## Investigation into the resistance of a filament lamp

## Aim

To investigate Characteristics of a filament lamp

### Background Knowledge

Filament bulb

The tungsten filament in a modern light bulb is supported by several molybdenum wires. The ends of the support wires are imbedded in a glass button at the top of the glass support rod. The copper and nickel lead-in wires, which carry the current to the filament, are supported by a glass support stem. One lead-in wire is soldered to the metal contact at the base of the bulb, while the other is electrically connected to the side socket contact. The contacts are separated by an insulating plate. An electrical current can pass in either direction through the filament.

The filament of a light bulb is a resistor. When a current passes through the resistor electrical energy is converted into heat. A light bulb converts current into power by the resistive element inside the light bulb. When the power level is sufficient to raise the temperature to above 1000 degrees Kelvin, visible light is produced, which can help us to estimate the temperature of the filament.

The length of time that you leave the light bulb on determines the total amount of energy that has been used. The hot filament then radiates a fraction of this energy in the form of light.

The current through the light bulb is not proportional to the voltage across it. This is because the current through the filament causes it to heat up, as the temperature of the filament changes the resistance also changes. The filament is therefore a non-ohmic device. We can use this change in the resistance of the filament to measure the temperature of the tungsten wire.

A light bulb is “turned on” by passing an electric current through its filament, heating the filament until it is “white hot.” The high temperature of the filament is up to 3,000 Kelvin, causes it to give off visible light by a process called incandescence. Incandescence is defined as the emission of visible light by a hot body. Any hot object gives off incandescent light. The higher the temperature of the object, the brighter the light given off. Examples of incandescent objects include hot coals in a campfire or barbecue grill, the sun, light bulb filaments and the burners on electric stoves, which glow dull red when their temperature is on high. (harison-toshiba.com)

##### Resistance

Definition:

The resistance between two points in a circuit is the ratio of the potential difference between the points to the current flowing between them.

Unit:

Ohms (Ω)

Ohm’s law and resistance:

Ohm’s law states that:

- For a fixed resistance value (R) the current flow (I) increases in direct proportion to the increase in voltage (V).

- In a circuit with a fixed resistance the current is directly proportional to voltage applied.

- If the Voltage is kept constant and the resistance is varied the current flow will be inversely proportional to the resistance.

(www.geocities.com/geomcamck/ohms_law.html)

This means that doubling the resistance will halved the current and also halving the resistance will double the current. We know that Voltage is directly proportional to the current and Resistance is inverse proportional to current.

I∝ V I ∝ 1

If we combine these two equations together we get I=V/R, if we re-arrange this formula we get ohm’s law V=I.R.

So to find resistance we can re-arrange V=I.R to get; R=V/I

What determines the resistance?

- The length of the material (Example- 70 cm of copper wire will have high resistance then 60cm of copper wire)

- The temperature (Example- the greater the temperature the greater the resistance, this is because the vibrating ions obstruct electrons at high temperature so therefore that increase the resistance)
- The impurities of a metal, the resistance of a impure metal is greater than that of a pure metal.

Current

Definition:

Current is the movement of electrical charge - the flow of electrons through the electronic circuit. Current is measured in AMPERES (AMPS, A or I). Current would be the flow of water moving through the tube (wire).

Unit:

Ampere (amp)

Formula:

I=V/R

Voltage

Definition:

Voltage is the electrical force, or "pressure", that causes current to flow in a circuit. It is measured in VOLTS (V). Voltage would be the force that is pushing the electrons forward.

Unit:

Volts