electricity revision notes

8/7/2019 Electricity Revision Notes

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Electricity:

Charge and Current:

Electrical Current is the net flow of charged particles.

In metals, the electric current is due to movement of electrons

In Electrolytes ( a conducting solution), the mobile charges are positive and negative

ions, which are attracted to the oppositely charged electrodes. It is the movement of these

ions which generate the electrical current in electrolytes

Conventional Current and Electron Flow:

Before electrons was discovered, it was assumed that positive charge flows from positive

to negative terminal. This is known as conventional current.

Conventional current is where current flows from the positive to negative terminal (In all A

level questions, assume conventional current unless stated otherwise)

However, we now know electrons flow around the circuit form the negative to positive

terminal, as electrons have a negative charge and are attracted to the positive terminal.

This is electron flow, where electrons flow from the negative to positive terminal.

Charge, Current, Coulombs, Ammeters:

Current is the rate of flow of charge. This is shown through equation:

or Q = IT

Current can be measured by connecting an ammeter in series in a circuit

One Coulomb is the charge that passes a point in a circuit when a current of 1 amp

flows for one second.

Elementary charge: Each electron has an elementary charge of 1.6 x 10 -19 C

Kirchoffs first law:

The sum of currents entering a junction is equal to the sum of currents exiting a

junction in a circuit.


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Current is rate of flow of charge. Charge cannot disappear or get used up as the number

of electrons stay constant

Kirchoffs first law is based on the idea that current is conserved, it cannot be created or

destroyed.

Drift Velocity:

Conduction electrons in a metal move around rapidly in random directions

When a voltage is applied, the electrons gain an additional velocity along the wire

The mean drift velocity v (of electrons/ charged carriers) is the average extra velocity

gained by the electrons

The greater the value of the current ( the greater the rate of flow of charge), the greater

the value of v (average additional velocity gained by electrons)

I = nAve Current does not only just depend upon the mean drift velocity of the electrons.

Current also depends on the cross sectional area of the metal. The greater the cross

sectional area, the greater the current (as more electrons can move past a point in the

circuit, increasing the charge)

Current also depends on number density n of electrons in metal. The greater the

number density of electrons in a metal, the greater the current ( as there are more

electrons flowing, increasing the charge)

Number Density in Conductors, Semi-Conductors and Insulators:

Metals have high number density of electrons, so drift velocities are low

Semi conductors have a far smaller number density (far fewer free electrons), so drift

velocities are much higher

For insulators, number density of electrons is close to zero

Circuit Symbols:


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EMF and PD:

Voltmeter must be connected across terminals of a component to measure the PD across

it; it must be connected in parallel with it

or W = VQ

1 volt is one joule of energy transferred per coulomb of charge.

Potential Difference- is the energy transferred per unit charge when electrical energy is

converted into another type of energy (e.g. lamp: electrical --> light + (heat))

Electromotive Force - is the energy transferred per unit charge when energy transferred

per unit charge when one type of energy is converted into electrical energy (e.g. battery:

chemical --> electrical). The electromotive force of a power supply is a measure of howmuch energy is given to each unit of charge or the work it does in pushing 1 C of charge

around a complete circuit

Combining EMFs - when two or more sources of EMF are connected in series, their

voltages add up. They must be connected positive - to - negative otherwise the E.M.F.s

subtract

Resistance:

Potential Difference is needed to push a current through a component

The Electrical Resistance of the component tells us how difficult to make charge flow

through it

The more Resistance there is, the more energy is needed to push the same number of

electrons through a part of the circuit


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One Ohm of resistance takes a p.d. of 1 volt to make a current of 1 amp to flow through

it. One Ohm is One Volt per Amp.

Ohms Law:

The current through a conductor is proportional to the potential difference across it,

provided physical conditions, such as temperature remains constant.

I-V Characteristics of Resistor at constant temperature:

At constant temperature, current through a resistor is directly proportional to the PD

across it. It obeys Ohms law

I-V Characteristics of a Filament Lamp:

The filament in a lamp is a fine metal wire inside the lamp

The lamp does not just emit light but also emits heat energy

An increase in current causes resistance of the filament to increase

As current in the filament increases, the temperature of the filament (and amount of light

energy) also increases

The increase in temperatures causes the positive ions in the metal filament to vibrate

more which makes it harder for conduction electrons to pass through the filament

Therefore, as the temperature increases, the resistance increases


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I-V characteristics of a Light Emitting Diode (LED):

A diode is a component which only allows current in one direction only, in reverse

direction only a very small current flows

A light emitting diode only emits light when a large enough current passes through it in

the correct direction known as the forward direction ( In diagram, current only flows in

forward direction when PD is above 0.4V)

Diodes are connected so that the triangle in their circuit symbol points in the direction

that conventional current flows


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Advantages of using LEDs over filament lamps:

Longer working life

Switch on instantly (unlike energy saving bulbs)

Operating with a low PD

Uses of LEDs:

Small scale - Power light on electronic goods

Large scale - Clusters of thousands of LEDs in traffic lights

Experiment to obtain the I-V characteristics of components:

An Ammeter connected in series measures the current through a component

A Voltmeter connected in parallel measures the PD across the resistor

Reversing the connections to the resistors makes the current flow through in opposite

direction (useful for LED characteristics)

Results of such an experiment can be plotted on an I-V graph

Resistivity:

Some materials resist the flow of electrical current more than others. This property is

known as resistivity

Resistivity is a property of a material, while Resistance is a property of an object

Resistivity depends on the material. The structure may make it easy or difficult for charge

to flow.

In general, resistivity also depends on environmental factors as well like temperature and

light intensity

Factors that determine resistance:


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1. Length (L) The longer the wire, the more difficult it is to make current flow, the higher

the resistance

2. Area (A) The wider the wire, the easier it will be for the electrons to pass along it, the

lower the resistance

3. Resistivity () The higher the resistivity, the more the material resists the flow of

electrical current, the higher the resistance

This can be summarized in the following formula for resistance:

R = l/A

Temperature also affects resistance, as seen earlier when looking at I-V characteristics

How Temperature affects Resistivity of Metals:

Increase in temperature means an increase in resistance, as positive ions vibrate

more vigorously when temperature increases which makes it harder for conduction

electrons to pass through the metal e.g. Filament

Therefore, an increase in the temperature increases the resistivity of the material

Resistivity is directly proportional to temperature (in kelvin)

Metals have a positive temperature coefficient i.e. as temperature goes up, resistivity

goes up

How Temperature affects Resistivity of Semi-Conductors:

Semi conductors e.g. thermistors, diodes, LDRs demonstrate opposite behaviour to

metals

As temperature increases, the number of free electrons also increases, as moreelectrons break free from their atoms

Resistance and therefore resistivity therefore decreases when temperatures goes up

Semi conductors have a negative temperature coefficient i.e. as temperature goes up,

resistivity goes down

Thermistors have high resistance when cold and low resistance when hot

Resistance - Temperature characteristics for Metals and Semi-conductors


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Pure metal wire: Resistance increases gradually as temperature is increased

NTC thermistor: Resistance decreases rapidly over a narrow range of temperature

Power:

Power is the rate of energy transferred or work done

P = IV or P = I2R or

The Kilowatt Hour:

W = IVT Electrical energy is measured in units of kWhr so people can be charged for the amount

of electricity they use. At this very moment, 1 kWhr of energy costs about 7p

Energy = power x time


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kWhr = kW x hr = 1000 x 60 x 60 = 3 600 000 J

This can be used to find out the costs of supplying electricity. First find out the energy

used in terms of kilowatt hours and then multiply this by the cost per kilowatt hour.

Fuses:

Electrical appliances are protected from dangerous overloading by the fuse in the plug

The fuse is made of thin copper wire in a ceramic casing

Above a certain value of current, the wire becomes so hot that it melts, breaking the

circuit

A fuse should therefore be chosen so that its rating is a little higher than the maximum

current drawn by the device when its operating correctly

Most household appliances use fuses of 3A, 5A or 13A

For example, if a plug of 1KW electric fire is connected to household mains (230V), what

fuse should be used?

I = P/ V = 1000/ 230 = 4.3 A

Therefore, a 5A fuse should be used. The fuse is rated slightly higher than maximum

current of 4.3A drawn by the device

Series and Parallel Circuits:

Kirchoffs second law:

The sum of the e.m.f.s around any closed circuit loop is equal to the sum of p.d.s

Kirchoffs second law is based on the conservation of energy. The total amount of energy

put in (sum of e.m.f.s) is equal to the total amount of energy taken out (sum of p.d.s)

Resistors in Series:


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But V = IR, so:

Cancel the I's:

Resistors in Parallel:

Cancel the V's to get:

Internal Resistance of Sources of e.m.f:

Current flows all the way round the circuit including the interior of the supply (from

negative to positive terminal - conventional current)

The interior of a supply is made up of chemicals or metal wire, and must have resistance

This is known as the internal resistance r of the power supply and is shown in the circuit


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Terminal PD + Lost Volts:

Terminal PD refers to the PD across the terminals of the supply/ battery

If the cell is not connected to a circuit, no current flows so the P.D. across the terminals

As soon as the cell is connected to a circuit, the terminal P.D. decreases because the cell

has its own internal resistance, and as soon as some current flows, some of the energy is

used within the cell to overcome the internal resistance

The energy wasted per coulomb overcoming the internal resistance is known as the lost

volts (r is internal resistance)

E.M.F = P.D. Across Terminals + Lost Volts

E = IR + Ir = I(R+r) = V+ Ir

Varying the value of R ( through a variable resistor), makes the current change. A graph

can be plotted to show this, showing the greater the current that flows from the supply, the

smaller its terminal PD, as the greater the current the greater the internal resistance

Y intercept = EMF Gradient = -R

Therefore, it is crucial that most power supplies have a low internal resistance to reduce

lost volts

The idea of internal resistance can also be applied to voltmeters. Voltmeters should have

high internal resistances so only a tiny negligible current will flow, so its reading willtherefore indicate the e.m.f. of the supply by making the lost volts as close to 0 as

possible


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Potential Dividers:

The P.D. provided by a supply can be reduced by connecting two resistors across its

terminal

The bigger resistor takes the bigger share of PD, the smaller resistor takes the smaller

share of PD. In other words, the Ratio of Resistances = Ratio of Voltages

Variable resistors can therefore be used in potential divider circuits to produce a variable

PD

How resistance of a LDR is affected by intensity of light:

Bright - Lower Resistance

Dark - Higher Resistance

Why and How thermistors and LDRs are used in Potential Divider circuits:

LDR - Shining light on the LDR will decrease its resistance, Vout will decrease

Therefore, LDRs can be used in potential divider circuits to monitor changes in light

intensity

This means that LDRs can be used in a potential divider control circuit to switch lights on

as it gets dark, as in the dark, the resistance increases, giving a higher voltage output so

the light will stay on. This is how street lights work

NTC thermistor - Warming the NTC thermistor will decrease its resistance, Vout will

decrease


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electricity revision notes

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