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Terms in this set (37)
Current
Rate of flow of charge (I) measured using an ammeter
Ammeter
They are put in a circuit in series and are designed with a very low resistance, so they do not effect the current they are measuring
Charge
Measured in coulomb (C) (one coulomb is the amount of charge that passes in 1 second if the current is 1 Ampere), charge flows from a high potential to a low potential
EMF
electrical energy produced per unit charge passing through a source
Equation for charge and current
Q = I*t, where Q = charge, I = current and t = time
Equation for charge and electrons
Q = n*e - where 'n' is the number of electrons and 'e' is the charge of one electron
Charge of one electron
e = -1.6 * 10^-19
Potential Difference (voltage)
Energy transferred (work done) per unit charge
Equations for p.d and charge
V = W/Q
Voltmeter
Measures potential difference across a component, they are placed into the circuit in parallel and they have a very high resistance so as not to disturb the circuit they are measuring
Resistance
V/I - can be thought of a measure of how difficult it is to get a current to flow through a component
Measuring resistance
Using a rheostat or a potential divider
Using a rheostat to measure resistance
Rheostat placed in series to the circuit, resistance is altered to measure the current and voltage, advantages are that it is easy to wire up however it does not give a good spread of readings
Using a Potential divider to measure resistance
Potential divider is placed into the circuit connected to the power supply, and then the voltmeter, ammeter and component are placed in parallel, advantages are that this gives a better spread of readings but the current drawn from battery is much bigger than the rheostat circuit as there are two routes for current to take so battery runs down more quickly
Explanation of resistance
Electrons which have a negative charge form the current in metal wires which are formed from positive lattice ions, as electrons flow down the wire, they bump into and rebound of the positive lattice ions which causes resistance
Resistance and temperature
resistance of a metal increases with temperature, this is because of the positive ions in the conductor vibrate more when its temperature is increased, the electrons therefore cannot pass through the metal as easily when p.d is applied across the conductor
Example of ohmic conductor
Carbon resistor at fixed temperature, metal wire at fixed temperature
Ohm's Law
At a constant temperature, the current through an ohmic conductor is directly proportional to the potential difference
Ohmic conductor resistance graph
The graph is a straight line with a positive gradient, passing through the origin, as the potential difference increases, the current increases at the same rate producing a uniform gradient
Resistance graph for a non ohmic conductor - Filament Lamp
The I/V graph for a filament lamp is a curve that starts steep but gets shallower as voltage rises (same shape at negative voltage and current), as current increases, temp of filament (coiled wire) increases, resistance increases with temperature, this means as voltage increases at set increments, current also increases but by smaller and smaller set increments, around origin there is a straight line obeying ohm's law as bulb is not yet hot
Diodes
Only let current flow in one direction
Forward Bias
The direction the current is allowed to flow
Between 0.0 and 0.7 volts
This is the point where the resistance of the diode is infinitely high so no current flows through the diode
Above 0.7 volts
This is known as the turn on voltage or threshold voltage in most diodes, this is where the resistance suddenly falls rapidly and a large current will flow
Break down voltage
Typically -50 to -500 volts, at this voltage a large current can flow, but most diodes are destroyed when this occurs
Resistance depends on what?
length (l) of the wire (longer the wire, higher the resistance), the cross sectional area of the wire (A), the higher the area of the wire, the lower the resistance, an the resistivity (row) which depends on the type of material and is how easy current can flow through that certain material
Resistivity Equation
Experiment to find resistivity
-First find cross sectional area of wire, use a micrometer and find diameter of wire at 3 points along wire (both ends and middle) and repeat at right angle to give six results, find mean and use it to calculate the area
-Set up circuit as shown in diagram
-Attach test wire to a ruler where it reads zero
-Attach fly clip (lead with crocodile clip at the end) to any point on the wire
-Record length of wire, voltmeter and ammeter readings
-Use this to calculate resistance for that length
-Repeat at many lengths, plot resistance against length
Superconductor
a wire of component made of material that has zero resistivity at and bellow a critical temperature, the component has zero resistance below the critical temperature so when a current passes through it there is no p.d so the current has so heating effect
Uses of superconductivity
used to make high powered electromagnets which generate a very strong magnetic field in devices such as MRI scanner and particle accelerators, also used in development of power cables which transfer electrical energy without energy dissipation
Power
Rate of transfer of energy, it is measured in watts, energy/time, a big power results in a heating effect in the resistor and increases the brightness of a lamp
In series, which bulb/resistor is hotter/brighter
The current is the same, this means the bulb/resistor with the biggest resistance is brighter/hotter because it uses up a higher proportion of the voltage and so more power
In parallel
The voltage is constant through each resistor/bulb, but the current splits and the smallest value resistor receives the most current and so uses up more power, this means it will get hotter/brighter
In series...
Current is the same, voltage is shared out in proportion to the value of the resistors
In Parallel...
Current splits between the resistors and voltage stays the same
Light dependant resistor
LDD, has a very high resistance in the dark but a lower resistance in the light
Thermistor
Has a very high resistance in cold temperatures and a low resistance in hot temperatures
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