulating materials are rubbed together, friction transfers electrons from one material to the other. • The material that loses electrons becomes positively charged, and the material that gains electrons becomes negatively charged.

Static Electricity: Sparking mechanism (Triple Only)

• As static charge builds up on an object, the potential difference between the object and the earth increases. • If the potential difference becomes high enough, electrons jump across the air gap, causing a spark.

Electric Fields definition (Triple Only)

• A region around a charged object where another charged object will experience a non-contact force. • Force is strongest close to the object and gets weaker as distance increases.

Electric Field Lines rules (Triple Only)

• Always point in the direction a positive charge would move (away from positive charges, towards negative charges). • Lines must meet the surface of a charged object at right angles (90°).

Electric Fields and Sparking (Triple Only)

• A strong electric field ionizes the surrounding air particles, turning the air from an insulator into an electrical conductor, allowing charge to flow through it as a spark.