**Electric field**

*Do you still remember what is electric field in **A2/A-level Physics**?*

**Electric fields** are created electric charges, in which the **field strengths** represent the force experienced per unit __positive__ charge. **Electric field lines** represent:

• **Direction of force**, through the **arrows** represented (positive repels if a charge which is relatively more negative, which will is attract)

• **Strength**, which is represented by the **gap **between each field lines, where closer the lines stronger the force

**Uniform fields**; occur between charged parallel plates. Charged plates can be created in a circuit, where **E****∝V**. As the distance between the charge plates increase, the strength of the electric field decreases, hence **E****∝****1/d**. joining these two proportionalities together we get the formula for uniform **electric field strength**.

**Radial fields**, occur in a point charges. Similar to gravitational field strength, **E****∝****1/r2**. Furthermore, **E****∝****Q** and one final factor that affect E is the medium (e.g. medium can have and vary charge, like water) knowing this **E****∝****1/4****πε**_{0}. Joining all these probabilities gives us the formula of **radial electric field strength**.

**The force** these electric fields exert on another charged particle is given by **multiplying** the **electric field strength by the charge** of the __new__ particle or plate. This is because **F=EQ** (derived from E=F/Q). **Coulomb’s law** is the Force experience between two point charges (radial electric force).

In a **uniform electric field, the motion of a charged particle** will:

•**Accelerate** **vertically** in direction of force acting on the particle

• if particle is positive accelerate towards the negative and vice versa.

• The** acceleration** is the force in a **uniform electric field** (where Q is the particle), divided by mass (F=MA)

• Travel at a **constant** horizontal velocity. The velocity is to be calculated before or given.

**Magnetic field**

**Magnetic field** is a region where particles with **magnetic properties** and __ moving__,

**charged particles**with experience a

**force**.

A magnetic field is induced by **two types** of magnets

• **Permanent magnets **with **North** and **South** pole that has a magnetic field around it that couldn’t “turn off”

• **Electromagnets (temporary) **magnets that produce a magnetic field by the flow of current. This can be turned off” if current flow is stopped.

**Magnetic flux density, B **(magnetic field strength/ intensity of field lines) is the **force (F)** acting **per unit current (I)** in a wire of a **unit length (l)**, which is __perpendicular__ to field __and__ wire (see electric field in wire). This is measured in **tesla (T)** where one tesla is **1 NA-1m-1**.

The force acting on a particle travelling in a perpendicular direction to field is given by **F=BQv**. This is because

**I= Q/t** and **IL= QL/t** but **L/t= v** hence **IL=Qv**. Since **sin (****θ****) = 1, F=BIL = F=BQv**

**Mass spectrometry** uses deflection of charged particles, using electromagnets to identify the mass of a sample.

• Smaple being ionised will then **be accelerated** towards an** electromagnet**.

• This exerts a magnetic force on the **different** particles.

• Particles of **larger mass accelerate less** (F=ma) relative to particles of lighter mass.

• This **degree of deflection** can be measured to identify the mass of the samples.

** Electronmagnetism**

*When it comes to **A2/A-level Physics** , it is important to understand electronmagnetism.*

Knowing that a current flow can induce a magnetic field, it is also possible to induce an emf (electron motive **force **- voltage (energy make **current** (electron) **flow**)). This is done by some way of “sweeping” a conductor across a magnetic field, where three thing affect the magnitude of EMF induced

• **Magnetic flux density**- as more field lines greater the magnitude emf induced

• **Area swept by conductor**- as greater the area the more field lines “swept”

• **Speed of the swiping**- more time the field lines are swept

• **Number of coils in conductor**- more coil more of the same area swept, hence it multiplies

**Magnetic flux (Φ**_{B}**)** is the product of magnetic flux density and the area swept by a conductor (field lines swept by conductor, which are perpendicular to each other). It is measured in Weber (Wb), where one weber NmA-1.

**Magnetic flux linkage (Φ)** is the product of magnetic flux and the number of turns in a coil. The number of turns in a coil increases the area being swept, essentially multiplying the emf induced.

Hence the only factor left is the **speed** of the “sweeping” , therefore if we divide magnetic flux linkage **by time**, we get the** emf **induced, this is **Faraday’s law** which states “emf induced is equal to the rate of change in flux linkage”.

**Lenz’s Law** states “emf is induced in a direction which opposes whatever causes the induction”,

• the emf induced follows the __ right hand rule __(rather than the left hand rule)

• create a force that **opposes the motion** of the conductor, by making current flow in the opposite direction.

• This alters faradays law by making the value for E (emf) __negative__.

**AC generator **creates an **alternating current,** to supply electricity for households around the country.

• It is created by a **taut coil** being rotated around a **uniform magnetic field**.

• When the motion of rotation is **perpendicular** to the field, it **induces** a current to **oppose the motion**

• this will **decrease** as the motion will become more and more parallel (where current become 0).

• As the coil **continue to rotate,** notice the will now be in the **opposite direction** to before

• this will induce a current in the **opposite direction**.

AC generator is used to supply electricity as it is more efficient, because resistance can be reduced by **transforming** electricity by:

• **Step- up**- number of turns of secondary coil is greater than primary. This increases voltage, decreases current (conservation of energy)

• **Step-down**- number of turns on primary is greater than secondary. This decreasing voltage, increase current

In order to transform a.c electricity, a **transformer** is used. It uses electromagnetism and an electromagnetic induction to

• Induce a **magnetic field** on the iron core from the **primary coil**

• induce an emf on the **secondary coil** from the magnetic field

Transforming electricity is done through changing the **turns-ratio**.

References:

- https://www.savemyexams.co.uk/notes/igcse-physics-cie-new/4-electricity-magnetism/4-2-electrical-quantities/4-2-2-electric-fields/
- https://qsstudy.com/physics/uniform-electric-field
- http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elefie.html
- http://www.coolmagnetman.com/maggallery.htm
- https://www.api.simply.science/images/content/physics/Electromagnetism/EM_induction/Concept_map/Lenz_Law.html
- https://www.pngwing.com/en/free-png-xcobq/download
- https://www.researchgate.net/figure/AC-generator-principles-Wayne-Storr_fig4_307570710

This is the end of the topic

Drafted by Brandon (Physics)