- Variable resistors
- Rotary resistors
- Slide resistors
Variable resistors
Variable resistors are used to control the strength of an electric current. They are needed in Hifi’s and radio’s to control the volume. Variable resistors are made in two ways.
Slide resistors
These consist of a coil of resistive wire with a sliding contact that can be moved along the wire. If the sliding contact is moved to the right, the current has to flow through a greater length of resistance wire. This increases the resistance and reduces the current. By moving the contact you can change the resistance.
Rotary resistors
The coil of resistance wire is bent round into a circle. The slide is replaced with a brass arm that touches the top of the coil. The arm is rotated clockwise to increase the resistance.
Measuring resistance (Ohm’s law)
Resistance is measured in ohms. A resistor has a resistance of one ohm (1Ω) if a voltage of one volt across it will push a current of one ampere through it.
The more resistance that a resistor has, the more volts are needed to push each ampere through. If the voltages across a resistor and current through it have both been measured, the resistance (R ohms) can be calculated using this equation:
V where V is voltage (volts)
R = ─
I I is currents (amperes)
This equations can also be used to calculate a voltage or current if the value of the
Resistance is already known.
V
─
I × R
In this experiment I’m going to try and find out the relationship between the diameter of the wire and resistance. I will be using three pieces of nichrome wire which are all different thickness.
My Hypothesis
I think the wire with the least diameter will have the most resistance because a thin piece of wire has more resistance than a thick piece.
Method
In this experiment I will be trying to find out the relationship between the diameter of the wire and resistance. I will set up the circuit like the diagram on the following page. I used a metre rule to measure the wire to 40cms I then cut it using wire cutters and wrapped it round a test tube to ensure it doesn’t catch on anything and affect my readings, I will ensure that the slider of the rheostat is at one end, with four cells in the holder. First of all I will have the wire connecting to the positive side to the battery. I will record the voltage (V) and current (I) respectively. I will move the slider about one tenth of the way along the rheostat, I will measure this using a ruler. I will record the new readings. I will then repeat this process again ten times. After that I will disconnect the battery and reconnect in reverse so that the negative end of the battery is connected to the rheostat. The meters should now give me negative readings. I will repeat the process as before and move the slider one tenth and record my readings. I will be doing this process for all the three wires.
Equipment
For this experiment I will need:
- Metre rule
- Wire cutters
- Rheostat
- 10A ammeter
- 20V voltmeter
- 4 1.5V cells
- Wires / clips
- Test tube
- Sellotape
- (40cm) Nichrome 28SWG, 32SWG, 36SWG.
Diagram
Safety
- When working with chemicals or other harmful substances every one must wear protective clothing e.g. lab coats.
- Also everyone must wear safety goggles in the lab during practical to eliminate a risk to the eyes.
- If you have long hair ensure it’s tied back securely to avoid catching on the equipment.
- The floor must be kept clear for easy access in the lab so keep bags and coats stored in the appropriate places provided.
Results
The results were as follows:
Nichrome 28 SWG
Positive Negative
Nichrome 32 SWG
Positive Negative
Nichrome 36 SWG
Positive Negative
Analysis
During this experiment I have found out that the thinner the wire the more resistance. Looking at my results I can see that the two thin pieces of wire have more resistance than the thick wire.
