# Electricity and magnetism (电与磁部分笔记归纳)

[VAMPIRE13] 2008-6-26 4:21:42

Physics

Electricity and magnetism

Circuit

Circuit Symbols

-          switch

-          lamp

-          cell

-          battery

-          voltmeter

-          ammeter

-          fuse

-          resistor

-          variable resistor

-          thermistor

-          Light Dependent resistor

-          Diode

-          There must be a complete circuit

-           There must be no short circuit

Series connections.

Components that are connected one after another on the same loop of the circuit are connected in series. The current (Moving electric charge) that flows across each component connected in the series is the same. When two or more cells are connected in series in a circuit, the total potential difference is the sum f their potential differences.

Parallel connections.

Components that are connected on separate loops are connected in parallel. The current is shared between each component connected in parallel. When two or more components are connected in parallel, the potential difference across them is the same.

Current and potential difference

You need to know how to measure the current that flows through a component in a circuit and potential difference or voltage across it.

Current

A current flows when an electric charge (the electrical state of an object which can be positively charged or negatively charged) moves around a circuit.

Measuring current.

-          It is measured in amperes.

-          Often abbreviated to amps. Or A

-          Measured using an ammeter

-          Must be connected in series with the component

Potential Difference

-          Measured in Volts. V

-          Measured using a voltmeter

-          Must be connected in parallel with the component.

Resistance and resistors

Resistance and Ohm’s law

An electric current flows when charged particles called electrons (subatomic particles with a negative charge and a negligible mass relative to protons and neutrons) move through a conductor. The moving electrons can collide with atoms of the conductor. This makes it more difficult for the current to flow, and cause resistance.

The resistance in a wire increases as

-          The length of wire increases

-          The thickness of the wire decreases

Ohm’s Law

Potential difference (V) = current (A) ∙ resistance (Ω)

When components are connected in series, their total resistance is the sum of their individual resistances.

Diodes are electronic components that can be used to regulate the voltage in circuits and to make logic gates.

Light-emitting diodes give off light and are often used for indicator lights in electrical equipment such as computers and television sets.

Thermistors are used as temperature sensors. Ex. Fire alarms. Their resistance decreases as the temperature increases.

-          At low temps, the resistance of a thermistor is high and little current can flow through them.

-          At high temps, the resistance of a thermistor is low and more current can flow through them.

LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases.

-          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.

Mains Electricity

Wiring a plug.

A mains electricity contains 2 or 3 inner wires. Each wire has a core of copper because copper is a good conductor. The other layers are flexible plastic because plastic is a good insulator (Material that is a poor conductor of electricity or heat). The inner wires are color coded.

Red: live

Blue: neutral

Green and yellow: earth

The features of plug:

-          Plastic or rubber case

-          3 pins made from brass

-          A fuse between the live terminal and the live pin

-          Cable secured in the plug by a cable grip

Blue goes left, red goes right, striped goes top.

Fuses and circuit breakers.

The fuse breaks the circuit if a fault in an appliance causes too much current flow. The fuse contains a piece of wire that melts easily.

Most common 3A.

The fuse should be rated at a slightly higher current than the device needs.

Circuit breakers

A spring-loaded push switch is held in the closed position by a spring-loaded soft iron bolt. An electromagnet (A magnet made by wrapping a coil of wire around an iron bar and passing an electric current through the coil.) is arranged so that it can pull the bolt away from the switch. If the current increases beyond a set limit, the electromagnet pulls the bolt towards itself, which releases the push switch into the open position.

Earthing and double insulation

Earth wire has low resistance, prevent electric shocks.

Some appliances do not have an earth wire because they have plastic casings or they have been designed so that the wired does not touch the casing.

Energy in circuits

Review

Power: The rate at which energy is transferred or work is done, measured in W.

Power = work done / time taken

Potential difference: The voltage between two points that makes an electric current flow between them.

Current: Moving electric charges, Ex. Electrons moving through a metal wire.

-          Energy is measured in joule. J

-          The rate of energy transfer (when energy is passed from one object to another or from place to place.) is called the power

-          Power is measured in watt, W.

The equation.

Power = potential difference * current

1000W=1KW

Charge, current and time.

Charge (C) = current (A) * time (sec)

(note to self: C. Coulomb)

Energy transferred (J) =potential difference (V) *charge (C)

The cost of using electricity

The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy is transferred is measured in kilowatt-hours, kWh. One unit is 1kWh.

Energy transferred (kWh) = power (kW) * time (h)

Total cost=number of units*cost per unit

Direct and alternating current supplies

If the current flows in only one direction it is called direct current, or dc. Batteries and cells supply dc electricity.

If the current constantly changes direction it is called alternating current, or ac. Mains electricity is an ac supply. Frequency: the number of waves per second. Hertz. Hz.

The potential difference of the live terminal varies between a large positive value and a large negative value. However, the neutral terminal is at a potential difference close to earth, which is zero.