Noadswood Science, 2012

To understand current-potential difference graphs

Tuesday, April 18, 2023

A graph of current against potential difference shows you how the current flowing through a component varies with the potential difference across it

The current is plotted on the vertical (y) axis; and the potential difference on the horizontal (x) axis

Using the class data, plot a graph of the current-potential difference for the length of wire (plot the data for all three types of wire)

Reading

Wire Type 1 Wire Type 2 Wire Type 3

Current (I) Voltage (V) Current (I) Voltage (V) Current (I) Voltage (V)

1 0 0 0 0 0.0 0

2 0.2 2 0.3 3 0.4 4

3 0.4 4 0.6 6 0.8 8

4 0.6 6 0.9 9 1.2 12

5 0.8 8 1.2 12 1.6 16

The current through a resistor (at constant temperature) is proportional to the voltage

Different wires have different resistances

The current flowing through a resistor at a constant temperature is directly proportional to the potential difference across it – it is said to follow Ohm’s Law

The filament lamp is a common type of light bulb (it contains a thin coil of wire called the filament)

The filament heats up when an electric current passes through it, and produces light as a result

The filament lamp does not follow Ohm’s Law – its resistance increases as the temperature of its filament increases

As voltage increases the bulb gets hotter, and its resistance increases

Diodes are electronic components that can be used to regulate the potential difference in circuits and to make logic gates

Light-emitting diodes (LEDs) give off light and are often used for indicator lights in electrical equipment

The diode has a very high resistance in one direction – this means that current can only flow in one direction

Resistance = Voltage ÷ Current

For the straight-line graphs the resistance of the component is steady and is equal to the inverse of the gradient of the line (1/gradient)

The steeper the graph, the lower the resistance

If the graph curves the resistance is changing – calculated by taking the voltage divided by the current

Resistance is anything which opposes an electric current

The resistance value of a resistor is shown by a series of coloured bands – each colour represents a number

Most resistors have 4 bands: -• The first band gives the first digit• The second band gives the second digit• The third band indicates the number of

zeros• The fourth band is used to shows the

tolerance (precision) of the resistor: -• Red band = 2%• Gold band = 5%• Silver band = 10%

Example – what is the resistor value?

This resistor has red (2), violet (7), yellow (4 zeros) and gold bands

Its value is 270’000 (270k)

Tolerance = 5%

R means ohm

k means kilo-ohm (1k = 1 kilo-ohm; 1k8 = 1.8 kilo-ohms)

M means mega-ohm (2M = 2 mega-ohms; 2M2 – 2.2 mega-ohms)