Electric & Magnetic Fields

Static Electricity Generation

Friction between two insulating materials causes negatively charged electrons to be scraped off one surface and transferred to the other.

Result of Electron Transfer

The material that gains electrons becomes negatively charged. The material that loses electrons becomes equally positively charged.

Why 'Static'?

Because the materials are insulators, the transferred charge is not free to move and remains built up on the surface, hence it is 'static' charge.

Electrostatic Forces

Like charges repel each other (e.g., positive repels positive). Opposite charges attract each other (e.g., positive attracts negative).

Force and Distance (Electrostatic)

The strength of the electrostatic force between two charged objects decreases as the distance between them increases.

Attraction by Induction

A charged object can attract a neutral (uncharged) object. This happens because the charged object induces a charge separation in the neutral object.

Mechanism of Induction

When a negatively charged rod is brought near a neutral insulator (like paper), it repels the electrons on the paper's surface. This leaves the surface of the paper nearest the rod with a net positive charge, resulting in an attractive force.

Cause of Static Sparks

A large build-up of static charge creates a high potential difference between a charged object and an earthed object (like the ground or a person).

Electric Field's Role in Sparks

The high potential difference creates a very strong electric field in the air gap between the charged object and the earthed object.

Ionisation of Air

The strong electric field is powerful enough to remove electrons from the atoms in the air, a process called ionisation. Ionised air is a conductor.

What is a Spark?

A spark is the flow of charge (a current) through the now-conductive, ionised air. This rapid discharge of electrons is seen as a flash of light.

Electrostatic Paint Sprayer - Principle

Paint droplets are given the same static charge as they leave the spray nozzle. The object to be painted is given the opposite charge.

Electrostatic Paint Sprayer - Advantages

1. The like-charged droplets repel, forming a fine, even mist. 2. The droplets are attracted to the oppositely charged object, reducing waste and coating the object evenly, even on the back ('wrap-around effect').

Danger of Static during Refuelling

Friction between the fuel and the pipe can cause a large static charge to build up. A spark could then occur, igniting the flammable fuel fumes and causing an explosion.

Preventing Refuelling Dangers

The aircraft and refuelling tanker are earthed by connecting them with a conducting wire. This provides a path for charge to flow safely to the ground, preventing any build-up.

Electric Field Definition

An electric field is a region around a charged object where another charged object will experience a non-contact force.

Electric Field Lines Direction

Field lines show the direction of the force on a positive charge. They always point away from positive charges and towards negative charges.

Electric Field Lines Strength

The strength of the field is shown by the spacing of the lines. Closer lines indicate a stronger field; lines further apart indicate a weaker field.

Electric Field of a Point Charge

The field lines radiate outwards from a positive point charge and inwards towards a negative point charge. They are always perpendicular to the surface of the object.

Uniform Electric Field

A uniform electric field exists between two oppositely charged parallel plates. The field lines are parallel, equally spaced, and point from the positive to the negative plate. The field strength is constant everywhere between the plates.

Magnetic Field Definition

A magnetic field is a region around a magnet or a current-carrying wire where another magnet or a magnetic material (iron, steel, cobalt, nickel) experiences a non-contact force.

Magnetic Field Lines

They show the shape and direction of the magnetic field. They always point from a North pole to a South pole. The field is strongest where the lines are most concentrated (at the poles).

Permanent vs. Induced Magnet

A permanent magnet (made of a magnetically 'hard' material like steel) produces its own field all the time. An induced magnet (made of a magnetically 'soft' material like iron) only becomes a magnet when placed in another magnetic field.

Magnetic Induction

When a magnetic material enters a magnetic field, it becomes an induced magnet. A north pole will induce a south pole in the material, and vice-versa. This is why the force is always attractive.

Magnetic Field of a Straight Wire

A current flowing through a wire creates a magnetic field consisting of concentric circles around the wire. The field's strength decreases with distance from the wire.

Right-Hand Thumb Rule

Used to find the direction of the magnetic field around a wire. Point your right thumb in the direction of the conventional current (+ to -). Your fingers will curl in the direction of the magnetic field lines.

The Motor Effect

A current-carrying wire placed in a magnetic field will experience a force, provided the wire is not parallel to the field. This is because the magnetic fields of the wire and magnet interact.

Fleming's Left-Hand Rule

Used to find the direction of the force in the motor effect. Thumb = Thrust/Force. First finger = Field (N to S). Second finger = Current (+ to -).

Force Equation (Motor Effect)

F = B × I × l where F is Force (N), B is Magnetic Flux Density (T, Tesla), I is Current (A), and l is the length of wire in the field (m). Maximum force occurs when the wire is at 90° to the field.

Solenoid

A long coil of wire. When current flows, it creates a magnetic field that is strong and uniform inside the coil, and shaped like a bar magnet's field outside. It is an electromagnet.

Increasing Electromagnet Strength

1. Increase the current. 2. Increase the number of turns per unit length. 3. Insert a soft iron core.

DC Electric Motor Operation

A current-carrying coil in a magnetic field experiences a force on each side (motor effect), creating a turning effect. A split-ring commutator reverses the current direction every half turn to keep the coil rotating continuously in the same direction.

Electromagnetic Induction

The process of inducing a potential difference (and a current if in a complete circuit) in a conductor which is experiencing a change in magnetic field.

Inducing a Potential Difference

A p.d. can be induced by: 1. Moving a conductor (e.g. a wire) through a magnetic field. 2. Moving a magnet relative to a coil. 3. Changing the magnetic field through a stationary coil (the principle of a transformer).

Factors Affecting Induced p.d.

The size of the induced p.d. is increased by: 1. Moving the wire/magnet faster. 2. Using a stronger magnet. 3. Increasing the number of turns on the coil.

Alternator (AC Generator)

Uses a rotating coil in a magnetic field (or rotating magnet in a coil) to induce a p.d. It uses slip rings and brushes, causing the output current to change direction every half turn, producing alternating current (AC).

Dynamo (DC Generator)

Also uses a rotating coil in a magnetic field. It uses a split-ring commutator which reverses the connection every half turn, so the output current always flows in the same direction, producing direct current (DC).

Transformer Principle

An alternating current in the primary coil creates a continuously changing magnetic field in the soft iron core. This changing field passes through the secondary coil, inducing an alternating p.d. across it. Transformers only work with AC.

Step-Up vs. Step-Down Transformer

A step-up transformer increases the p.d. and has more turns on the secondary coil than the primary (Ns > Np). A step-down transformer decreases the p.d. and has fewer turns on the secondary coil (Ns < Np).

Transformer Equations

Turns Ratio: Vp / Vs = Np / Ns. Power (100% efficient): Vp × Ip = Vs × Is.

The National Grid

A network of cables and transformers that connects power stations to consumers. It uses transformers to manage the potential difference of the electricity.

Why the Grid Uses High Voltage

Power is transmitted at very high voltages (e.g., 400,000 V) and low currents. This minimises energy loss as heat in the transmission cables, as power loss is calculated by P = I²R. A lower current (I) drastically reduces power loss.

Transformers in the National Grid

Step-up transformers at power stations increase the p.d. for efficient transmission. Step-down transformers are used locally to reduce the p.d. to a safe, usable level (e.g., 230 V) for homes.

Microphone Operation

Sound waves cause a diaphragm to vibrate. The diaphragm moves a coil of wire in a magnetic field, which induces a p.d./current via electromagnetic induction. The electrical signal matches the sound wave.

Loudspeaker Operation

An AC electrical signal is passed through a coil attached to a cone, which is in a magnetic field. The motor effect causes a varying force on the coil, making the cone vibrate and create sound wave