IGCSE Physics Section 2- Electrcity

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Safety features in plugs

Plugs are wired in a certain way:

• The correct colour of wire is attatched to each pin, firmly screwed.
• No bare wires are shown inside the plug.
• The cable grip is tightly fastened over the outer layer.
• The metal parts are made of goodd conductors such as copper or brass.
• The insulating materials are made of rubber or plastic, because they do not conduct electricity.

Appliances are insulated or earthed:

The neutral wire is always 0 V, and the live wire alternates between high positive and high negative V. Electricity normally flows through the live wire, and out through the neutral wire. The earth wire and fuse are just for safety.

All appliances with metal cases must be earthed to reduce the danger of a shock. This means that the casing is attached to the EARTH WIRE. An earthed conductor can never be live. If there is a plastic casing, this is double insulation, as it stops the current flowing.

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Safety precautions when using plugs

Check the there is no damage to the plug, since live parts could be exposed, resulting in a shock.

Check that the cable isn't frayed, or live parts could be exposed, and this is potentially dangerous.

Check the cables are not too long, as this could be a trip hazard.

Check that the plug is away from water, as water is an excellent conductor of electricity.

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Earthing appliances

Earthing and fuses prevent fires and shocks from happening. If a fault occurs and the live wire is touching the metal casing, the big current flows in through the live, conducted by the metal casing, and then out through the earth wire.

This surge in current blows the fuse, which cuts off the live supply.

This isolates the appliance and you cannot recieve a shock from the casing. This also prevents a fire by isolating the appliance when there is a large current flowing, since there are problems with over heating.

Circuit breakers.

They are used in circuits for safety, so that there isn't too much current flowing through as a possible fire hazard, or damaging the circuit. When the current flowing through the circuit is too large, the circuit breaker opens a switch via a magnet, to break the switch. It can easily be reset by flicking a switch on the device. Therefore they can be more convinient than fuses because they need not be replaced after the act.

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RCCBs

Residual Current Circuit Breaker

Usually an equal current flows through the live and returns through the neutral wire. An RCCB compares the current going in to the current going out. If something goes wrong in the appliance, the current wil flow through the earth wire, therefore there will be less current flowing through the neutral wire than the live wire. This difference is the residual current. The RCCB detects this difference and cuts off the power by opening a switch.

RCCBs operate much faster than fuses, and they work for even small current changes, as these can still be fatal. They are thus more effective at protecting against electrocution.

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Resistors and electrical power

A resistor restricts the flow of current in a circuit, and provides resistance (the ability of a material to resist the flow of current, measured in ohms.) They can be used to protect components from damage due to an overload of current. When the current flowss through the resistor, there is an energy transfer of heat. The heating increases the resistor's resistance, so less current will flow. The heating can cause components in the circuit to melt, which stops it working properly, as a safety measure. This heating can also be useful when you want to heat something, for example toasters have coild of wire which glow and toast food when the current passes through.

ELECTRICAL POWER: is the rate at which an appliance transfers energy. This energy comes from the current flowing through the circuit, so a higher power rating will draw up a large current from the supply. Most electrical appliances show their power and volatge rating.

ELECTRICAL POWER= CURRENT X VOLTAGE

P = I x V

Fuses used should be slightly higher than the normal operating current. You can work out the fuse you need by working our the current the oobject normally uses.

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Electrical appliances transfer electrical energy

ENERGY TRANSFERRED = CURRENT x VOLATGE x TIME

When the current is flowing through a component, the energy is transferred into other forms of energy, for example in a buzzer, it is transferred to mainly sound energy.

This amount of energy transferred depends on how long the component is on for, the current, and the voltage.

Example:

If a buzzer is part of a circuit with a 1.5V cell, and 5A is flowing for 10 minutes, how much energy has been transferred by the buzzer?

E= I x V x t

1.5 x 5 x (10 x 60) = 4500 J

time is in seconds

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Circuits

• CURRENT- The rate of flow of charge in the circuit. Current will only flow when voltage is applied across the component. Measured in amperes (A).
• VOLTAGE- "electrical pressure" is the driving force that pushes current. Measured in  Volts (V).
• RESISTANCE- anything that slows the flow down. The more components, the greater the resistance. Measured in ohms (Ω)
• The relative sizes of the voltage and resistance decide the current.
• Increase in voltage = more current will flow.
• Increase in resistance = less current will flow.
• A test circuit: The Ammeter measures the current and must be placed in series to the component being tested. However, it can never be placed parallel to the voltmeter. The voltmeter  measures the voltage, and must be placed parallel to the COMPONENT UNDER TEST.

This test is the most basic way of getting results for VI graphs. The variable resistor, component and ammeter are in series, so they can be in any order, but the voltmeter must be around the component. The variable resistor will vary the current (x-axis.) Therefore you can get readings for V and I and plot them on a graph.

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Resistance and types of current

Mains supply of electricity (230 V) is a.c alternating current, therefore the current is always changing direction. However, cells and batteries supply d.c direct current. This means that the current only flows in the same direction.

Voltage = current x resistance

V= I x R

Example: A 5 ohm resistor is part of a circuit where the voltage across it is 10V. What is the value of the current flowing through the resistor?

10= I x 5  ----> I= 10/5 -----> I = 2 A

This equation is useful to work out the resistance for a pair of value on a VI graph. The gradient of the graph = 1/R. The steeper the graph, the lower the resistance. A straight line graph has a constant resistance. When the graph curves, the resistance is changing.

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Voltage- Current Graphs

Metal Filament Lamp: When the temperature of the metal filament increases, the resistance increases, shown by the curve.

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Voltage- Current Graphs

A wire: A current through a wire (which is at a constant temperature,) is propotional to voltage.

Different resistors: At a constant temperature, the resistor will show a straight line because the current is proportional to the voltage. However, varying resistors have different resistances, so therefore the gradient of the line is different.

THE STEEPER THE GRAPH THE LOWER THE RESISTANCE.

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Voltage- Current Graphs

A diode:

Current only flows through the diode in one direction. When the voltage is positive, it conducts the electricity.

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LEDs, LDRs and thermistors

• LEDs: Light-emitting diodes emit light when a current flows through them in the forward direction. They can be used for numbers of digital clocks, traffic lights, and in remote controls.They are useful as unlike other lamps, as they do not have a metal filament which can burn out. They can be used to indicate the presence of a current in a circuit, and are often used for this in appliances.
• LDRs: Light-dependent resistors change their resistance depending how much light falls onto them. Bright light= resistance falls, darkness= resistance increases. This is useful for a camera lens cover, or security lighting.
• Thermistors: hot =resistance drops, cool= resistance increases. This is useful for mechanisms such as a car engine temperature sensor.
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Series and parallel circuits

In a series circuit, the components are connected in a line, and all the components must work. If one is disconnected, the circuit breaks, and it will not work.

The current is the same through all parts of the circuit. The total resistance of the circuit depends on the number of components and type of components used.

A1=A2 where ever the component or ammeter is in the circuit.

Parallel: ISOLATION

Each component is separately connected to the +ve and -ve of the supply. The components can be operated separately, and if one is faulty, it will not break the whole circuit. This is how electricity works in a house; each light is connected in a parallel circuit so that they can be switched on or off independently.

HOWEVER

Ammeters are always connected in series, even in parallel circuits, and voltmeters are always connected in parallel to the component being tested even in a series circuit.

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Charge Voltage and Energy change

Current is the rate of flow of electrical CHARGE. In solid metal conductors, charge in carried by negatively charged electrons.

Charge (Q) = current (I) x time (t) in seconds

More charge flows in a circuit when there is a larger current.

CHARGE IS MEASURED IN COULOMBS (C)

PAPER TWO- When the charge goes through a change in voltage, the energy is transferred. Energy is supplied at the cell to 'raise' the charge through voltage. The charge gives up this energy when it passes through a component where the voltage is lower. The bigger the change in voltage, the more energy is transferred. A bigger voltage cell supplies more energy to the circuit for every cooulomb of chargewhich flows around it.

VOLTAGE IS THE ENERGY TRANSFERRED PER UNIT CHARGE PASSED.

THE VOLT IS A JOULE PER COULOMB.

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Static electrcity

PAPER TWO- Build up of static electricity is caused by friction, which leaves a positive electrostatic charge on one, and a negative electrostatic charge on the other. This depends on the two materials involved.

POLYTHENE ROD

Electrons move from the duster to the rod. The rod is therefore negatively charged, and the duster has an equal positive charge (from losing electrons.)

ACETATE ROD

Electrons move from the rod to the duster. The rod is therefore positively charged, and the duster has an equal negative charge (from gaining electrons.)

A charged conductor can be safely discharged if it is connected to the earth via a metal strap. the electrons flow down the strap to the earth is the charge is negative, to lose electrons. However, if the charge is positive, the electrons flow up the metal strap to neutralise the charge. ONLY ELECTRONS MOVE.

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Static electrcity

PAPER TWO- Alike charges repel, opposite charges attract.

Gold leaf electroscope:

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Static electrcity

There are two thin pieces of gold leaf. When an object with a KNOWN charge is brought near to the metal cap/disk, depending on the charge, it will either attract or repel. This in turn induces a charge on the gold leaves. If the charge is the same, they will repel eachother and rise until the object is taken away.

Another way to experiment with charges is the suspend a rod with a known charge by thread. Then bring another object next to it and if tghey attract, they are opposite etc.

Van de Graaff generator: Is a rubber belt moving round plastic rollers, underneath a metal dome. A metal charge is built up on the dome as the belt goes round, and if you stand on a insulated chair, the electrons move between your body and the dome which gives your body a charge. The charge can make your hairs repel eachother and stand on end.

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Uses of static electricity.

PAPER TWO-

1) Inkjet printer: Tiny droplets of ink are forced out of a fine nozzle which makes them charged.

2) The droplets are deflected between two metal plates , where a voltage is applied on them,  one is +ve and one -ve charged. Therefore it repels from one and attracts to the other. The size and direction of the voltage across the plates so that each droplet is deflected to land on different parts of the paper. These dots make up the image.

1) Photocopier: The image plate is positively charged. Whiter bits of the images make more light fall on those places, and the charge leaks away. Charged bits attract to negatively charged black powder, which is transferred onto positively charged paper.

2) The paper is heated so the powder sticks.

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Problems with static electricity

PAPER TWO-

1) Lightning: Rain drops and ince rub together knocking off electrons. The top of the cloud is positively charged, and the bottom is negatively charged. This creates a huge voltage and a massive spark.

2) Fuel filling: When fuel flows out the filler pipe, static can build up, which can easily lead to a spark in a dusty or fumy place. Therefore they make the nozzle out of metal so that the charge is conducted away, instead of building up and leading to a huge spark. It is also useful to have earthing straps between the fuel tank and the fuel pipe.

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