Capacitors Flashcards
(18 cards)
Capacitance?
The capacitance of an object is the amount of charge it is able to store per unit of potential difference across it. It is measured in farads, F, and is the equivalent of one coulomb per volt
Capacitor?
An electrical component which can store electrical charge. They are made of two plates separated by an insulator otherwise a dielectric
Capacitor connected to circuit?
Charge builds up on its plates with one becoming positively charged and the other negatively charged. The separation of the plates from the insulator means no charges move between the plates which creates a potential difference between the plates
Voltage on capacitors?
The voltage rating of a capacitor is the maximum potential difference that can safely be put across the two plates. A capacitor only builds charge up to the voltage of the power supply it is connected to
Investigating charge and voltage relationship for capacitors?
On a capacitor the capacitance will be fixed which means its voltage and charge have a directly proportional relationship. In a circuit with constant current maintained with adjusting a variable resistor, at time intervals record the potential difference until it equals that of the battery. Then plot a current-time graph, using the area of this graph identifies the charge which allows a charge-voltage graph to be plotted to show this relationship
Uses of capacitors?
Capacitors can only store relatively small amounts of charge, significantly less than a battery. They only provide power for a short time as they store charge until its needed and then discharge it all in a fraction of a second whereas a battery may take much longer to discharge it
Electric Potential Energy for capacitors?
As the charge builds up on a capacitor one plate becomes negative and the other positive. Like charges repel so to keep the charges on the plate electric potential energy is supplied by the power supply and this is supplied for as long as they are held, until they are released where this electrical potential energy is released. This is worked identified by using the area on a charge-voltage graph or the equation E=1/2 x Q x V
Energy used with capacitors?
Since the energy stored by the capacitor is E=1/2 x Q x V it means half the energy supplied to the capacitor is lost due to resistance in the circuit and the internal resistance
Permittivity?
A measure of how difficult it is to generate an electric field in a medium. The higher the permittivity the more charge that is needed to generate an electric field of a given size
Relative Permittivity?
Relative permittivity is the ratio of permittivity of a material compared to the permittivity of free space otherwise εr = ε1 / ε0 where ε1 is the permittivity of the material and ε0 is the permittivity of free space. It is sometimes called the dielectric constant and is the reason capacitance can be different for capacitors made from different materials as different materials have different relativity permittivity values
Polar Molecules?
Polar molecules are what a dielectric is made up of. When there is no charge stored there is no electric field so they are aligned randomly. When charge is applied their negative end attracts to the positive plate and the positive end attracts to the negative plate. They have their own field which opposes the direction of the field of the capacitor. The larger the permittivity the larger this opposing field is so capacitance increases as it results in a lower potential difference to transfer a charge to the capacitor
Investigating capacitance with dielectrics?
This can be experimented by altering how much the capacitor plates overlap, using different materials as the dielectric or stacking multiple layers of same material to investigate separation on capacitance. These factors test the equation: C = Aε0εr / d where “d” is the separation distance between the capacitor plates and “A” is the area the plates overlap each other
Charging capacitors?
A current flows in the circuit until it is fully charged where it becomes 0A. This is because as more charge builds up the electrostatic repulsion makes it harder for electrons to be deposited which decreases current and when the potential difference is same as supply it becomes 0A. Electrons flow from the negative terminal to build a negatively charged plate. Electrons are repelled from positive plate as negative charge builds up on negative plate so electrons are repelled and attracted to positive terminal. The same number of electrons are repelled as built on negative plate which results in equal and opposite charge on each plate which causes the potential difference between the plates
Charging and Discharging Capacitor Curves?
For charging the equations for Voltage and charge are Q = Q0 ( 1 - e ^ -T/RC ) and for current is I = I0 e ^-t/rc. For discharging voltage, current and charge all have the equation Q=Q0e^-t/rc
Experimentally investigating discharging capacitors?
Have a circuit with a resistor, capacitor, switch, power supply, voltage sensor and data logger. Close the switch and charge the capacitor. Then open the switch and remove the power supply. When the switch is then closed the capacitor will discharge. When the ammeter reaches zero stop the plot in the data logger. Using the data it has collected will allow discharge curves of current, voltage and charge to be plotted
Factors affecting discharging and charging of capacitors?
The time taken for a capacitor to charge or discharge depend on the capacitance of the capacitor as this affects the amount of charge transferred at a given voltage, the other factor is resistance of the circuit as this affects the current in the circuit
Time constant?
The time taken for the charge on a discharging capacitor to fall to 37% of the initial charge or for a charging capacitor is the time it takes for the capacitors charge to rise 63% compared to the initial charge. When a circuit has more resistance it takes longer for the capacitor to charge or discharge
Time to halve proof?
Q = 1/2 QO
This means:
Q=Q0e^-t/rc -> 1/2 = e ^-t/rc
T = ln2 RC
