Electric Revision

Current flow versus Electron flow

Conventional current flows this way.

Electrons flow this way.

What formula relatesCharge, 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

Substituting into Q = It

gives Q = 0.15 A x 60 s

= 12 C

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 Law2 A 8 Ω

V = ?

V IxR = 2A x 8

= 16 V

Ammeters and VoltmetersAmmeters measure current and are placed in series

in a circuit.

Voltmeters measure voltage and are placed Voltmeters measure voltage and are placed in parallel in a circuit.in parallel in a circuit.

A

V

Rules forResistors in SERIES

RTotal R R R1 2 3

Examples on Resistors in Series

6 Ω 9 Ω

Ans. = 15 Ω

4Ω 6 Ω 3 Ω

Ans. = 13 Ω

No. 1

No. 2

Rules forResistors in PARALLEL

1 1 1

R

1

R R R

This formula is shortened to

RR R

R R

oduct

Sum

Total 1 2 3

Total1 2

1 2

Pr

Examples on Resistors in Parallel6 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 Ω6 Ω

12 Ω

10 Ω

3 Ω

10 Ω 2 Ω

2 Ω

No. 4

No. 5

Rules for SERIES CIRCUITS

• Same current but ……

• split voltage between them.

18 V

6 V 6 V 6 V

?

Equal resistors share the voltage between them!!

Rules for PARALLEL CIRCUITS

• Same voltage but ……

• split current between them.

? A

? 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 WCurrent = ----------- = 0.25 A 240 V

ElectricalElectrical Power = Potential difference * current Power = Potential difference * current

Watts Volts AmpsWatts 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 coil Secondary coil

Input voltage

(a.c.)

Output voltage (a.c.)

All transformers have three parts:

1. Primary coil – the incoming voltage Vp

(voltage across primary coil) is connected across this coil.

2. Secondary coil – this provides the output voltage Vs (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.

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

voltage across the primary coil

voltage across the secondary coil

number of turns on primary

number of turns on secondary

V

V

N

Np

s

p

s

Where Vp = primary voltage

Vs = secondary voltage

Np= Number of turns in primary coil

Ns = Number of turns in a secondary coil.

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

Step-up transformer!

Solution

VV

NN

Substituting12V

180540

Crossmultiplying180.V 12 x 540

V 12 x 540180

V 36 V

P

S

P

S

S

S

S

S

Worked example No. 2A 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?

VP = 230 V

NP = 1380VS = 6 V

NS = ?

Obviously, a Step-down transformer!!

Solution

VV

NN

Substituting2306

1380N

Crossmultiplying2300.N 6 x 13800

N 6 x 1380230

N 36 turns

P

S

P

S

S

S

S

S