Effects of an electric current

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Effects of an Electric Current:

Effects of an Electric Current Chap 24

Types of effects:

Types of effects Heating effect Chemical effect Magnetic effect

Heating effect:

Heating effect When current flows in a wire, the wire heats up (e.g. Kettle) Exp. To show the heating effect of an electric current p 273 Joule studied this effect and found experimentally that the amount of heat energy given out by a wire depends on several factors-

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W (heat energy )  I 2  R  t (time) THUS W  R I 2 t W = k R I 2 t For SI units the constant =1 W = R I 2 t

Joule’s Law:

Joule’s Law Power = rate at which work is done = rate at which heat is produced = W /t P = R I 2 Joule’s Law The rate at which heat is produced in a conductor is directly proportional to the square of the current provided the resistance is constant P  I 2 Questions p274

Experiment:

Experiment To verify Joule’s Law (i.e. to verify that the rise in temperature (which is a measure of the power generated) of a wire in a given time is proportional to the square of the current p275

High Voltage energy transmission.:

High Voltage energy transmission. Electricity is generated at power stations and transmitted to consumers via wires. As the electricity passes along the wires, the wires will heat According to Joule’s law, the rate at which they heat (power) is proportional to the (current) 2

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The electrical energy which heats the wires is useless, and so should be minimised Thus, before transmission the electricity is transformed to a very low current (and consequently a very high voltage).

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When this travels through the wires a small amount is lost as heat in the wires (since heat (power) generated is proportional to the current squared). This high voltage signal is too dangerous for domestic use, so it is transformed back to lower voltages, higher currents before it enters houses.

Chemical effect of an electric current:

Chemical effect of an electric current When an electric current passes through a liquid it may cause a chemical reaction to occur in the liquid-electrolysis The liquid is called the electrolyte The metal plates placed in the liquid are called the electrodes (+ plate = anode, -plate = cathode)

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The entire system is called a voltameter See exp. P277 If the electrodes take part in the chemical reaction they are called active electrodes, if they don’t, they are called inert or inactive electrodes

Electrolysis of water:

Electrolysis of water Hydrogen and oxygen form at the two electrodes in the ratio 2:1 (see diagram p278)

Uses of electrolysis:

Uses of electrolysis Used for electroplating-coating one metal with a thin layer of another metal in order to prevent rusting and also to improve the appearance Used to purify metals and to extract metals from their ores Used to coat a thin layer of dielectric on to the plate of an electrolytic capacitor

Experiment:

Experiment To demonstrate the chemical effect of an electric current p277 To investigate the variation of current with pd for copper sulphate solution with copper electrodes p278

I and V for different conductors:

I and V for different conductors 1. a metallic conductor-if the temperature remains fairly constant, the resistance is constant and I is directly proportional to V I V

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2. A Filament bulb As current flows through the filament it heats and so the resistance increases, so increasing V does not result in as big an increase in I I V

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3. A semiconductor (e.g. a thermistor) As the semiconductor heats up its resistance drops so that a given change in voltage will produce a bigger change in current I V

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4. An electrolyte-in this case the resistance stays constant, so there is a straight line relationship. For active electrodes the graph obeys Ohm’s Law I V Electrolyte with active electrodes

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5. If the electrodes are inert then the system behaves as a cell, with an emf. The applied voltage must be bigger than this emf before current will start to flow I V Electrolyte with inert electrodes

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6. A Gas - + - Gas at low pressure Discharge tube Due to cosmic rays and background radiation there are always some ions being formed in the tube, which recombine over time. If a p.d. is applied across the tube, these ions move towards the electrodes, and so a current flows. As the p.d. increases the no. of ions and thus the current increases (region O→A in graph)

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By point A every ion produced in the tube is reaching the electrodes. I O A B Between A and B no further ions are being produced, they are just being accelerated more, hence the current stays constant V C

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If the voltage is increased still more beyond B, the ions are travelling so fast that they begin to create other ions from collisions with atoms of gas in their path. Thus, this increased no. of ions means that the current increases again.

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6. A vacuum A vacuum will not conduct electricity as it has no charge carriers. If the cathode is heated, thermionic emission occurs and a current will flow (see cathode rays in later chapter)

Domestic Circuits:

Domestic Circuits Two wires bring electricity into your house-the live wire carries the current, with voltages varying from -325V to 325V. The other is neutral and at around 0V. When the live wire enters the house it first goes through the house’s main fuse, and switch.

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It then goes through a meter measuring the energy used, then to the distribution box. From here both live and neutral go to lights, sockets etc. Big appliances (cooker, electric shower) have their own live and neutral wires coming directly from the distribution box. Each such circuit is called a radial circuit and has its own fuse.

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Lights don’t take much current, so several can be connected together-in most houses all the lights are in series and this circuit has one fuse (i.e. if the fuse blows you lose all the lights)

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Sockets. These are connected in a ring circuit. This means that the live connections of all sockets are connected in a ring. Thus power can travel in either direction along the ring. Similarly all the neutrals are on a ring

Switches:

Switches Switches should always be connected to the live wire (i.e. to disconnect the appliance from the live) Similarly the fuse is ALWAYS connected to the live, so that if something goes wrong the fuse blows thus disconnecting the device from the live.

MCBs:

MCBs Miniature circuit breakers can be used instead of fuses. They contain a bimetallic strip and an electromagnet. If the current exceeds a preset value one of these two devices pulls the contacts apart and stops the current

Bonding:

Bonding In a house all metal pipes, taps, sinks etc are connected to earth. This is a safety precaution-if the metal should accidentally become connected to live, this bonding ensures that it stays at zero volts rather than 230V of the live wire

Earthing.:

Earthing. In any device the current flows in through the live and out through the neutral. The presence of an earth connection is simply a safety device. It provides an easy path to ground. Thus, if a fault develops and the device becomes connected to live (230V) the earth connection will provide an easy path to earth, rather than the power choosing a path through you

Electricity Costs:

Electricity Costs Electricity is charged per unit. 1 unit = 1 kilowatt hour The no.of kilowatt hours = power in kilowatts x time in hours E.g. a 2 kW heater is used for 15 mins. Find the cost of this if electricity is charged at €0.22 per unit -Do questions p 283