Circuits: Symbols, Charge, Current, Potential

Circuit Symbols

• Specific circuit symbols are used to represent electrical components in circuit diagrams.

Electrical Charge

• For electrical charge to flow, a circuit must be closed, meaning there are no open switches.
• A source of potential difference, such as a battery or cell, is required for charge to flow.

Electrical Current

• Current is defined as the flow of electrical charge.
• A greater rate of charge flow results in a greater current.
• Charge flow (Q) is calculated as current (I) multiplied by time (t): Q = It, where Q is in coulombs (C), I is in amperes (A), and t is in seconds (s).
• In a single closed loop, the current is the same at any point.
• The current through a component depends on its resistance and the potential difference across it.
• Higher resistance leads to a smaller current for a given potential difference.
• Potential difference (V) is calculated as current (I) multiplied by resistance (R): V = IR, with V in volts (V), I in amperes (A), and R in ohms (Ω).

Resistors

• For an ohmic conductor with constant resistance, current is directly proportional to potential difference, resulting in a linear graph.
• The resistance of components like lamps, diodes, thermistors, and LDRs is not constant and changes with the current, leading to nonlinear graphs.
• A filament lamp's resistance increases as its temperature rises.
• A diode allows current to flow in one direction only, offering high resistance in the reverse direction.

How Resistance Changes

• As current increases, electrons gain energy and collide with atoms in the resistor, causing the atoms to vibrate more and impeding electron flow, thus increasing resistance.
• In normal wires, higher temperatures cause atoms to vibrate, increasing resistance.
• Thermistors have lower resistance at higher temperatures and are used in temperature detectors and thermostats.
• Longer wires have greater resistance because electrons must navigate through more atoms.
• Light Dependent Resistors (LDRs) decrease in resistance with increasing light intensity and are used in automatic night lights.
• Diodes allow current to flow freely in one direction but have high resistance in the opposite direction.
• The gradient of a current-voltage graph is 1/resistance, so a steeper gradient indicates lower resistance.

Series and Parallel Circuits

• Series Circuits:
  • Current follows a single path in a closed circuit.
  • The current is the same at all points in the circuit.
  • Total resistance is the sum of individual resistances: Rtotal = R1 + R2 + ...
• Parallel Circuits:
  • Current splits into multiple paths in a branched circuit.
  • The total current entering a junction equals the total current leaving it.
  • The potential difference is the same across each branch.
  • The total resistance for two resistors in parallel is less than the resistance of the smallest resistor.

Series Circuits Explained

• Components are connected end to end.
• All the current flows through each component.
• All components can only be switch off at once
• The potential difference of the power supply is shared across each component.
• The current is the same through all parts of the circuit.
• Total resistance is the sum of individual resistances, meaning the total resistance is greater than any single resistor.

Parallel Circuits Explained

• Components are connected separately to the power supply.
• Current flows through each component independently.
• Each component can be switched off individually.
• The potential difference is the same across all branches.
• Current is shared between each branch.
• The total resistance is less than the branch with the smallest resistance as charge has multiple paths to take.

Domestic Uses and Safety

• Mains electricity is an AC supply with a frequency of 50 Hz and a voltage of approximately 230 V in the UK.
• AC stands for alternating current, where the current direction varies continuously.
• DC stands for direct current, where charge moves in one direction only, as supplied by cells and batteries.

Cabling

• A standard plug contains three wires: live, neutral, and earth.
• The live wire (brown) carries the alternating potential difference from the supply at 230V and can be dangerous even when the circuit is off.
• The neutral wire (blue) is at 0V and completes the circuit.
• The earth wire (green and yellow) is at 0V and is a safety wire that carries current only in the event of a fault.
• The earth wire connects to the earth and the appliance casing to prevent the casing from becoming live.

Power

• Power is the energy transferred per second and is directly proportional to current and voltage.
• Power loss is proportional to resistance and the square of the current.
• Energy is transferred from chemical potential in batteries to electrical energy in wires and then to other forms of useful energy in devices.
• Energy transferred is calculated as power multiplied by time: E = Pt.
• Energy transferred is also calculated as charge multiplied by potential difference: E = QV.
• Power (P) is measured in watts (W), potential difference (V) in volts (V), current (I) in amperes (A), and resistance (R) in ohms (Ω).

Energy Transfers in Everyday Appliances

• Electrical energy can be transferred into kinetic energy in a motor or thermal energy in a kettle.
• Work done is the energy transferred when charge flows through a circuit.
• The power rating of an appliance indicates the power it uses in watts, with a greater power rating indicating greater energy usage.

National Grid

• The National Grid is a network of cables and transformers that connects power stations to consumers across the UK.
• Electrical power is transferred using this grid.

Transformers

• Transformers change the potential difference.
• Step-up transformers increase the potential difference from the power station to the National Grid, reducing current (P=VI) and energy loss.
• Step-down transformers decrease the potential difference from the National Grid to consumers for safety.

Charge

• Charge is a fundamental property of matter.
• Positive and negative charges exist; equal amounts result in a neutral body.
• Like charges repel each other.
• Opposite charges attract each other.
• Insulators do not conduct electricity because their electrons are fixed.
• Conductors can conduct electricity because their electrons are delocalized and can flow.

Static Electricity (Physics only)

• When two insulators are rubbed together, electrons are transferred from one to the other.
• This transfer creates a positive charge on the object that loses electrons and a negative charge on the object that gains electrons.
• Conductors remain neutral when rubbed because electrons flow in or out, cancelling any effect.
• Sparking occurs when enough charge builds up and jumps through the air between objects to balance the charges.
• Charged objects exert an electrostatic force of attraction or repulsion on each other.
• A greater charge results in a greater force.
• Force is inversely proportional to the square of the distance between charges, and it's a non-contact force.

Electric Fields

• Electric fields are similar to magnetic fields but apply to charges.
• Fields point in the direction a positive charge would move: away from positive charges and towards negative charges.
• Field lines are at right angles to the surface.
• Stronger charges have more field lines and exert a stronger force.
• The force felt is stronger closer to the charge.

Solar System (Physics only)

• Our solar system consists of one star (the Sun), eight planets, dwarf planets, and natural satellites (moons).
• The Sun is at the center of our solar system (heliocentric model).
• Dwarf planets, asteroids, and comets also orbit the Sun.
• Our solar system is part of the Milky Way galaxy.
• The order of the planets is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
• Smaller planets are primarily made of rock, while larger planets are primarily made of gas.
• All planets orbit the Sun on the same plane and rotate at different speeds.
• Some planets rotate in the opposite direction or on a skewed axis due to past collisions.
• Larger planets have rings due to their strong gravitational fields attracting debris.

Solar System Model (Physics only)

• The initial model was geocentric, with Earth at the center and the planets, Moon, and Sun orbiting it in perfect circles.
• The heliocentric model, with the Sun at the center, was later developed, supported by Mars' retrograde motion.
• Galileo's observation of moons orbiting Jupiter showed not everything orbited Earth.
• Kepler demonstrated that planets orbit in ellipses, not circles.

Planetary Orbits (Physics only)

• As a planet orbits the Sun, the gravitational force constantly changes its direction, causing acceleration without increasing speed.
• For a stable orbit, if a planet moves closer to the Sun (decreasing orbital radius), the gravitational attraction increases, and so does the orbital speed.

Life Cycle of a Star (Physics only)

• Stars form from dust and gas clouds within a galaxy.
• Gravitational attraction draws gas and dust particles together.
• The cloud becomes concentrated, increasing temperature and pressure.
• Eventually, pressure allows gas/dust particles to fuse (hydrogen to helium), releasing energy.
• The energy release opposes gravitational collapse, forming an equilibrium and creating a stable star for billions of years.
• Eventually, the star runs out of gas to fuse and collapses.
• If the star is massive: It collapses, increasing core pressure and temperature, allowing heavier elements to fuse, leading to a supernova, leaving behind a neutron star or black hole.
• If the star is normal-sized: It collapses, with less fusion occurring, producing a planetary nebula and leaving behind a white dwarf.

Red Shift (Physics only)

• Light from galaxies moving away from Earth is red-shifted
• The change in speed with the distance shows expansion of the universe
• As the universe expands, the wavelength of light from a galaxy appears stretched, shifting it towards the red end of the spectrum.
• Frequency appears to decrease as each wavelength is emitted from a further distance.

Evidence for the Big Bang (Physics only)

• Red Shift: Indicates that the universe is expanding, suggesting it originated from a single point.
• CMB (Cosmic Microwave Background Radiation):
  • The early universe was hot and emitted short-wavelength radiation.
  • As the universe expanded, this radiation stretched into microwaves.
  • CMB is present throughout the sky, proving that the hot, young universe has cooled and expanded.
• The Big Bang model is the most accepted due to accounting for all experimental evidence.
• Dark matter and dark energy mean that parts of the universe are still not fully understood.

Magnets

• Magnets have North and South Poles.
• Like poles repel each other.
• Opposite poles attract each other.

Permanent Magnets

• Permanent magnets always exhibit magnetic properties and have fixed poles.

Induced Magnets

• Induced magnets are materials that become temporarily magnetic when exposed to a magnetic field, without fixed poles.
• Stroking certain materials (iron, nickel, cobalt) with a permanent magnet can align their domains, creating temporary magnets.

Magnetic Fields

• Field lines point from North to South.
• The strength of the magnetic field decreases with distance from the magnet.
• Field direction always points to the south pole and away from the north pole at any point.
• Plotting compasses indicate the direction of the magnetic field at a specific point.

Earth's Core

• Earth's core is magnetic, creating a large magnetic field.
• A freely suspended magnetic compass aligns with Earth’s field lines and points North.
• The magnetic north pole is located over North Canada, not the geographic North Pole.
• Earth’s magnetic pole above Canada is a magnetic South Pole, as opposite poles attract.

Current

• Current produces a magnetic field around a wire.
• The direction of the magnetic field is determined by the "right-hand grip rule”.
• The plotting compass through which the wire is pierced will demonstrate this

Strength of Magnetic Field

• The magnetic field is stronger with the greater the current.
• The magnetic field is weaker with greater distance from the wire.

Solenoid

• A solenoid's magnetic field resembles that of a bar magnet.
• Coiling the wire enhances the magnetic effect by aligning the fields.
• An iron core increases the solenoid's strength because magnetic field lines pass through it more easily than air.
• Solenoid strength is affected by:
  • Size of current.
  • Length.
  • Cross-sectional area.
  • Number of turns (coils).
  • Use of a soft iron core.

The Motor Effect

• Two magnets interact, experiencing a magnetic force of attraction or repulsion.
• A magnet and a wire also exert a force on each other due to the interaction of their magnetic fields.
• The wire experiences a force at right angles to both the wire direction and the field direction.

Fleming's Left Hand Rule

• Each direction (Force, Field, Current) is 90° to each other.
• Use this rule to determine the unknown factor among force, field, and current.
• Conventional current moves in the opposite direction to electron flow.
• F = BIL, where:
  • B is the magnetic flux density in Tesla.
  • I is the current.
  • L is the length of the conductor in the magnetic field.

How Electric Motors Work

• Permanent magnets are in fixed positions.
• A coil of current-carrying wire lies on an axis between the magnets.
• The force on one side of the coil moves it up.
• The force on the other side (with current flowing in the opposite direction) moves it down.
• This causes the coil to rotate.

Electromagnetic Induction (Physics only)

• Relative movement between a conductor and a magnetic field induces a potential difference across the conductor.
• A change in the magnetic field also induces a potential difference.
• Current flows if the conductor forms a complete circuit.
• The induced current produces its own magnetic field, opposing the change that induced it.

How Electric Generators (Dynamos) Work (Physics only)

• A turbine spins a coil of wire situated between two permanent magnets
• The changing magnetic field induces a potential difference.
• With a complete circuit, an alternating current (AC) flows, functioning as a basic alternator
• Direct current (DC) is produced with a split-ring commutator, reversing the current each half-rotation to maintain a positive current, known as a dynamo.

Transformers (Physics only)

• AC in the primary coil creates a changing magnetic field, which cuts through the secondary coil, inducing AC current
• A DC primary current produces a constant magnetic field, which does not induce current in the secondary coil
• More coils on the secondary coil result in increased voltage (step-up transformer)
• Fewer coils on the secondary coil result in decreased voltage (step-down transformer)
• The ratio of turns and voltage is given by: (number of turns on primary / number of turns on secondary) = (voltage on primary / voltage on secondary), assuming 100% efficiency, unless this stated, assume it is not 100% efficient.

How Dynamic Microphones Work (Physics only)

• They produce a current that is proportional to the sound signal.
• A fixed magnet is at the centre, and a coil of wire around the magnet is free to move.
• Pressure variations in the sound waves cause the coil to move, inducing current in the coil.
• This current is sent to a loudspeaker.

Loudspeakers (Physics only)

• The setup is similar to a dynamic microphone but functions in reverse.
• Current flows into the coil.
• The interaction between the magnetic field from the magnet and the current causes the coil to move.
• This movement of the coil moves the cone, producing pressure variations and sound.