Current flow versus Electron flow Conventional current flows this way. Electrons flow this way.

What formula relates Charge, Current and Time? A current of 1 Ampere is flowing when 1 Coulomb of charge flows past a point in a circuit in 1 second. Charge = current x time (C) (A) (s) If a current of 5 A is flowing then 5 C of charge pass a point in 1 second. In general, if a steady current I (amperes) flows for time t (seconds) the charge Q (coulombs) passing any point is given by Q = I x t

Worked example A current of 150 mA flows around a circuit for 1minute. How much electrical charge flows past a point in the circuit in this time? Solution

1.Convert the following currents into amperes: a) 400 mA b) 1500 mA. Ans. = a) 400 mA = 0.4 A b) 1500 mA = 1.5 A 2. What charge is delivered if a current of 6A flows for 10 seconds? Ans. = 60 C 3.What charge is delivered if a current of 300 mA flows for 1 minute(60 seconds)? Ans. = 18 C For you to do!!

What is Ohm’s Law? The voltage dropped across a resistor is directly proportional to the current flowing through it, provided the temperature remains constant. Voltage (V) = Current (A) x resistance (Ω) V = I x R What is the formula for Ohm’s law?

Worked example on Ohm’s Law 2 A 8 Ω V = ?

Ammeters and Voltmeters Ammeters measure current and are placed in series in a circuit. Voltmeters measure voltage and are placed in parallel in a circuit. A V

Rules for Resistors in SERIES

Examples on Resistors in Series 6 Ω 9 Ω Ans. = 15 Ω 4Ω4Ω6 Ω3 Ω Ans. = 13 Ω No. 1 No. 2

Rules for Resistors in PARALLEL

Examples on Resistors in Parallel 6 Ω 12 Ω Ans. = 3 Ω 6 Ω Ans. = 6 Ω 12 Ω No. 1 No. 2

For you to do!!!! 16 Ω 6 Ω 16 Ω Ans. = 14 Ω No. 3

Ans. = 9 Ω Ans. = 6 Ω 6 Ω 12 Ω 10 Ω 3 Ω 10 Ω2 Ω No. 4 No. 5

Rules for SERIES CIRCUITS Same current but …… split voltage between them.

18 V 6 V ? Equal resistors share the voltage between them!!

Rules for PARALLEL CIRCUITS Same voltage but …… split current between them.

? A 4 A ? A Equal resistors What will be the currents flowing through each ammeter?

Electrical Power E.g. A study lamp is rated at 60 W, 240 V. How much current is the bulb carrying? Solution 60 W = 240 V * Current 60 W Current = = 0.25 A 240 V Electrical Power = Potential difference * current Watts Volts Amps Watts Volts Amps

A transformer is a device for increasing or decreasing an a.c. voltage.

Structure of Transformer

Circuit Symbol for Transformer

How Transformer works Laminated soft iron core Primary coilSecondary coil Input voltage (a.c.) Output voltage (a.c.)

All transformers have three parts: 1.Primary coil – the incoming voltage V p (voltage across primary coil) is connected across this coil. 2.Secondary coil – this provides the output voltage V s (voltage across the secondary coil) to the external circuit. 3.Laminated iron core – this links the two coils magnetically. Notice that there is no electrical connection between the two coils, which are constructed using insulated wire.

Two Types of Transformer A step-up transformer increases the voltage - there are more turns on the secondary than on the primary. A step-down transformer decreases the voltage - there are fewer turns on the secondary than on the primary. To step up the voltage by a factor of 10, there must be 10 times as many turns on the secondary coil as on the primary. The turns ratio tells us the factor by which the voltage will be changed.

Formula for Transformer Where V p = primary voltage Vs = secondary voltage N p = Number of turns in primary coil N s = Number of turns in a secondary coil.

Worked example No. 1 The diagram shows a transformer. Calculate the voltage across the secondary coil of this transformer. Step-up transformer!

Solution

Worked example No. 2 A transformer which has 1380 turns in its primary coil is to be used to convert the mains voltage of 230 V to operate a 6 V bulb. How many turns should the secondary coil of this transformer have? V P = 230 V N P = 1380 V S = 6 V N S = ? Obviously, a Step-down transformer!!

Solution