IN THE SPOTLIGHT!!!.

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Presentation transcript:

IN THE SPOTLIGHT!!!

Quick Questions. What is a capacitor made of? What is their purpose? What is the circuit symbol for the capacitor? What happens when a capacitor is connected to a battery/emf source? What happens when a load is connected across a charged capacitor? What does a current-time graph look like when a capacitor is charged? How do you obtain a value for charge stored from a current-time graph? How are charge stored and p.d. across a capacitor linked? How do the charge-time and pd – time graphs compare to current-time?

Investigating the link between charge and pd: Charging at Constant Current. 470μF Set your current using the variable resistor to approx. 60µA. Remove the shorting lead and immediately start timing. Whilst fiddling with the variable resistor so that current remains at 60µA record the time taken for the pd across the capacitor to reach 0.5V increments. Repeat for 220µF capacitor

Once you’ve got your results….. Grade Outcome Checklist E I can state the link between potential difference and charge for a capacitor. Plot a graph of potential difference (x) against charge (y) for both sets of results. What is the relationship between p.d and charge? C I can derive the quantity capacitance from a series of measurements. Compare your results for both capacitors, note the capacitance of each from the components themselves and using your graph, attempt to come up with a formula and definition for this quantity “capacitance” and a description of how it can be found from your graph. A I can explain capacitor charging by electron motion and relate this to capacitance and energy transfer. Assuming the capacitor had a capacitance of 200 F, what was the maximum charge stored? What other feature of the graph could be calculated here? What quantity does that represent and can you derive an expression for it? (NB: Think back to early work on electricity)

Capacitance We have seen that The constant is called capacitance and is the charge stored per volt across the capacitor. Capacitance = Charge stored Pd across capacitor

Key Points to remember Capacitance is measured in Farads (F) Capacitance is the gradient of the Q-V graph Example: A 1000μF capacitor is charged upto 12V, how much charge is stored? JUMP!

E = ½ QV

E = ½ QV E

What’s missing? Charge Potential Energy 1000 µF 16 V 10 mF 0.01 C Capacitance Charge Potential difference Energy 1000 µF 16 V 10 mF 0.01 C 1.0 F 100 J 2.0 mC 5000 V 100 V 50 mJ 33 000 µF 2.0 J 1 2 4 3 5 6 Don’t do this as it is on the sheet. 8 7 9 10 11 12

A 1.0 mF capacitor is charged to a p.d. of 10 V. 1. Calculate the charge on the capacitor. 2. How much charge flowed through the battery during charging? 3. How many electrons flowed through the battery during charging? 4. Calculate the energy stored by the capacitor. 5. How much energy was transferred from the battery during the charging process? 6. (Harder) You should have found different answers for questions 4 and 5. Explain this difference.

Group Discussion on results Plenary 1 Group Discussion on results

I can show my understanding of effects ideas and relationships… Plenary 2 Learning Review – use the tick sheet to grade yourself – today we did the statements: I can show my understanding of effects ideas and relationships… decay of charge on a capacitor modelled as an exponential relationship between charge and time, with the rate of removal of charge proportional to the quantity of charge remaining I can sketch, plot and interpret graphs of: decay of charge, current or potential difference with time for a capacitor (plotted both directly and logarithmically)