True or False MHW7A transformer does not work from

a DC supplyA transformer changes AC into DCYour mobile phone charger

contains a transformerIn step-up transformers the output

voltage is less than the input voltage

T

F

F

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 - TheoryA 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 constructionTwo metal platesSeparated by insulating

material ‘Sandwich’ construction

‘Swiss roll’ structure

Difference between a capacitor and a batteryBatteries 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

V or I

t

Voltage and charge characteristics

Charging Discharging

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.

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.

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)

   

appliedvoltage

storedeChceCapaci

argtan

C = Q / Vor

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.

1F = 1x10-6F and 1pF = 1x10-12F

What applied voltage would charge a 1F capacitor to 1.0x10-5 C?

V = Q / C = 1.0x10-5 C / 1x10-6FV = 10V