True or False MHW7 A transformer does not work from a DC supply A transformer changes AC into DC Your mobile phone charger contains a transformer In step-up transformers the output voltage is less than the input voltage A transformer does not work from a DC supply A transformer changes AC into DC Your mobile phone charger contains a transformer In step-up transformers the output voltage is less than the input voltage T T F F F F T T

P6 – Electricity for Gadgets Lesson 16 – Capacitors Learning aim: Demonstrate an understanding of how capacitors work

Learning Objectives Success Criteria Recognise full-wave rectification. Understand how capacitors act in smoothing circuits. Understand how a bridge circuit works. Recognise full-wave rectification from a voltage–time graph.(Grade E-D) Recognise and draw the symbol for a capacitor and describe its function.(Grade E-D) Explain why many devices need a more constant voltage supply. (Grade E-D) Recall how diodes are used in full-wave rectification. (Grade C) Understand how the flow of current changes with time when a conductor is connected across a charged capacitor.(Grade C) Describe how a capacitor is charged/discharged (Grade C/A) Describe how a bridge circuit provides full-wave rectification(Grade C-A) Explain how a capacitor behaves in a smoothing circuit(Grade C-A)

The Capacitor The symbol for a capacitor is: In reality it looks like: Pg262 –Draw the symbol and describe its function

Capacitors - Theory A capacitor is an electronic component that stores charge which can be discharged later Capacitance is charge stored per volt Capacitance is measured in farads F (big unit so nF, mF and  F are used) A capacitor is an electronic component that stores charge which can be discharged later Capacitance is charge stored per volt Capacitance is measured in farads F (big unit so nF, mF and  F are used)

Capacitor construction Two metal plates Separated by insulating material ‘Sandwich’ construction ‘Swiss roll’ structure

Difference between a capacitor and a battery Batteries use a chemical reaction to create a current. Capacitors are charged by pumping in electrons to charge the plates. These are then released on discharge. A capacitor can be charged and discharged much quicker than a battery – they are used for things like camera flashes and to start some motors.

V or I t t Voltage and charge characteristics ChargingDischarging

AA Two large metal plates very close together but not touching. Close the switch and a tiny current flows - the capacitor is charged. Remove the cell from the circuit and then close the switch again and a tiny current flows back the other way - the capacitor discharges. Close the switch and a tiny current flows - the capacitor is charged. Remove the cell from the circuit and then close the switch again and a tiny current flows back the other way - the capacitor discharges.

Full wave rectification (add notes to sheet to explain how this circuit works) HT

Uses of capacitors (smoothing circuits) One use of a capacitor is to smooth the output of a full wave rectifier circuit

direct, flash, stores charge, releasing, smooth, plates, charge, negative, positive

capacitor questions 1.Describe the flow of current in a circuit when a conductor is connected across a charged capacitor 2.Once the capacitor is completely discharged, it is placed in a circuit containing a battery. Describe how the p.d. across the capacitor changes when the circuit is complete. 1.Describe the flow of current in a circuit when a conductor is connected across a charged capacitor 2.Once the capacitor is completely discharged, it is placed in a circuit containing a battery. Describe how the p.d. across the capacitor changes when the circuit is complete.

Multiple-choice quiz Multiple-choice quiz

Learning Objectives Success Criteria Recognise full-wave rectification. Understand how capacitors act in smoothing circuits. Understand how a bridge circuit works. Recognise full-wave rectification from a voltage–time graph.(Grade E-D) Recognise and draw the symbol for a capacitor and describe its function.(Grade E-D) Explain why many devices need a more constant voltage supply. (Grade E-D) Recall how diodes are used in full-wave rectification. (Grade C) Understand how the flow of current changes with time when a conductor is connected across a charged capacitor.(Grade C) Describe how a capacitor is charged/discharged (Grade C/A) Describe how a bridge circuit provides full-wave rectification(Grade C-A) Explain how a capacitor behaves in a smoothing circuit(Grade C-A)

C = Q / V or Q = CV The unit of capacitance is the farad (F). It is a large unit. You will more often meet the  F and the pF. 1  F = 1x10 -6 F and 1pF = 1x F What applied voltage would charge a 1  F capacitor to 1.0x10 -5 C? V = Q / C = 1.0x10 -5 C / 1x10 -6 F V = 10V