Some Electronic circuits are varied in resistance using variable resistors (rheostat). These are used in lighting circuits (dimmer switches) and hi-fi systems (volume control) to vary the current flow. I will use one in my experiment because without one the amperes are too high and the reading goes of the scale

Factors Affecting Resistance

Length

Cross-Sectional Area

Materials – The atom structure is different so it easier for electrons to move is some the others.

Temperature

OHM'S LAW

Current flows in an electric circuit in accordance with several definite laws. The basic law of current flow is Ohm's law, named after its discoverer, the German physicist George Ohm. Ohm's law states that, over a wide range of circumstances and materials, the amount of current flowing through a conductor is directly proportional to the electromotive force applied between the ends of the conductor. If resistance is defined as the ratio of electromotive force to current, then V = IR, where I is the current in amperes, V is the electromotive force in volts (see Electrical Units), then Ohm's law is equivalent to saying that R (which is measured in ohms) is a constant in the specified circumstances. A material for which this holds true is described as ohmic. Ohm's law can apply to electric circuits for both direct current (DC) and alternating current (AC), but additional principles must be invoked for the analysis of complex circuits and for AC circuits involving inductances and capacitances.

A series circuit is one in which the devices or elements of the circuit are arranged in such a way that the entire current passes through each element without division or branching into parallel circuits.

When two or more resistances are in series in a circuit, the total resistance may be calculated by adding the values of such resistances. If the resistances are in parallel, the total value of the resistance in the circuit is given by the formula

In a parallel circuit, electrical devices, such as incandescent lamps or the cells of a battery, are arranged to allow all positive (+) poles, electrodes, and terminals to be joined to one conductor, and all negative (-) ones to another conductor, so that each unit is, in effect, on a parallel branch. The value of two equal resistances in parallel is equal to half the value of the component resistances, and in every case the value of resistances in parallel is less than the value of the smallest of the individual resistances involved. In AC circuits, or circuits with varying currents, circuit components other than resistance must be considered.

Example Of Resistance

A 12 Resistor has a PD of 6V across it, what is the current through the resistor?

V=6, R= 12 and I is to be found

I = V/R = 6/12 = 0.5A

Resistors

There are many different electrical resistor components such as:

- Variable Resistors (rheostats) – Are used for varying current. The one on the right controlling the brightness of the bulb

- Thermistors – Have a high resistance when cold but much lower resistance when hot. They contain semiconductor materials (e.g. Silicon) Some electronic thermometers use a thermistor to detect temperature change

- Light-dependant Resistors (LDRs) – have a high resistance in the dark but a low resistance in 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

Fair Test: To make a fair test we will use the same cells for each experiment. All readings will be taken at the same time twice by different people to get the most accurate results. We will not change the circuit or the wires to avoid problems. We will make sure that everyone is accurate and pays attention to get precise results. We will use the same people as well. We will not move the settings on the variable resistor or change the Nichrome wire to keep it fair

Variables:

The possible variables in this experiment are:

- The Amount Of Cells in the Circuit
- The Length Of The Wire
- The Settings Of The Variable Resistor

The independent variables for this experiment are the Number Of Cells and the length of the wire

The dependant variable is the amount of resistance in the circuit.

Safety Precautions:

Because we are dealing with electricity it is important to take safety precautions such as:

- Don’t Have Wet Hands When Constructing The Circuit.
- Make Sure Before We Start the experiment know how to use each piece of equipment safely to avoid shocks
- Don’t Play with the circuit layout and batteries to avoid shocks

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Apparatus: 3 cells, 1 variable resistor, connecting wires, crocodile clips,

### Nichrome wire, Ammeter, Voltmeter

Circuit Diagram:

### Method:

- We set up the circuit with 1 cell in
- We fixed the variable resistor so we could take a reading
- We tested 10mm of nichrome wire and took readings of the volts and amps
- We tested 20mm, 30mm and so on until we got to 1m of wire
- We set up the circuit with 2 cells in
- We took new readings on the wires
- We set up the circuit with 3 cells in
- We took new readings on the wires

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