What is a capacitor?
A device designed to store charge
What does a capacitor consist of?
It consists of two conducting surfaces seperated by an insulator called a dielectric.
Draw the circuit symbol for a capacitor?
pg380
What happens when a capacitor is connected to a DC power supply?
- Electrons in the circuit move through the battery and flow from the negative terminal of the battery on to the plate connected to it causing this plate to become negatively charged.
- At the same time an equal number of electrons flows from the other plate towards the positive terminal of the battery, causing this plate to become positively charged.
- As a result each plate gains an equal and opposite charge, resulting in a potential difference across the plates which in turn generates a uniform electric field.
- Capacitors do not allow the flow of current through it due to the gap.
What happens to the current when a capacitor is connected to a DC supply?
- Initially the current through the circuit is high. (Rate of flow of charge is high in the circuit)
- However as charge builds up on the plate, the increasing electrostatic repulsion from the negative plate, makes it harder and harder for electrons to be deoposited. (Therefore rate of flow of charge decreases). Therefore the current in the circuit decreases.
- When the potential difference across the capacitor is the same as the potential difference of the supply, current falls to zero.
- Capacitor is fully stored.
*Note: the poles of a battery are equal and
opposite in charge
* The potential difference across the capacitor increases as the magnitude of the charge difference increases. The potential difference will continue to increase until it equals the potential difference of the supply.
What is the capacitance C of a capacitor?
The capacitance C of a capacitor is defined as the charge stored per unit pd.
What is the equation to calculate capacitance?
For a capacitor that stores charge Q at pd V, its capacitance is given by:
C = Q/V
C = Capacitance (F, Farads) Q = Charge (Coloumbs, C) V = Potential Difference (V, Volts)
Applications of capacitors?
Smoothing circuits (smoothing out unwanted variations in voltage)
- Backup power supplies
- Filter circuits that remove unwanted frequencies
- Pulse-producing circuits (circuits which switch on and of repeatedly).
One of the ways we can alter the capacitance of a capacitor is by changing the _______ _______ seperating the two ______ _______.
…by changing the dielectric material seperating the two conducting materials.
What is a dielectric?
This is the electrically insulating material between the plates.
What is the effect of placing a dielectric* between oppositely charged parallel plates connected to a battery?
- a dielectric is used to replace the gap between the plates (this gap may have just been a vacuum or dry air)
- dry air is a dielectric but this question means placing a dielectric with a larger relative permittivity - this is just how the question is phrased.
A dielectric (with a large relative permittivity), when placed between two parallel conducting plates of a capacitor, can allow it to store more charge at any given pd. In other words, its effect is to incease the capacitance of the capacitor.
*relative to the dielectric used previously
Examples of dielectrics?
- Polythene
- Waxed paper
- Dry air
- Water
- Vacuum is not a dielectric - check with sir
- Some have a larger relative permettivity than thers
How does a dielectric actually increase the charge stored by a capacitor?
As the conducting parallel plates of a capacitor become oppositely charge, a potential difference occurs across them generating a uniform electric field. This electric field polarises each molecule of the dielectric. This means that the electrons in the molecule of the dielectric are pulled slightly closer to the positive plate, giving that end of the molecule a slight negative charge, whilst the other end of the molecule gain a slight positive charge (due to the absence of electrons) and thus are attracted to the negative plate. This means the surface of the dielectric facing the positive plate gains a negative chage and the surface of the dielectric facing the negative plate gains a positive charge. As a result, more charge is stored on the plates because (1) the positive side of the dielectric attracts more electrons from teh battery onto the negative plate and (2), the negative side of the dielectric plate pushes electrons back to the battery.
What is relative permittivity, εᵣ?
This is the ratio of charge stored with the dielectric to the charge stored without the dielectric.
How to calculate relative permittivity, εᵣ?
εᵣ = Q/Q₀ = C/C₀
εᵣ = relative permittivity (no units as its a ratio) Q = Charge stored by parallel-plate capacitor at a given pd when space is completely filled with a dielectric substance Q₀ = Charge stored by parallel-plate capacitor at the same pd as Q, when space is completely empty C = Capacitance of parallel-plate capacitor at a given pd when space is completely filled with a dielectric substance C₀ = Capacitance of parallel-plate capacitor at the same pd as Q, when space is completely empty
Why can the ratio of capacitance also be used to calculate εᵣ?
Using equation C = Q/V, charge is proportional capacitance when potential difference remains the same.
What other way can you calculate εᵣ?
εᵣ = ε / ε₀
εᵣ = relative permittivity of material 1 ε = permittivity of material 1 in Fm⁻¹ ε₀ = permittivity of free space, 8.85x10⁻¹² Fm⁻¹
What is the relative permmitivity of a substance also known as?
Its dielectric constant.
What actually is permettivity?
This is a measure of how difficult it is to generate an electric field in a medium. The higehr the permittivity of a material, the more charge charge needed to generate an electric field of a given size through it.
The large the relative permittivity/dielectric constant of the dielectric used, the ….?
…the more charge it can store and the higher the capacitance will be (WHY THO? LINK WITH DEFINITION OF PERMITIVITY.)
Does the molecules of a dielectric always have to be polarised?
No. In some dielectric substances, the molecules are already polarised, so when an electric field occurs across the parallel conducting paltes of a capacitor, all they do is rotate so that that the positive side of the molecule face the negative plate and the negative side of the moelcule face the positive plate.
When a capacitor has a dielectric field, between it, a new equation is needed to calculate its capacitance. What is this equation?
C = Aε₀εᵣ/d
C = Capacitance (Farads, F) A = Surface of a plate (m²) d = spacing between the plates (m) εᵣ = relative permittivity of the dielectric ε₀ = permittivity of free space, 8.85x10⁻¹² Fm⁻¹
From the equation, a large capacitance can be achieved by?
- Making the area of a plate as large as possible
- Making the plate spacing d as small as possible
- Filling the space between the plates with a dielectic which has a relative permittivity as large as possible
We have gone through how to charge a capacitor. How does a capacitor discharge e.g. through a resistor?
You must connect it to a closed circuit with just a resistor (+ ammeter to measure current in circuit, adn voltmeter, to measure pd of capacitor).
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