Magnetism

12PHYS - Electricity

Finn LeSueur

2019

Mahi Tuatahi 1

If an object has a charge of \(0.03C\), how many electrons has it lost?

Hint: Charge of one electron \(=-1.6\times10^{-19}C\)

Mahi Tuatahi 1 Answer

\[\begin{aligned} & n = \frac{0.03}{1.6\times10^{-19}} \newline & n = 1.875\times10^{17} \end{aligned}\]

Mahi Tuatahi 2

There is \(80mA\) of current flowing through a \(2k\Omega\) resistor.

  1. How many electrons are going through the resistor in one second?
  2. What is the power output of the resistor?

Mahi Tuatahi 2 Answer

\[\begin{aligned} & I = \frac{q}{t} \newline & It = q \newline & q = 0.08 \times 1 = 0.08C \newline & n = \frac{0.08}{1.6\times^{-19}} = 5\times10^{17} \end{aligned}\]
\[\begin{aligned} & P = IV, V = IR\newline & P = I^{2}R \newline & P = 0.08^{2} \times 2000 = 12.8W (Js^{-1}) \end{aligned}\]

Pātai: What is an electric field?

Think, pair and share!

  • A region in which a charged object experiences a force

What is a magnetic field?

  • A region in which a moving charged object experiences a force
Source

The force (\(F\)) that the charge experiences as it moves through the field depends on three things:

  • Magnetic field strength (\(B\), measured in Tesla (\(T\)))
  • Charge of the object (\(q\), measured in Coulombs (\(C\)))
  • Velocity of the object (\(v\), measured in \(ms^{-1}\))
\[\begin{aligned} F = Bqv \end{aligned}\]

Let’s summarise:

Electric Field: A region in which a charged object experiences a force \(F=Eq\)

Magnetic Field: A region in which a moving charged object experiences a force \(F=Bqv\)

Why Do Fields Form?

  • A model that will help us understand is as follows:
  • The valence electrons of each atom have some small magnetic component to them
  • When all of the atoms are aligned, their magnetic fields all add up to a much stronger/larger field
Source

Ferromagnets and Paramagnets

  • Ferromagnet: A material where the atoms are all aligned to create a permanent magnetic field
    • E.g. Iron, nickel, cobalt, and their alloys
  • Paramagnets: A material with disorderly atoms, but that can become aligned when exposed to a strong external magnetic field.
    • E.g. Platinum, aluminium, oxygen

Question

A narrow beam of protons (\(1.6\times10^{-19}C\)) moving at a speed of \(2.0\times10^{-6}ms^{-1}\), enters a uniform magnetic field of strength \(0.20T\).

Calculate the magnetic force applied on each proton.

Answer

\[\begin{aligned} & F = Bqv \newline & F = 0.2 \times 1.6\times10^{-19} \times 2.0\times10^{-6} \newline & F = 6.4 \times 10^{-26}N \end{aligned}\]

Pātai: Is force a scalar or vector? How do we know the direcito of the force?

Right Hand Rules

Collect and glue in your sheet of right hand rules somewhere you can find it. These are very important and useful!

Right Hand Slap Rule (+ve Charges)

Thumb in the direction of positive charge velocity, finger-tips indicate the \(B\) field strength, and the palm shows the direction of force on the positive charge.

Source

Back-Hand Slap Rule (-ve Charges)

Thumb in the direction of NEGATIVE charge velocity, finger-tips indicate the \(B\) field strength, and the back of the hand shows the direction of force on the NEGATIVE charge.

Pātai

A charged object (\(q=1.6\times10^{-19}C\)) moves across a magnetic field with a speed of \(4.0\times10^{3}ms^{-1}\). The magnetic field strength is \(12T\).

  1. Draw a diagram and illustrate the magnetic field lines
  2. Calculate the force applied to the charged object
  3. Describe/draw the direction of the force applied

Which direction is the force acting in?

Whakakite: Cathode Ray Tube

  • Worksheet 1 & 3

Formula: F=BvL

Instead of thinking about the velocity of the moving charges, we can think about the current and length of wire in the magnetic field.

\[\begin{aligned} Force &= B Field \times Current \times Length \newline F &= BIL \end{aligned}\]
  • Mahi Kāinga booklet Q16, Q17