物理学专业英语A4

A4 Charges and circuits
• Conductors and insulators • Current flows easily through metals and carbon. These materials are good conductors because they have free electrons which can drift between their atoms (see A3). • Most non-metals are insulators. They do not conduct because all their electrons are tightly held to atoms and not easily moved. Although liquids and gases are usually insulators, they do conduct if they contain ions. • Semiconductors, such as silicon and germanium, are insulators when cold but conductors when warm.
A4 Charges and circuits
• A charged object will attract an uncharged one. On the right, the charged rod has extra electrons. Being uncharged, the foil has equal amounts of – and + charge. The - charges are repelled by the rod and tend to move away, while the + charges are attracted. However, the force of attraction is greater because of the shorter distance. • Charge which collects in one region because of the presence of charge on another object is called induced charge.
A4 Charges and circuits
• Current • In the circuit above, chemical reactions in the cell push electrons out of the negative (-) terminal, round the circuit, to the positive (+) terminal. This flow of electrons is called a current.
A4 Charges and circuits
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A4 Charges and circuits
• Static electricity • If two materials are rubbed together, electrons may be transferred from one to another. As a result, one gains negative charge, while the other is left with an equal positive charge. If the materials are insulators (see right), the transferred charge does not readily flow away. It is sometimes called static electricity.
A4 Charges and circuits
• Ohm's law and resistance • If a conductor obeys Ohm's law, then the current I through it is directly proportional to the PD V across it, provided the temperature is constant. • Metals obey Ohm's law. If a graph of I against V is plotted • for a metal conductor at constant temperature, the result is as on the right. Expressed mathematically: • V/I = constant • The resistance R of a conductor is calculated like this: • Resistance = PD/current In symbols: R = V/I • The SI unit of resistance is the ohm (Ω).
A4 Charges and circuits
• Thermistors These components have a resistance which changes considerably with temperature (e.g. high when cold, low when hot). They contain semiconducting materials. • Light-dependent resistors (LDRs) These have a high resistance in the dark but a low resistance in the light. • Diodes These 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.
A4 Charges and circuits
• Resistance components • Heating elements If a conductor (such as a wire) has resistance, then electrical energy is changed into heat when a current passes. This effect is used in heating elements . • Resistors These are components specially designed to provide resistance. In electronic circuits, they are needed so that other components are supplied with the correct current. • Variable resistors These have a control for varying the length of resistance material through which the current passes.
A4 Charges and circuits
• • Temperature A hot wire has more resistance than a cold one. • • Type of material A nichrome wire has more resistance than a copper wire of the same dimensions. • Note: • • While the resistance of a metal increases with temperature, that of a semiconductor decreases.
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A4 Charges and circuits
• For example, if a PO of 1 V causes a current of 1 A, then the resistance is 1 Ω. (This is how the ohm is defined.) • Similarly, if a PD of 12 V causes a current of 4 A, then the resistance is 3 Ω. • The resistance of a metal conductor (such as a wire) depends on various factors: • • Length A long wire has more resistance than a short one. • • Cross-sectional area A thin wire has more resistance than a thick one.
A4 Charges and circuits
• Voltage (PO and EMF) • In the circuit below, several cells have been linked in a line to form a battery The potential difference (PD) across the battery terminals is 12 volts (V). This means that each coulomb (C) of charge will 'spend' 12 joules (J) of energy in moving round the circuit from one terminal to the other
A4 Charges and circuits
• Charge • Charge can be calculated using this equation: • charge = current x time
• The SI unit of charge is the coulomb (C).
• For example, if a current of 1 A flows for 1 s, the charge passing is 1 C. (This is how the coulomb is defined.) Similarly, if a current of 2 A flows for 3 s, the charge passing is 6 C.
A4 Charges and circuits
• The voltage produced by the chemical reactions inside a battery is called the electromotive force (EMF). When a battery is supplying current, some energy is wasted inside it, which reduces the PD across its terminals. For example, when a torch battery of EMF 3.0 V is supplying current, the PD across its terminals be might be only 2.5 V.
A4 Charges and circuits
• The PO across the bulb is also 12 V. This means that, for each coulomb pushed through it, 12 J of electrical energy is changed into other forms (heat and light energy). • PD may be measured using a voltmeter as shown above. • PD, energy, and charge are linked by this equation: • energy transformed = charge x PD • For example, if a charge of 2 C moves through a PD of 3 V, the energy transformed is 6 J.