# Magnetism

12PHYS - Electricity

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