# Physics - Resistance and Ohm's Law

> Source: https://ollybritton.com/notes/a-level/physics/topics/ohms-law/ · Updated: 2021-01-20 · Tags: physics, circuits, year-1, school

##### What is Ohm's Law??
The current through an ohmic conductor is proportional to the potential difference across it when at a constant temperature.

##### What is the symbol formula for resistance??
$$
R = \frac{V}{I}
$$

##### What is the word formula for resistance??
$$
\text{resistance} = \frac{potential difference}{current}
$$
##### What is the formula for potential difference $V$ in terms of resistance $R$ and current $I$??
$$
V = IR
$$

##### What is the formula for current $I$ in terms of resistance $R$ and potential difference $V$??
$$
I = \frac{V}{R}
$$

##### What are the units for resistance??
Ohms, $\Omega$

##### What does $1\Omega$ mean??
The resistance of a component when a potential difference of $1V$ is produced per ampere of current.

##### In an Ohmic conductor, what happens if you double the potential difference??
The current also doubles.

##### How can you tell from a voltage-current graph that something is an Ohmic conductor??
The line is linear.

##### How could you describe resistance in a component??
The amount that a component resists the flow of charge carriers through it.

##### What is the resistance of a component $R$ defined as??
The ratio between $V$ and $I$.

##### ![PHOTO VOLTAGE RESISTANCE GRAPH](voltage-resistance-graph.png) Which of these components is an Ohmic conductor??
B

##### ![PHOTO VOLTAGE RESISTANCE GRAPH](voltage-resistance-graph.png) Which of the components shows a decreasing resistance at higher currents??
A

### 2021-01-22
##### What is an Ohmic conductor??
A component where the current is always directly proportional to the potential difference.

##### What is the threshold voltage in a diode??
The voltage at which a diode starts letting current through.

##### ![PHOTO RESISTOR IV CHARACTERISTIC](resistor-iv-characteristic.png) What component is this??
A resistor.

##### ![PHOTO RESISTOR IV CHARACTERISTIC](resistor-iv-characteristic.png) How can you tell this component in an Ohmic conductor??
The current is directly proportional to the potential difference.

##### ![PHOTO RESISTOR IV CHARACTERISTIC](resistor-iv-characteristic.png) What is the gradient equal to??
$$
\frac{1}{R}
$$

##### What is on the $y$-axis of an $IV$ characteristic??
$$
I
$$

##### What is on the $x$-axis on an $IV$ characteristic??
$$
V
$$

##### ![PHOTO LARGER RESISTANCE](larger-resistance.png) Which graph, left or right, has a higher resistance??
Left.

##### Why does a smaller gradient mean a larger resistance??
Because the gradient is $\frac{1}{R}$ it's the opposite to what you expect.

##### ![PHOTO FILAMENT BULB IV CHARACTERISTIC](filament-bulb-iv-characteristic.png) What component is this??
A filament bulb.

##### ![PHOTO FILAMENT BULB IV CHARACTERISTIC](filament-bulb-iv-characteristic.png) How can you tell that this component is not an Ohmic conductor??
The current is not directly proportional to the potential difference.

##### ![PHOTO FILAMENT BULB IV CHARACTERISTIC](filament-bulb-iv-characteristic.png) What happens to resistance as voltage increases??
It begins to increase.

##### ![PHOTO LED IV CHARACTERISTIC](led-iv-characteristic.png) What component is this??
A diode.

##### ![PHOTO LED IV CHARACTERISTIC](led-iv-characteristic.png) Effectively, what is the resistance on the left hand side of the graph??
$$
\infty
$$

##### ![PHOTO LED IV CHARACTERISTIC](led-iv-characteristic.png) Is a diode an Ohmic conductor??
No.

##### ![PHOTO IV DATA COLLECTION CIRCUIT](iv-data-collection-circuit.png) What is this circuit used to measure??
How current varies with voltage.

##### ![PHOTO FILAMENT BULB IV CHARACTERISTIC](filament-bulb-iv-characteristic.png) How can you take data for "negative voltages"??
Flip the polarity of the battery.

### 2021-03-02
##### In series, what is the formula for the total resistance $R_T$ given $R_1, R_2, R_3, ...$??
$$
R_T = R_1 + R_2 + R_3 + ...
$$

##### In parallel, what is the formula for the total resistance $R_T$ given $R_1, R_2, R_3, ...$??
$$
\frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + ...
$$

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