Static Electricity & Charge

12PHYS - Electricity

2021

Akoranga 1 Mahi Tuatahi

On the board, brainstorm all the things you can remember about electricity from Year 10!

Te Whāinga Ako

1. Explain how static electricity is created by the removal or addition of electric charge (and is measured in Coulombs)

Write the date and te whāinga ako in your book

Static Electricity

Static electricity has many effects - some dangerous, some hilarious, but they all depend on charge!

Charges: Positive & Negative

From Year 10 we should all remember that, much like magnets, opposites attract and likes repel.

Charge Carriers

Recall: the atom. What element is this? What are the three subatomic particles that make it up?

Draw this diagram in your book and label the particles and the charge they carry.

• Electrons: Negatively charged
• Protons: Positively charged
• Neutrons: No charge

Pātai: What happens when an atom loses or gains electrons?

Ions

• Electrons are extremely light and move very fast. Therefore they can sometimes escape an atom.
• An ion is formed when an atom gains or loses electrons.
• Losing one or more electrons makes you positively charged and is called a cation.
• Gaining one or more electrons makes you negatively charged and is called an anion.

Pātai Tahi

What did we do in Year 10 Pūtaiao to remove charges from one object and put them onto another?

Whakatika Tahi

We applied friction!

Akoranga 2 Mahi Tuatahi

Working with the person next to you, describe the process of generating a spark using the Van der Graaff Generator. Start with the motor, the brush, the dome and then go to the wand and spark.

Te Whāinga Ako

1. Explain how static electricity is created by the removal or addition of electric charge (and is measured in Coulombs)

Write the date and te whāinga ako in your book

What is Charge?

• In Physics the symbol for charge in equations is $$q$$ or $$Q$$.
• Unit: Coulombs (C)
• We use it to describe how positive or negative an object is.

So, What is a Coulomb?

• We know that electrons are negatively charged. In fact, they have a charge of $$-0.000,000,000,000,000,000,16C$$
• Pātai: Write this number in scientific form (e.g. $$1\times10^{5}$$)
• $$-0.000,000,000,000,000,000,16 C =-1.6 \times 10^{-19}C$$.
• Pātai: How many times does this number fit into $$1$$? Therefore, how many electrons are needed to carry $$1C$$ of charge?
\begin{aligned} num_{e} &= \frac{Q_{total}}{q_{e}} \newline num_{e} &= \frac{1}{1.6 \times 10^{-19}} \newline num_{e} &= 6,250,000,000,000,000,000 = 6.25 \times 10^{18} \end{aligned}

$$6.25 \times 10^{18} electrons = -1 C$$

$$1C$$ is the amount of charge carried by $$6.25 \times 10^{18}$$ electrons.

Pātai Rua

If a balloon has charge of $$-3C$$: did the balloon lose or gain electrons, and how many?

Whakatika

If a balloon has charge of $$-3C$$: did the balloon lose or gain electrons, and how many?

\begin{aligned} & \text{Negative C} \rightarrow \text{electrons gained (-ve charge)} \newline & num_{e} = 3 \times (6.25 \times 10^{18}) \newline & num_{e} = 1.875 \times 10^{19} && \text{ electrons gained} \end{aligned}

Akoranga 3 Mahi Tuatahi

If Charlotte has charge of $$0.2C$$ did she lose or gain electrons and how many?

Whakatika

If Charlotte has charge of $$0.2C$$ did she lose or gain electrons and how many?

\begin{aligned} & \text{Positive C} \rightarrow \text{electrons lost (+ve charge)} \newline & num_{e} = 0.2 \times (6.25 \times 10^{18}) \newline & num_{e} = 1.25 \times 10^{18} && \text{ electrons lost} \end{aligned}

Ngā Whāinga Ako

1. Be able to define current
2. Be able to calculate current using charge and time
3. Recall the properties of conductors and insulators

Write the date and ngā whāinga ako in your book

Charge and Current

• Current is the flow of charge (electrons, ions etc.)
• Like a river, current is the amount of charge passing a point each second
• Therefore we can simply give an equation relating the three like this:
\begin{aligned} current &= \frac{charge}{time} \newline I &= \frac{q}{t} \newline A &= \frac{C}{s} \end{aligned}

Alternative definition of a Coulomb:

A current of $$1A$$ will carry $$1C$$ of charge past a point in $$1s$$.

Worksheet One Q1-4

Conductors

• A conductor is a material through which charge can move freely
• E.g. Metals because they have a delocalised sea of electrons

Insulators

• A material through which electrons cannot easily flow
• The electrons encounter a large friction force when trying to move
• This friction causes heat to build up
• e.g. Wood can catch fire if current is applied