Mahi Tuatahi ^🔗

1. If an object has a charge of $0.03C$, how many electrons has it lost?
2. There is $80mA$ of current flowing through a $2k\Omega$ resistor. How many electrons are going through the resistor in one second?
3. What is the power output of the resistor?

Whakatika Tahi ^🔗

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

Whakatika Rua ^🔗

$$ \begin{aligned} & I = \frac{q}{t} \newline & It = q \newline & q = 0.08 \times 1 = 0.08C \newline & n = \frac{0.08}{1.6\times^{-19}} = 5\times10^{17} \end{aligned} $$

Whakatika Toru ^🔗

$$ \begin{aligned} & P = IV, V = IR\newline & P = I^{2}R \newline & P = 0.08^{2} \times 2000 = 12.8W (Js^{-1}) \end{aligned} $$

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

Magnetic Force: Particles ^🔗

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 Magnetic 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

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, copper

Pātai ^🔗

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.

Whakatika ^🔗

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

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.

Back-Hand Slap Rule (-ve Charges) ^🔗

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

By default we use the +ve charge rule because we tend to think about conventional current (the flow of +ve charge).

Pātai ^🔗

For each of these four situations, apply the RH rule and figure out what direction the current feels a force. Fingers go in direction of field and thumb in direction of current.

A cross means current going into the page down a wire; a dot means current coming out of the page.

Whakatika ^🔗

1. Force out of the page
2. Force into the page
3. Force down
4. Force up

Pātai ^🔗

A charged object ($q=1.6\times10^{-19}C$) moves to the left across a magnetic field (going bottom to top of the page) 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

Cathode Ray Tube & Magnets ^🔗

• Collect a Cathode Ray Tube and Magnets sheet
• Come up the front where you can observe the CRT monitor.
• Use our discussion and information at the top to answer Task 1-2.

Whakawai ^🔗

Textbook page 235 Q1-2