# Resistance & Ohm’s Law

12PHYS - Electricity

2019

## 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

1. 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.
2. When $$9V$$ is applied to a resistor, $$0.03mA$$ of current flows through it. Calculate the resistance of the resistor.
3. 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}