## Mahi Tuatahi / Starter

Homework booklet **Electric Fields Question Five: Static Electricity**

Previously we have learned about current:

\begin{aligned} & current = \frac{charge}{time} \newline & I = \frac{q}{t} \end{aligned}

and voltage:

\begin{aligned} & voltage = \frac{\text{Electric field strength}}{charge} \newline & V = \frac{E}{q} \end{aligned}

### Pātai

What is the third variable that we are missing so far?

## Resistance

**Pātai/Question:**What carries the charge in a circuit?**Whakatika/Answer:**Typically, electrons ($e^{-}$)

**Pātai:**Why are they able to flow in metals?**Whakatika/Answer:**Because electrons exist in a “sea”, not bound to one atom but able to move around the solid

**Pātai:**What determines how fast the electrons can flow?**Whakatika**:*Resistance*is the measure of how much electrons are impeded in a circuit. How much they are*slowed down*.

### Symbol & Units

**Resistance**has symbol**R**in equations and has the unit**Ohms**($\Omega$, the Greek letter omega).- Resistance is determined by the components in the circuit, and is not usually variable
- Resistance changes when components are added/removed or when a rheostat (variable resistor) is altered
- Usually voltage is also fixed by the power supply, so usually only the current change

## Resistance & Heat

When current moves through a material with resistance the electrons *bump* into other atoms. This causes energy to be transferred in the form of vibrations (**heat**)!

- The higher the resistance, the more heat produced!
- The higher the current, the more heat produced!

## Ohm’s Law

\begin{aligned} & V = IR \newline & voltage = current \times resistance \newline \end{aligned}

- Voltage is measured in:
- Current is measured in:
- Resistance is measured in:

### Pātai

- The resistance of a light bulb is $1.5k\Omega$. Calculate the current through the bulb when it is connected across a $12V$ power supply.
- When $9V$ is applied to a resistor, $0.03mA$ of current flows through it. Calculate the resistance of the resistor.
- How much voltage is required to produce $180\mu A$ of current flowing through a $0.6M\Omega$ resistor?

#### Whakatika Tahi

The resistance of a light bulb is $1.5k\Omega$. Calculate the current through the bulb when it is connected across a $12V$ power supply.

\begin{aligned} & V = IR \newline & I = \frac{V}{R} \newline & I = \frac{12}{1500} \newline & I = 0.008A \end{aligned}

#### Whakatika Rua

When $9V$ is applied to a resistor, $0.03mA$ of current flows through it. Calculate the resistance of the resistor.

\begin{aligned} & V = IR \newline & R = \frac{V}{I} \newline & R = \frac{9}{0.00003} \newline & R = 300000\Omega \end{aligned}

#### Whakatika Toru

How much voltage is required to produce $180\mu A$ of current flowing through a $0.6M\Omega$ resistor?

\begin{aligned} & V = IR \newline & V = (180 \times 10^{-6}) \times (0.6 \times 10^{6}) \newline & V = 108V \end{aligned}