electricity

Circuit Symbols

Electric Charge and Current

• for charge to flow:

  • circuit must be closed

  • there must be a source of potential difference

• current = the flow of charge

• charge = current x time

  • charge in coulombs (C), current in amperes (A) and time in seconds (secs)

• in a single close loop, the current has the same value at every point

• the current through a component is dependent on the resistance and potential difference through it

• the greater the resistance, the smaller the current (for a given pd)

• potential difference = current x resistance

• potential difference in volts (V), current in amperes (A) and resistance in ohms (Ω)

Resistors

• if the resistance is constant (ohmic conductor = device that follows ohm’s law) current is directly proportional to potential difference so the graph is linear

• if the resistance of components such as lamps, diodes, thermistors and LDRs is not constant it changes with the current through the component so the graph is not linear

  • eg. resistance of filament lamp increases as temp increases

  • eg. current through a diode flows in only one direction as it has a high resistance in the reverse direction

How does the resistance change?

• with current

  • as current increases, electrons have more energy

  • when electrons flow through a resistor, they collide with the atoms in the resistor

  • this makes it more difficult for electrons to flow through the resistor

    so resistance increases and current decreases

• with temperature

  • normal wires - the same process above occurs as atoms vibrate when hot

  • thermistor = as temperature increases, resistance decreases - these are often used in temperature detectors and thermostats

• with light

  • LDR (Light Dependant Resistor) = the greater the intensity of light, the lower the resistance - these are used in automatic night lights

• with length

  • the greater the length, the more resistance

  • electrons have to make their way through more resistor atoms, so it is hard than using a shorter wire

• with voltage

  • diodes = allow current to flow freely in one direction but have high resistance in the other direction so no current can flow

Series and Parallel Circuits

• series

  • closed circuit

  • current only follows a single path and is the same everywhere

  • total resistance = R₁ + R₂ + … (R of each component)

  • potential difference is shared across the whole circuit

  • can only switch everything off at once

• parallel

  • branched circuit; current splits into multiple paths

  • current is shared across branches (not always equally)

  • the potential difference is the same across each branch

  • two resistors in parallel will have a smaller overall resistance than just one because charge has more than one route to take so only some will flow along each branch

Domestic Uses and Safety

Mains

• mains electricity is an AC supply - in the UK the domestic electricity has a frequency of 50 Hz and voltage of 230 V

• AC = alternating current; current continuously varies from +ve to -ve - current changes direction

• DC = direct current; current moves in one direction only - supplied by calls and batteries

Cabling

Power

• the amount of energy transferred per second; directly proportional to current and voltage

  • energy is transferred from chemical potential in the batteries to electrical energy in the wire to any form of useful energy in the device they power

energy transferred = charge x potential difference

• energy in joules (J), charge in coulombs (C) and potential difference in volts (V)

energy transferred = power x time

• energy in joules (J), power in watts (W) and time in seconds (secs)

Energy Transfers in everyday appliances

Electrical energy can be transferred by the appliance in different ways:

• kinetic energy for a motor, thermal energy in a kettle

work done = when charge flows through a circuit; is equal to energy transferred as all the electrical energy (ideally) gets transferred to the appliance

• power rating shows power used in watts so higher power rating means more energy used

National Grid

• a system of cables and transformers linking power stations to consumers across the UK

• electrical power is transferred from power stations to consumers using the National Grid

Transformers

• these change the potential difference

• step-up transformers

  • increase voltage from the power station to the National Grid

  • as power is constant, current decreases so less energy is lost

• step-down transformers

  • decrease voltage from the National Grid to the consumer for consumer safety

Charge

• a property of all matter

  • opposite charges attract and the same charges repel

Insulators don’t conduct electricity

• their electrons cannot flow throughout the material; they’re fixed

Conductors can conduct electricity

• their electrons can flow; they’re delocalised

Static Electricity

• when two insulators are rubbed together:

  • electrons are transferred from one object to the other

  • this forms a +ve charge on one and a -ve charge on the other so they attract

• if conductors were rubbed:

  • electrons would flow in/out of them cancelling out any effect

• sparking occurs when enough charge builds up, the objects are close but not touching

  • the “spark” is when the charge jumps from the highly negative object to the highly positive object to balance out the charges

Forces Exerted

• the charged objects experience a force - electrostatic force of attraction / repulsion

• greater charge = greater force

• closer together = greater force

  • non-contact force (can be felt when object aren’t touching)

Electric Fields

• arrows represent the direction a positive charge would go

  • closer to the charge = stronger force felt

  • more field lines = stronger charge and force felt