# Physics 2.3

- Created by: Laura
- Created on: 11-01-13 18:03

## How can insulating materials gain static?

Some insulating materials can become statistically charged when they are rubbed together. Static energy builds up when electrons are 'rubbed off' one material to another. The material which gains electrons is becomes negatively charged, and the one which loses electrons becomes positively charged.

Materials with the same charge will repel eachother and materials with opposite charges will attract eachother.

## Will statically charged objects repel or attract?

When two objects rub together and become statistically charged one of the objects will gain electrons, so will have a negative charge, and the other will lose electrons, so will have a positive charge.

Objects with similar static charges, either positive and positive or negative and negative, will repel eachother. Whereas objects with different static charges, positive and negative, will attract eachother. If one object is statically charged and the other is not the objects will also attract (think of a balloon rubbed on hair will pull hair up and stick to the wall, but will push away another rubbed balloon).

Remember when you rub two objects together and they gain static electricity their charges will be equal and opposite.

## Which materials can electrical charges move in?

Electrical charges move easily in metals and other good conductors. This is because the atomic structure of the materials allows the electons to move freely around the structure, instead of being tied to an atom. This allows good conduction as the charge cab be passed quickly and freely through the atoms.

## What is the equation for current?

Current = Charge in Coulombs / Time Taken I = Q / t

I = the current in amperes (amps, A)

Q = the charge in coulombs (C)

t = the time in seconds (s)

An electric current through a circuit is a flow of electric charge. The size of the electric current is the rate of flow of electric charge.

## What is the equation for potential difference?

Potential Difference = Work Done V = W W

Charge Q V | Q

Potential Difference is measured in volts (V).

Work Done is the energy transfered and is measured in joules (J).

Charge is measured in coulombs (C)

## Current-potential difference graph for a resistor?

At a constant temperature the current (amps) is directly proportional to the potential difference (volts) in a resistor because if you have a fixed resistor and fixed potential-difference then the current too must be fixed. This means that on a graph you will have a constant positive gradient.

## How do you measure the resistance of a component?

The resistance across a component can be found by measuring the current flowing through it and the potential difference across it.

Or you could use an ohmetre.

Or you could use the equation R = V

I

Where.. R is resistance (ohms, Ω)

V is potential difference (volts, V)

I is current (amps, A)

Resistance is measured in ohms (Ω).

## What is the relationship between .....

**the current through a resistor (at a constant temperature) and the potential difference across the resistor?**

The current through a resistor at a constant temperature and the potential difference across the resistor are directly proportional. This is becuase if you have a fixed resistance and a fixed potential difference then you must have a fixed current, so all are inter-linked.

## How do you use the equation V = I x R?

Potential Difference (volts, V) = Current (amps, A) x Resistance (ohms, Ω)

V = I x R

Using this triangle ..... V

I | R

You can work out the potential difference, current and resistance.

For potential difference (volts, V) you use V = I x R

For current (amps, A) you use I = V / R

For resistance (ohms, Ω) you use R = V / I

## Current-potential difference graph for a bulb?

The current-potential difference graph for a filament lamp graph is shown below, and is not directly proportional. It has the curve because as the bulb gets brighter the temperature of the bulb increases, which also increases the resistance of the lamp because more of the electrons in the current are slowed by the greater vibrations in the metal ions of the bulb (as the electrons collide with the ions), so the resistance increases as the voltage (push power) is reduced.

## Current-potential difference graph for a diode?

As a diode only allows current to flow in one direction through it the current-potential difference graph is 0 on the negative side of the x axis (as the resistance against the one way is too high for any current to flow), and only on the positive side of the x axis does the curent increase with the potential difference (because the current can flow through the one way diode).

## What is the link between resistance and current?

At a constant potential difference the link between resistance and current is that the higher the restistance the smaller current which flows through, as the current has more resistance blocking its path. The lower the restistance the larger the current which flows through, as the current has less blocking its path.

## What are the rules of a series circuit?

- The total resistance (R) is the sum of the resistance of each component.
- The current remains the same through each component.
- The total potential difference of the supply is shared between the components, so after one it will reduce and after the next it will reduce more, and it will always return to the battery as 0V

## What are the rules of a parallel circuit?

- The total resistance is the sum of the resistance of each of the components
- The current at the start of the circuit and end will be the same, where the circuit becomes parallel the current will split to be proportional to the resistance, so the more resistance the less current and the less resistance the more current, if the resistance is equal so will be the current
- The voltage at the start of the circuit will be the same as the voltage in all parts of the parallel, but after the parallel the votage will either be decreased (if there is further resistance) or 0V, but either way the voltage at the battery will be 0V.

## What is the use of a light dependant resistor?

A light dependant resistor or LDR is used mainly in street lamps or automatic garden lights, security lights or christmas decorations.

LDRs are used in these products because their resistance depends on the amount of light falling on it. In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it. In bright light, the resistance of an LDR is low and more current can flow through it. So as either the light intensity or resistance increases the other decreases.

## What is the use of a thermistor?

Thermistors are temperature sensors, and can be used in fire alarms and maybe in ovens (to keep the oven at the right temperature).

The resistance of a thermistor depends on its temperature. At low temperatures, the resistance of a thermistor is high and little current can flow through it. At high temperatures, the resistance of a thermistor is low and more current can flow through it. So as either the resistance or temperature of a thermistor increases the other decreases.

## What is the use of light emitting diode?

Light emitting diodes or LEDs are often used as indicators for electrical equipment and as they use a smaller current, so is more cost efficient, than other forms of lighting their use as a light is increasing.

LEDs work by emitting light when a current flows through it but only in a forward direction, and not in a backwards direction. This is because when the current flows in a forwards direction through the LED the resistance is low so it allows a lot of current to pass through and the light to emit light. However when the current flows in a backwards direction through the LED the resistance is high so no current flows through and the light does not emit light.

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