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12 Cards in this Set
- Front
- Back
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What is current |
-Current is the flow of electric charge around the circuit. Current will only flow through an electrical component if there is a potential difference across that component, and if the circuit is compete. The unit is ampere (A) Charge = current x time. Q/ I x T |
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What is the potential difference |
This is the driving force that pushes the charge round. The unit is volt (V) |
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What is resistance |
Is anything that slows the flow down. The unit is ohm (omega symbol) |
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Current-Potential difference Graphs |
Back (Definition) |
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What is a LDR |
-An LDR is a resistor that is dependent on the intensity of light. -In bright light, the resistance falls. -In darkness, the resistance is highest. -They have lots of applications including automatic night lights, outdoor lighting and burglar detectives |
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What is a thermistor |
-A thermistor is a dependent resistor. -In hot conditions the resistance drops. -In cool conditions, the resistance goes up. -Thermisyors male useful temperature detectors. |
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Series circuits |
-Components are connected in a line, except for voltmeters which have to be in parallel. -If you remove in component, the circuit is broken and all stop working. -Current same everywhere. Size of current depends on total Pd and total resistance. -PD is shared among, so all add up to equal total. Total resistance increase as you add resistors. This is because the two resistors have to share the Pd. The pd across each resistor is lower, so the current through each repository is also reduced when a resistor is added. This means the total resistance of the circuit increases. The bigger a components resistance, the bigger it’s share of total pd. |
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Parallel circuits |
-Every component is separate, if you disconnect one component, the others will still work. -How most things are connected. -Current adds up to equal total. The total current entering a junction is the same when leaving. -PD is different -Total resistance of circuit decreases if you add a second resistor. Their is because in parallel, both resistors have the same pd across them as the source. This means the pushing force making the current flow is the same as the source of for each resistor you add. But by adding another loop, the current had more than one direction to go in. This increases the total current that can flow through the circuit. Using V=IR, an increase in current means a decrease in the total resistance of the circuit. |
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Energy in circuits |
E=IVT. The larger the current through, or Pd across, a component has more energy transferred to it. Heating in a circuit isn’t always bad. It generally reduces efficiency. If temperature gets too high, it can cause the components in the circuit to melt. Fuses use the effect to protect circuits- they melt and break the circuit if it gets to high. The heating effect can have other advantages. Eg a toaster as the coil of wires goats the bread. |
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Power in circuits |
Energy transferred depends on how long the appliance is on for and it’s power. The power can be found using Power (w) = energy transferred (j) / time (s). P=E/T. Power ratings use this. Appliances are labelled with the maximum safe power that they can operate at. The power rating tells you the maximum amount of energy transferred between stores per second when the appliance is in use. The helps customers chose between models- the lower the power rating, the less electricity an appliance uses in a given time so the cheaper it is to run. Power also depends on current and pd. use P=IV. You can use this equation to work out the fuse that should be in an appliance. |
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Electricity in the home |
Mains supply is ac, battery supply is dc. In cables, there are 3 wires, each a core of copper and a coloured plastic coating. 1) live wire- brown. The live wire carries the pd. It alternates between a high +ve and -ve voltage of about 230v. 2) neutral wire- blue the neutral wire completes the circuit- when the appliance is operating normally, current flows through the live and neutral wires. It is around 0v. 3) Earth wire- green and yellow. The earth wire is for safety and protecting the wiring. It carries the current away if something goes wrong and stops the appli enact casing becoming live. It’s also at 0v. Touching the live wire gives you an electric shock as your joy is at 0v. Their means that if you touch the live wire, a large pd is produced across you body and a current flows though you. This causes a large electric shock which could injure or kill you. |
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Fuses and earthing |
The earths wire and a fuse prevents surges of current melting appliances or causing a fire. This is how they work: -if a fault develops in which the live wire somehow touches the metal case, then beasts the case is earthed, too great a current flows trough the live wire, the case and the earth wire. This surge in current melts the fuse. Fuses are connected to the live wire so that breaking the fuse breaks the circuits and cuts off the live supply. This isolated the whole appliance, making it impossible to get an electric shock from the case. It also prevents the risk of fire cause by the heating effect of a large current. Fuses should be rated as near as possible but just higher than the normal operating current. The larger the current, the thicker the cable you need to carry it (to stop the va le getting too hot and melting). That’s why the fuse rating needed for cables usually increases with cable thickness. As well as the fuses in plugs, there are also household fuses. These work in the same way, but protect the wiring in the house, not just the appliance. Circuit breakers are even safer than fuses. Instead of melting a fuse, a large current may instead ‘trip’ a circuit breaker. Circuit breakers turn if quicker than the time taken for a fuse to melt. They can also be reset, which is much more easier than having to replace a fuse. Gower’s, they are more expensive than fuses. |
