Physics 2

Static electricity builds up when electrons are "rubbed off" from one material onto another.

It only happens between insulating materials.

The material:

• receiving electrons becomes negatively charged.
• giving up electrons becomes positively charged .

When two charged materials are brought together the exert a force on each other.

• materials with the same charge repel each other.
• materials with different charges attract each other.

Electrostatic Smoke Precipitator

1. Solid smoke particles become positively charged as they pass a charged metal grid.
2. These "like" particles repel each other causing the particles to move away from the grid.
3. The particles are then attracted to a negatively charged collecting plates where they stick to form a layer of "soot which can then be removed.

Photocopier

1. An image of the page to be copied is projected on to an electrically charged plate (usually positive) .
2. Light causes charge to leak away leaving and electrostatic image on the page.
3. The charged image on the plate attracts tiny specks of black powder. The powder is then transferred from plate to paper. 

A charged conductor (positive or negative) can be discharged. Any charge is removed by connecting it to the earth with a conductor.

The flow of electrons through a solid object is an electric current.

Potential Difference

The greater the charge on an isolated object the greater the potential difference between the object and the earth.

If the potential difference between the object and a nearby earthed conductor becomes high enough, the air molecules can ionise and there is a spark as discharge occurs.

An electric current will flow through an electrical component (or device) if there is potential difference (voltage) across the ends of the component.

In a circuit:

• The potential difference is measured in volts using a voltmeter connected in parallel
• The current id measured in amperes, using a ammeter connected in series.

The amount of current that flows through the component depends on:

• The potential difference across the component.
• the resistance across the component.

The greater the potential difference across a component, the greater the current that flows through the component.

Components resist the flow of current through them. They have resistance (measures in Ohms). 

The greater the resistance of a component:

• The smaller the current that flows for a particular potential difference.
• The greater the potential difference needed to maintain a particular current.

Resistance is a measure of how hard it is to get a current through a component at a particular potential difference.

Current-Potential Difference graphs show how the current through the component varies with the potential difference across it.

Potential difference, current and resistance are related by the formula:

Potential Difference = Current X Resistance 

A direct current (d.c) always flows in the same direction
Example:

• Cells and Batteries 

An alternating current (a.c) changes the direction of flow back and forth continuously .

The frequency is the number of complete cycles of reversal per second. The UK mains electricity is about 50 cycles per second (hertz).

The UK mains supple has a voltage of about 230 volts. This voltage, if isn't used safely, can kill.

Most electrical appliances are connected to the mains electricity supply using a cable and a three pin plug.

The materials used for the plug and cable depend on their properties:

• The inner cores of the wire are made of copper because its a good conductor.
• The outer layer is made of flexible plastic because its a good insulator.
• The pins of the plug are made from brass because its a good conductor.
• The casing is made from plastic or rubber because bother are good insulators.

The live terminal of the mains supply alternates between a positive and negative voltage with respect to the neutral terminal.
The neutral terminal stays at a voltage close to zero. 

If an electric fault causes a current that is too high, the current will be switched off by a circuit breaker or fuse.

A circuit breaker is a safety device which automatically breaks a circuit if it is too high. Circuit breakers can be easily reset.

If a current becomes too high:

• An electromagnet increases in strength.
• An electromagnet separates a pair of contacts.
• The circuit is broke (trips).
• The appliance or user is protected.

A fuse should always be part of a live circuit. It is a short, thin piece of wire with a low melting point.

The current rating of the fuse must be just above the normal working current of the appliance for the safety system of the appliance to work properly.

If the current running through the appliance is greater then the current rating of the fuse:

• The fuse wire gets hot and melts or breaks.
• The circuit is broken.
• The current is no longer able to flow
• The appliance or user is protected.

Electrical appliances with outer metal cases are  usually earthed. The outer case of an appliance is connected to the earth pin in the plug through the earth wire.

The earth wire and fuse work together to protect the appliance and the user.

If a fault in the appliance connects the live wire to the case:

• The case will become live.
• The current will then "run to earth" through the earth wire as this offers the least resistance.
• This overload of current will cause the fuse to melt (or the circuit breaker to trip)
• The appliance and user is protected.

The amount of electrical charge which passes any point in circuit is measured in coulombs (C).

You can calculate charge using this formula:

Charge (coulombs) = Current (amps) x Time(seconds)

As charge passes through a device, energy is transformed.

The amount of energy transformed for every coulomb of charge depends of the size of the potential difference. The greater the potential difference, the more energy transformed per coulomb.

Energy Transformed = Potential Difference x Charge

An electric current is the rate of the flow of charge. A current transfers electrical energy from a battery or power supply to components in a circuit.

The rate of flow is measured in Amperes.

The components transform some of this electrical energy into other forms of energy. For example, a resistor transforms electrical energy into heat energy.

The rate of which energy is transformed by a component or appliance is called the power.

You can calculate power by using:

Energy Transformed = Power (watts) x Time (seconds)