L6 - Resistance 1

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To make a current flow in a conductor, there must be a potential difference (voltage) across it. Copper connecting wire is a good conductor and a current passes through it easily.
However, a similar piece of nichrome wire is not so good and less current flows for the same p.d. The nichrome wire has more resistance than the copper.

For example, if a p.d. of 6 V is needed to make a current of 3 A flow in a wire: resistance = 6 V/3 A = 2 Ω.

With a lower resistance, a lower p.d. would be needed to give the same current.
Even copper connecting wire has some resistance.
However, it is normally so low that only a very small p.d. is needed to make a current flow in it, and this can be neglected in calculations.

The resistance of a conductor depends on several factors:
- Length Doubling the length of a wire doubles its resistance.
- Cross-sectional area Halving the ‘end on’ area of a wire doubles its resistance. So a thin wire has more resistance than a thick one.
- Material A nichrome wire has more resistance than a copper wire of the same size.
- Temperature For metal conductors, resistance increases with temperature. For semiconductors, it decreases with temperature.

There is a heating effect whenever a current flows in a resistance.
This principle is used in heating elements, and also in light lamps with filaments.
The heating effect occurs because electrons collide with atoms as they pass through a conductor.
The electrons lose energy.
The atoms gain energy and vibrate faster.
Faster vibrations mean a higher temperature.

8.06---Nichrome-Heating-Element.mp4

Heating elements are normally made of nichrome - a video showing how they work

Resistors are specially made to provide resistance.
In simple circuits, they reduce the current. In more complicated circuits, such as those in radios, TVs, and computers, they keep currents and p.d.s at the levels needed for other components (parts) to work properly.
Resistors can have values ranging from a few ohms to several million ohms. For measuring higher resistances, these units are useful;
1 kilohm (kΩ) = 1000Ω 1 megohm (MΩ) = 1 000 000 Ω
Like all resistances, resistors heat up when a current flows in them. However, if the current is small, the heating effect is slight.

Resistors

Variable resistors (rheostats) are used for varying current.

The one on the right is controlling the brightness of a lamp. In hi-fi equipment, rotary (circular) variable resistors are used as volume controls.
Moving the slide control of the variable resistor to the right increases the length of resistance wire in the circuit. This reduces the current and dims the lamp.

Thermistors have a high resistance when cold but a much lower resistance when hot.

They contain semiconductor materials. Some electrical thermometers use a thermistor to detect temperature change.

Light-dependent resistors (LDRs) have a high resistance in the dark but low resistance in the light.

They can be used in electronic circuits which switch lights on and off automatically.

Diodes have an extremely high resistance in one direction but a low resistance in the other.

In effect, they allow current to flow in one direction only. They are used in electronic circuits

8.6----Understanding-Resistance.mp4

8.06---Resistance.docx

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