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56 Cards in this Set
- Front
- Back
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What is electric current? |
electric current is the rte of net flow of any electrically charged particles. |
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What is the unit for current? |
Ampere (A) |
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What is the quantity symbol of current? |
I |
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What is conventional current? |
Conventional current is the direction in which a unit of positive would go; positive terminal to negative terminal. |
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What direction is electron flow? |
Negative terminal to the positive terminal. |
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What is a resistor? |
A component which has a fixed value of resistance to control the current flow. Often made from carbon or carbon film. Left symbol is American. |
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What is a variable resistor / rheostat? |
A resistor, whose resistance can be changed by rotating a knob or moving a slider. |
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What is a battery? |
A component which supplies energy to charge carriers to make the current flow. A battery is made by joining a number of cells together. +ve terminal is the longest line. |
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What is a switch / circuit breaker? |
A component which interrupts the current, usually by introducing an air gap into the circuit (breaking the circuit). Some switches can be used to switch the current between alternative circuits. |
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What is a voltmeter? |
A component which measures the p.d. between two points in the circuit. |
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What is an ammeter? |
A component which measures the current at a point in the circuit. |
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What is an LED? |
Light Emitting Diode. Made of a material which emits light when an electric current is passed through. |
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What is an LDR? |
Light Dependent Resistor. A component whose resistance depends on the light intensity. |
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What is a thermistor? |
A component whose resistance depends on temperature. |
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What is an indicator? |
A component that lights up when the current or p.d. is at a suitable level. Can be an LED or a filament bulb for example. |
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What is the equation to calculate current involving Q and t? |
I = Q/t |
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An ideal ammeter would have... |
0 resistance |
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Ammeter has to be connected in... |
Series |
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Define the Coulomb |
The product of current of 1 ampere and of time 1 second, gives a total charge of 1 coulomb. |
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What is the charge of an proton (and an electron)? |
(-)1.6e-19C |
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What is the equation for Total charge? |
Total Charge = No. of electrons x elementary charge |
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Define charge carriers. |
Charged particles that move through a substance when a p.d. is applied across it. |
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Electric current through a metal is due to the flow of... |
electrons. |
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Define potential difference |
The energy per unit charge, transferred from electrical to other forms. |
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What is the equation of p.d. or voltage in terms of E and Q? |
V = E/Q OR V = W/Q |
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What happens if you substitute It = Q into W = VQ |
W= VIt |
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Define the Volt |
1 volt is equal to 1 joule per coulomb. |
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An ideal voltmeter would have... |
infinite resistance. |
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A voltmeter has to be connected in... |
parallel (across a component). |
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Define resistance. |
Ratio of the potential difference across the resistor to the current through it. |
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What is the equation for resistance? |
R = V /I |
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Define the Ohm |
1 ohm is equal to 1 volt per ampere. |
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Describe what causes electrical resistance. |
The resistance of a component in a circuit is a measure of the difficulty of making the current pass through the component. Resistance is caused by repeated collisions between the charge carriers in the material with the fixed positive ions of the material. As the temperature increases the positive ions vibrate more, which increases the chance of an electron collision, therefore the resistance increases. |
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State Ohm's law. |
For a metallic conductor, the p.d. across it is proportional to the current through it, when other physical conditions are constant. |
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State what is meant by the term ohmic device. |
An Ohmic device obeys Ohm's law. Plotting a graph of I - V gives a straight line through the origin. Resistance is constant. |
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State what is meant by the term non-ohmic. |
A non-ohmic device doesn't obey Ohm's law. Plotting I - V on a graph doesn't produce a straight line. Resistance is not constant. |
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How would you determine the resistance from an I - V graph. |
The resistance can be determined at any point on the graph by determining the ratio of V to I, at that point. |
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Describe and explain how current through a filament lamp varies with p.d. |
The increase in p.d. is greater than the increase in the current. Hence the ratio of p.d. to current increases. Resistance is increasing, therefore it doesn't obey Ohm's law (it's a non-Ohmic device). This is because, as you increase the current through the filament, the temperature of the filament increases. Hence the rate of electron-ion collisions increases. |
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State the characteristics of a diode |
A diode only allows current to flow in one direction.
Upto ~ 0.6 V, there's high resistance, giving a small current. After 0.6 V resistance decreases rapidly and therefore current increases. Reverse Bias: Very high resistance gives a very small current. With diodes, we use conventional current. |
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What is the resistance of a material dependent on? |
Its dimensions. |
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What is the equation for resistivity? |
ρ = RA/L |
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What is resistivity of a material dependent on? |
its temperature |
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What is resistivity of a material independent of? |
its dimensions |
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Define Resistivity |
Resistivity = Resistance x Cross-sectional area / Length |
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What are the units for resistivity? |
Ωm |
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What is the shape of the cross-section of a wire? |
Circle
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Describe how current through a NTC thermistor varies with p.d. |
A thermistor is a negative temperature coefficient semi-conductor. As the temperature increases, the resistance decreases. It can be used as a temperature sensitive sensor. |
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Describe and explain the V - I graph for a NC thermistor. |
The increase in p.d. is less than the increase in the current. Hence the ratio of p.d. to current decreases. Resistance is decreasing, therefore doesn't obey Ohm's law. This is because as the current through the thermistor increases, the temperature of the thermistor increases. Hence the number of free electrons increases therefore the resistivity decreases. So the resistance of the thermistor decreases. |
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Describe and explain the effect of temperature on the resistance of metal conductors. |
The greater the temperature, the greater the resistance. Hence, the greater resistivity. this is because, as the temperature of a metal wire increases, the ions oscillate more (since they have more KE), Therefore, the rate of electron-ion collisions increases. |
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Describe and explain the effect of temperature on a negative temperature coefficient thermistor. |
The greater the temperature, the lower the resistance. Hence the lower resistivity. This is because, as the temperature of the thermistor increases, the number of free electrons increases, so the resistance of the thermistor decreases. |
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Explain how how a thermistor can be used in an on/off circuit. |
To turn a device on or off a certain temperature, e.g. thermostats in: -Central heating systems - Refrigerator -Electric oven |
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Explain how how a thermistor can be used in an thermal cutoff circuit. |
To turn a device off over a certain temperature to prevent overheating. E.g. Hairdryer, Tumble Dryer, Electric Oven. |
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Explain how how a thermistor can be used in an temperature measurement circuit. |
To determine the exact temperature from the potential difference across the thermistor. E.g. Digital thermometer, Car engine management system, central heating temperature display. |
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Explain what is meant by a superconductor. |
A superconducting material has zero resistance (resistivity) below a certain critical temperature. Once below this temperature, due to zero resistance, massive currents can flow without losing any energy as heat. This is achieved by cooling the material with liquid elements such as nitrogen or helium. |
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Describe how superconductors can be used to produce strong magnetic fields and to reduce energy losses in the transmission of electric power. |
Superconductors can be used as electromagnets to produce massive magnetic fields, in: -MRI scanner -Particle Accelerator -Maglev rains -Nuclear fusion reactors Superconductors can be used to improve efficiency of electricity production, in: -Electrical generators -Transformers -Transmission We use superconductors in these scenarios because they have no resistance therefore there's no energy dissipation. |
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Why are superconductors used in the LHC in CERN? |
Because without the superconducting material, the electromagnets would have been too large and would have lost too much energy as heat. |
