Electricity and Magnetism

9.1 Potential Difference in Circuits

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 A current will flow through  an electrical component (or device)  only if there  is a voltage or potential difference (p.d.), across its ends. The bigger the potential difference across a component, the bigger the current that flows through it.

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 Components resist a current flowing through them. The bigger their resistance, the smaller the current produced by a particular voltage, or the bigger the voltage needed to produce a particular current.

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 The p.d. across a component in a circuit is measured in volts (V) using a voltmeter connected across the component.

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 The current flowing through a component in a circuit is measured in amperes (A) using an ammeter connected in series with the component.

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Current-voltage graphs are used to show how the current through a component varies with the voltage across it.

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A resistor at constant temperature.   A filament lamp.   A diode.

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 When components are connected in series:

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 * their total resistance is the sum of their separate resistances

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 * the same current flows through each component

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* the total potential difference of the supply is shared between them.

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 When components are connected in parallel:

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 * there is the same potential difference across each component

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* the current through each component depends on its resistance; the greater the resistance of the component, the smaller the current

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* the total current through the whole circuit is the sum of the currents through the separate components.

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 The potential difference provided by cells connected in series is the sum of the potential difference of each cell separately (bearing in mind the direction in which they are connected).

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Candidates should be able  to interpret and/or  draw circuit  diagrams using  standard symbols.  The following standard symbols should be known.

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switch (open),  lamp, switch (closed), fuse, cell, voltmeter, battery, ammeter, diode, resistor  variable resistor

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 thermistor,  LDR 

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 Resistance, potential difference and current are related as shown: 

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   Resistance (ohms) = Voltage (or potential difference) (in Volts) / Current (in Amps)

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The current through a resistor (at constant temperature) is proportional to the voltage across the resistor.

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The resistance of a filament lamp increases as the temperature of the filament increases.

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The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.

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The resistance of a light dependent resistor decreases as the light intensity increases.

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The resistance of a thermistor decreases as the temperature increases.

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[i.e. knowledge of negative temperature coefficient thermistors only is required]

9.2 Energy in Circuits

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 As an electric current flows through a circuit, energy is transferred from the battery or power supply to the components in the electrical circuit.

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 An electric current is a flow of charge.

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 When electrical charge flows through a resistor, electrical energy is transferred as heat.

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 The rate of energy transfer (power) is given by:  Power (Watts) = Voltage( Volts) x Current (Amps)

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1 watt is the transfer of 1J of energy in 1s.

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 The higher the voltage of a supply, the greater the amount of energy transferred for a given amount of charge which flows.

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Candidates should be able to use [but will not be expected to recall] the relationship:

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   [This relationship will be provided when required]

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The amount of electrical charge which flows is related to current and time as follows:

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Charge (coulombs) = Current (Amps) x time (seconds)

9.3 Mains Electricity

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 The UK mains supply is about 230 volts. Mains electricity can kill if it is not used safely.

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 Most electrical appliances are connected to the mains using cable and a 3-pin plug. To make them safe to use:

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 cable comprises:

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 * two or three inner cores of copper, because copper is a good conductor;

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 * outer layers of flexible plastic, because plastic is a good insulator.

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a plug has:

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* a plastic or rubber case, because plastic and rubber are good insulators;

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* pins made from brass, because brass is a good conductor;

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* a fuse;

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* an earth pin;

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* a cable grip.

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The fuse in a plug should always be the same as the one recommended by the manufacturer of the appliance. Appliances with metal cases are usually earthed.

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When connecting an appliance to a 3-pin plug:

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* the blue wire is connected to the neutral terminal;

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* the brown wire is connected via a fuse to the live terminal;

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* the green/yellow wire (when fitted) is connected to the earth terminal;

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* the cable should be secured in the plug by the cable grip;

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* a fuse of the current value (rating) should be in place.

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 Candidates should be able, when provided with appropriate diagrams:* to recognize errors in the wiring of a mains (3-pin) plug;

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* to recognize dangerous practice in the use of mains electricity.

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 An alternating current (a.c) is one which is constantly changing direction. Mains electricity is an a.c. supply. It has a frequency of 50 cycles per second or 50 hertz (Hz).

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 Cells and batteries supply a current which always flows in the same direction. This is called a direct current (d.c.)

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 If a fault in an electrical circuit or an appliance causes too great a current to flow, the circuit is switched off by a fuse or a circuit breaker.

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The fuse should have a value higher than, but as close as possible to, the current through the appliance when it is working normally.

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When the current through a fuse wire exceeds the current rating of the fuse the wire becomes hot and will (eventually) melt breaking the circuit and switching off the current.

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Candidates should be able to explain how one type of circuit breaker works.

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 Appliances  with metal  cases  need  to  be earthed.  The  earth pin is  connected  to  the  case via  the yellow/green wire. If a fault in the appliance connects the case to the live wire, and the supply is switched on, a very large current flows to earth and overloads the fuse.

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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 with respect to Earth.

9.4 The Cost of Using Electrical Appliances

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 Much of  the energy transferred  in homes  and industry is  electrical energy.  This is because  electrical  energy is readily transferred as:

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* heat (thermal energy);

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* light;

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* sound;

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* movement (kinetic energy)

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Candidates should be able:

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* to specify the energy transfers everyday electrical devices are designed to bring about;

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* to give examples of everyday electrical devices designed to bring about particular energy  transfers.

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  How much electrical energy an appliance transfers depends on:* how long the appliance is switched on;

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* how fast the appliance transfers energy (its power).

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The power of an appliance is measured in watts (W) or kilowatts (1kW = 1000W).

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 The amount of energy transferred from the mains is measured in kilowatt-hours, called Units.

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 Candidates should be able to use [but will not be expected to recall] the relationship: 

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Energy (units in Kilowatt hours) = Power (kilowatts) x time (hours)

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 Candidates should be able, when provided with suitable diagrams of a digital domestic electricity meter, to calculate the number of Units used.

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 The cost of this energy can be calculated using: Cost = number of units (kilowatt hours) x cost of one unit

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 The total amount of energy transferred by an electrical device can be calculated as follows: Energy (Joules) = Power (Watts) x Time ( seconds)

9.5 Electric Charge

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  When certain different materials are rubbed against each other they become electrically charged. Electrically charged objects attract small objects placed near to them.

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 When two electrically charged objects are brought close together, they exert a force on each other. Two charged objects may either pull towards each other (attract) or push each other away (repel).

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These observations can be explained in terms of two types of charge called positive (+) and negative (-). Two objects which have the same type of charge repel. Two objects which have different types of charge attract.

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When two different materials are rubbed against each other, electrons, which have a negative charge, are rubbed off one material on to the other. The material which gains electrons becomes negatively charged; the material which loses electrons is left with an equal positive charge.

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Candidates should be able to describe one example of how electrostatic charges are used in everyday life.

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A charged conductor can be discharged by connecting it to the Earth with a conductor.

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 Candidates should be able to describe one situation in which static electricity is dangerous and explain how precautions can be taken to ensure that the electrostatic charge is discharged safely.

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 In solid conductors, an electric current is a flow of electrons.

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 When some chemical compounds are melted or dissolved in water they conduct electricity. These compounds are made up of electrically charged particles called ions. The current is due to negatively charged ions moving to the positive terminal (electrode) and the positively charged ions moving to the negative electrode. Simpler substances are released at the terminals (electrodes). This process is called electrolysis.

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  The greater the charge  on an isolated object, the greater the  voltage (potential difference) between the object and the Earth. If the voltage becomes high enough, a spark may jump across the gap between the object and any earthed conductor which is brought near it.

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 Metals are good conductors of electricity because some of the electrons from their atoms can move freely throughout the metal structure.

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 During electrolysis the mass and/or volume of the substance deposited or released at the electrode increases:

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* when the current increases;

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* when the time for which the current flows increases.

9.6 Electromagnetic Forces

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If two magnets are attracting each other and one of them is turned the opposite way round, they will repel

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 If a magnet is free to move it comes to rest pointing in a north-south direction.

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 The end of a magnet which points north (or south) is called the north (or south) seeking pole. Like poles repel; unlike poles attract.

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 A magnet exerts a force on any piece of magnetic material including iron and steel, or another magnet which is placed near it. (There is a magnetic field around the magnet).

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 A coil of wire acts like a bar magnet when an electric current flows through it. One end becomes a north-seeking pole and the other end a south-seeking pole.

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This is called an electromagnet.

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The strength of an electromagnet can be increased:

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* by placing an iron core inside the coil;

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* by increasing the number of turns on the coil;

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* by increasing the size of the current flowing through the coil.

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Reversing the current in an electromagnet reverses the poles of the electromagnet.

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When a wire carrying an electric current is placed in a magnetic field, it may experience a force. The size of the force can be increased by:

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* increasing the strength of the magnetic field;

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* increasing the size of the current.

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The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed.

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Candidates should be able - when provided with diagrams and/or other appropriate information - to explain how electromagnetic effects are used in everyday devices, including electromagnets, simple d.c. motors, loudspeakers, circuit breakers and relays.

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[Details of the split ring, for reversing the current to a d.c. motor each half turn, will not be required.]

9.7 Electromagnetic Induction

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  If a magnet is moved into a coil of wire which is part of a complete circuit a current is produced (induced) in the wire.

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 If the magnet is moved out of the coil, or the other pole of the magnet is moved into the coil, the direction of the induced current is reversed.

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 Transformers  are used  to change  the voltage of an a.c. supply.  At power stations,  transformers are used to produce very high voltages before the electricity is transmitted to where it is needed through power lines (National Grid). Local transformers reduce the voltage to safer levels before the electricity is supplied to consumers.

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Electricity can be generated by rotating a coil of wire in a magnetic field or by rotating a magnet inside a coil of wire. This is how a generator works.

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If a wire, or coil of wire 'cuts through' a magnetic field, or vice-versa, a voltage (potential difference) is produced between the ends of the wire. This induced voltage causes a current to flow if the wire is part of a complete circuit.

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The size of the induced voltage increases when:

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* the speed of the movement increases;

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* the strength of the magnetic field is increased;

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* the number of turns on the coil is increased;

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 * the area of the coil is greater.

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 Candidates should be able, when provided with a diagram, to explain how an a.c. generator works, including the purposes of the slip rings and brushes.

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 A changing magnetic field will also produce an induced voltage in a coil. This is how a transformer works.

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The higher the voltage, the smaller the current needed to transmit energy at the same rate. Less energy is wasted by heating up the power lines.

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A transformer consists of two separate coils wound around an iron core. When an alternating voltage is applied across one coil (the primary) an alternating voltage is produced across the other coil (secondary).

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The voltages across the primary and secondary coils of a transformer are related as shown: