# Physics Isa

I have chosen to investigate a factor that affects the output of a potential divider circuit. A potential divider is resistors connected in series across a voltage source; used to obtain a desired fraction of the voltage. A resistor is a device having resistance to the passage of an electric current which affects the output voltage (Vout). The factors that I could investigate are changing the resistance of r2 and record the output (Vout), or use a light-dependent resistor (LDR) and change the resistance of r2 and record the output (Vout). Theories

Resistors restrict or limit the flow of current in a circuit. The ability of a material or component to resist current flow is measured in ohms. There are three main types of resistor: * Fixed resistors - are the most common type of resistor. They have three important uses: protecting components, dividing voltage between different parts of a circuit, and controlling a time delay. * Variable resistors (or potentiometers) - There are two types of variable resistor: Variable resistors, which are altered continually as they work: e.g. for volume control in a radio, and pre-set potentiometers, which have a resistance control that is adjusted and then fixed. These resistors would normally be adjusted once only. * Special resistors, such as thermistors and light-dependent resistors (LDRs): Thermistors change resistance as temperatures change. Most thermistors have a negative temperature coefficient, meaning their resistance falls as temperature increases. Thermistors are used in temperature-sensing circuits. Light-dependent resistors (LDRs) change resistance as light levels change. The light levels are detected by a photo-sensitive plate on the resistor. Most LDRs have a negative light coefficient, meaning that their resistance falls as the amount of light falling on them increases. LDRs are used in light-detection circuits.

Potential divider circuit

In a potential divider, a fixed resistor is used to split voltage between different parts of the circuit. Potential dividers (or voltage dividers) are used, for example, with LDRs in circuits which detect changes in light. Timing applications is when a fixed resistor can be used in series with a capacitor to control a time delay.

This is the circuit for a potential divider. As you can see it is a very simple one, just two resistors of resistance R1 and R2 connected in series. Assume that the input p.d. to the circuit is fixed at Vin. The same current passes through both resistors so the p.d. across each is proportional to their resistance.

Fixed resistors in series

You can calculate the value of the output voltage from a potential divider using the formula: Vout = ( R2 / R1+R2 ) x Vin

Vout is the output voltage in volts

R1 is the value of resistor R1 in ohms

R2 is value of resistor R2 in ohms

Vin is the input voltage in volts

The potential difference across a component can be calculated using Ohm's Law: Potential difference (V) = current (I) x resistance (R)

V = I x R

Calculate the resistance of resistors in series:

The combined resistance of two resistors in series is the sum of the resistance values of the two resistors in ohms. The formula is: R total = R1 + R2

R total = combined resistance value (in ohms)

R1 = value of resistance in first resistor (in ohms)

R2 = value of resistance in second resistor (in ohms)

Variations on the potential divider circuit:

Variable resistors are which are altered continually as they work. For example:

Vin =9.0v

2.0kΩ

2.0kΩ

Vout

This circuit has a 9.0v supply and is connected to a fixed value 2.0kΩ resistor and a variable 2.0kΩ resistor. The output p.d. across the variable resistor will be 4.5v at its maximum value. Both resistors will have equal values so each will have half the total p.d. As the value of the variable is reduced down to zero, more of the total p.d. will be across the fixed resistor. If the value of the variable resistor is zero it...

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