Electricity

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Current

Electric current is a flow of electric charge. It will only flow around a complete curcuit if there is a source of potential difference, the driving force that pushes the current around. The unit of potentail difference is the volt, V, and the unit for current is ampere, A.

In a single-looped circuit, the current is equal everywhere and depends on both the potential difference and resistance. Resistance is anything that slows the current down, measured in ohms. The greater the resistance across a component, the smaller the current that flows for a given potential difference.

The size of the current is the rate of flow of charge. When current flows past a point in a circuit for a length of time then the charge that has passed is given by the formular:

Charge = Current x Time

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Resistance

Potential difference (V) = Current (A) x Resistance (ohms)

Factors Affecting Resistance Practical.

The resistance of a circuit can depend on the components being in series or parallel or the length of the wire. This is to investigate how the wire affects it:

  • Set up a circuit with a switch, a battery, an ammeter in series and a volmeter in parallel with a 1m test wire next to a 1m ruler.
  • Attach a crocodile clip to the wire with 0cm on the ruler and the second 10cm away. Record the distance between the clips.
  • Close the switch then record the current through the wire and the potential difference.
  • Open the switch then move the second crocodile clip 10cm down the wire, close the switch and record the new length, current and pd.
  • Repeat this until reaching 1m and use your results to calulate the resistance, using R = V / I.
  • Plot a graph of resistance against wire length. The results should be directly proportional. If the line of best fit does not go through the origin the first clip may not have been attached at 0cm, this is a systemic error.
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Resistance and I-V Characteristics

The resistance of ohmic conductors, e.g. a wire or a resistor, does not change with the current. At a constant temperature, the current flowing through is directly proportional to the potential difference across it. However, the resistance of some components does change, for example, a diode or a filament.

When an elecectric current flows through a filament lamp, it transfers some energy to the thermal energy store of the filament, which is designed to heat up. Resistance increases with the temperature, so the current increases, increacing the resistance and temperature.

For diodes, the resistance depends on the direction of the current. Current can flow in one direction but there is a very high resistance in the opposite direction.

Required Practical.

I-V charcteristic refers to a graph which shows how the current flowing through a component changes as the potential difference across it increases.

  • Set up a circuit with a battery, an ammeter, a voltmeter, a viable resistor, and the componet you are testing.
  • Begin to vary the variable resistor to alter the current and potential difference and take readings from the voltmeter and ammeter to see how the current changes.
  • When measuring the diode, swap the wires connected to the cell over so the direction of the current is reversed.
  • Plot a graph of current against voltage.
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Circuit Devices

Light dependent resistor - An LDR is a resistor that is dependent on the intensity of light. In bright light, the resistance falls and in darkness the resistance is at its highest. This is useful for automatic night lights and burglar detectors.

Thermistor - A thermistor is a temperature dependent resistor. In hot conditions, the resistance drops and in cool conditions the resistance goes up. Thermistors make useful temperature detectors, e.g. car engine temperature sensors and electric thermostats.

Sensing circuits can be used to turn on or increase the power of a component dependent on the conditions. For example, if a fan was connected to a resistor in parallel and a themistor in circuit, as a room got hotter, the resistance of the thermistor will decease and so the potential difference across the circuit will rise, making the fan go faster.

Swapping the thermistor and fixed resistor and raplacing the bulb with a fan, the pd across both the LDR ad the bulb will be high when its dark and the LDR's resistance will be high and so the bulb will get brighter in low light.

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Series Circuits

Components are connected in a line, meaning if ne is disconnected, the circuit is broken and everything stops. You can use this design to measure quantities and test components, e.g. the sensor circuits.

Potential difference is shared between the various components, so the potential difference round a a series circuit always add up to equal the source pd: Vtotal = V1 + V2 + ...

In series circuits the same current flows through all components, the size of the current is determined by the total pd of the cells and the total resistance of the circuit, i.e. I = V / R.   I1 = I2 + I3 + ...

Resistance is the total resistance of all the components. This is because resistors share the total pd. The potential difference acoss each resistor is lower, so the current through each resistor is always lower. The current in the circuit is reduced when a resistor is added, this means that resistance increases. The bigger a component's resistance, the bigger its share of total pd.    Rtotal = R1 + R2 + ...

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Parallel Circuits

In parallel cicuits, each component is seperately connected to the cell and so the disconection of one will hardly affect the others. This is used in most things like cars and houshold electrics. Eveyday circuits often contain a mixture of both parallel and series circuits.

Potential difference is the same and equal to the source of the potential difference.V1 = V2 = ...

In parallel circuits the total current is equal to the total of all the currets through the seperate components. If two identical components are connected in parallel then the same curent will flow through each component. Itotal = I1 + I2 +...

If two resistors are in parallel, their total resistance is less than the resistance of the smallest of the two resistors. This is because the current has more than one direction to go in, increasing the total current that can flow around the circuit. Using V = IR, an increase in current means a decrease in total resistance.

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Investigating Resistance

In Series...

  • Build a simple circuit with one resistor, one bettery and one ammeter.
  • Measure the current and calculate the resistance.
  • Add another resistor in series and calculate the resistance again.
  • Repeat 4-5 times and plot a graph.

In parallel...

  • Use the same equiptment and follow the same steps, however connect the resistors in parallel.

Results - It should be found that adding resistors in series increases the total resistance of the circuit as the current decreases. The more resistors, the higher the current. When you add resistors in parallel, the total current increases and so the resistance decreases. The more you add, the smaller the resistance will be.

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Electricity in the home

In ac supplies, the current is constantly changing direction. This is produced by an alternating voltage in which the positive and negative ends keep alternating, for example the mains which is at 230v. The frequency id 50Hz. Cells and batteries produce a direct current, produced by a direct voltage, and which always flow in the same direction.

Most cables have three cores (wires) made of copper and a colored plastic to show the cable's purpose and to be able to tell them apart.  The three wires are:

  • Neutral - blue. Compleates the circuit and carries away current. It is around 0v.
  • Live - brown. Provides the alternating potential difference from the mains, it is aout 230v.
  • Earth - green and yellow. Protects the wiring and stops the appliance from becoming live. Also at 0v.

Your body is at 0v so a large current will flow through it if you touch the live wire, causing an electric shock which could injure or kill you. Even without a current,an open socket could be dangerous as it carries a large potential difference. Your body could provide a link between the live and earth which may still cause a large current through your body. If the link creates low resistance path to the earth, a huge current will flow which could result in a fire.

The pins are made of brass as it is a good conductor and does not rust or oxidise. The case is plastic as it can be shaped so no wires and pins touch each other, also it is a good electrical insulator. The plug contains a fuse between the live pin and wirewhich melts and cuts the live wire off if a current too high passes through it. Copper is used for the wires as it is a good electrical conductor and bends easily. Plugs which carry a higher current must have larger cables so as prevent too much resistance.

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Power

Electrical appliances are designed to transfer energy to the components in a cicuit when a current flows (a charge transfers energy from the work against the resistance of the circuit).The higher the current of an appliance, the more energy is transferred to the thermal energy stores of the components and then to the surroundings.

Energy transferred (J) = Power x Time (S)

Appliances are often given a power rating - the maximum safe power they can operate at. This is the maximum amount of energy transferred between stores per second when the appliance is in use, however a higher rating does not necessarily mean it transfers more energy usefully.

When an electrical charge goes through a change in potential difference, energy is transferred. Energy is supplied to the charge at the power source to 'raise' it through a potential. The charge gives up this energy when it 'falls' through a drop in components elsewhere in the circuit:

Energy transferred (J) = Charge flow (C) x Potential difference (V)

This means that a battery with a biiger pd will supply more energy to the circuit for every coulomb of charge which flows round it, because the charge is raised higher at the start.

Power (W) = Potential difference (V) x Current (A)

Power (W) = Current^2 x Resistance (coloumbs)

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The National Grid

The national grid is a giant system of cables and transfomers that covers the UK and connects power stations to consumers, transferring electrical power.

Throughout the day electricity demands vary and so power stations can predict these increases and adapt. They often run much below their maximum outpt so they have spare incase of an emergency. There are also smaller power stations nearby which can start up if necessary.

The national grid has a very high voltage (4000 000v) and a low current for maximum power (P=VI). This keeps the current low so not as much energy is lost through heating.

Transfomers and big pilons with huge insulators get the potential difference so high. Step up transformers step up the potential difference at one end (from about 25 000V to 132 000V) and step down transformers bring it back down to a safe, useable level.

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