Revision Tips - Electricity 8th May You will need paper, pen and a calculator Use the chat pod to ask questions

Capacitors - What you need to know? 1. Capacitors and the relationship between capacitance, charge and potential difference. 2. The total energy stored in a charged capacitor is the area under the charge against potential difference graph. 3. Use the relationships between energy, charge, capacitance and potential 4. Variation of current and potential difference against time for both charging and discharging. 5. The effect of resistance and capacitance on charging and discharging curves.

A classic capacitor design A simple capacitor consists of 2 parallel metal conducting plates separated by an electrical insulator such as air. A graph of charge versus potential difference for a capacitor illustrates the direct proportionality between charge and pd. The gradient of this line (the ratio between Q and V) is defined as the capacitance (C) of the capacitor. This leads to the relationship C= Q/V.

Charging a capacitor To pull electrons from the positively charged plate and push them onto the negatively charged plate requires energy. The cell below must do work against the potential difference between the capacitor plates. WORK MUST BE DONE TO CHARGE A CAPACITOR

EXAMPLE

Current and Voltage time graphs for a charging capacitor

Current and Voltage time graphs for a discharging capacitor

Energy stored in a capacitor The negatively charged plate will tend to repel the electrons approaching it. In order to overcome this repulsion work has to be done and energy supplied. This energy is supplied by the battery. Note that current does not flow through the capacitor, electrons flow onto one plate and away from the other plate.

contd

EXAMPLE A 40 mF capacitor is fully charged using a 50 V supply. Calculate the energy stored in the capacitor.   energy = ½CV2 = ½ × 40 × 10–6 × 502 = 0.05 J

Your Turn

Wheatstone Bridges A potential divider is a circuit consisting of a number of resistors (often only two) in series, connected across a supply, that is used as a source of fixed or of variable p.d. Here, the potential difference across each of the resistors R1 and R2 is a fixed fraction of the potential difference of the supply (Vs). This means that the fixed potential of point P is determined by the values of the two resistors R1 and R2. The ratio of the potential differences across the resistors in a potential divider circuit is the same as the ratio of the resistances of the resistors. The following relationships hold for all potential divider circuits that consist of two resistors, V1/ V2 = R1/ R2

Wheatstone Bridge The Wheatstone bridge circuit shown is balanced when R2 is set to 384 Ω. Calculate the resistance of R1

Answer At balance:

Quiz The Wheatstone bridge circuit shown is used in the questions in this quiz.

Question 1 The Wheatstone bridge circuit shown is balanced. What is the relationship between the resistances

Question 2 The Wheatstone bridge circuit shown is balanced when R2 = 60 Ω, R3 = 30 Ω and R4 = 90 Ω. What is the value of R1? 20 30 60 90 120

Question 3 R3 80 Ω; R4 120 Ω R3 120 Ω; R4 80 Ω R3 400 Ω; R4 400 Ω   Question 3 The Wheatstone bridge circuit shown is balanced when R1 = 600 Ω and R2 = 400 Ω. The resistances of R3 and R4 could be R3 80 Ω; R4 120 Ω R3 120 Ω; R4 80 Ω R3 400 Ω; R4 400 Ω R3 400 Ω; R4 600 Ω R3 600 Ω; R4 600 Ω

Semiconductors and Band Theory In Short: Metals- highest occupied band is not full so electrons are free to move Insulators – Highest band (valence) is full, electrons are not free. First unfilled band is the conduction band, the large gap between them makes it unlikely that electron will gain enough energy to ‘jump’ at room temperature Semiconductors- Is similar to an insulator but the gaps between the valence and conduction band is smaller, making it possible for electrons to jump, at room temperature or when heated

True or False Electrons in atoms are contained in energy levels. In insulators some of the electrons are in the conduction band. In conductors all of the electrons are in the valence band. The first band above the valence band is known as the conduction band. The greater the band gap, the greater the energy required by an electron to move from the valance band to the conduction band.

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