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Unit 10.04

Current ElectricityUnit 10.04Unit 10.04

Current ElectricityCurrent Electricity

Prepared in Dec 1998Second editing March 2000

At the end of this unit you should be able to :

1. show understanding that current is a rate of flow of charge measured in ampere.

2. use the equation I = Q/t.

3. use and describe the use of an ammeter with different ranges including a milliampere range.

Learning objectivesLearning objectives

Simple Experiment Simple Experiment

The next 4 slides are illustrated:

The concept of electric currentis the rate of flow of electrons.

Simple ExperimentSimple Experiment

• The ping-pong ball is coated with aluminium paint.

• When the electrostatic generator is switched on, the metal plates becomes oppositely charged.

(continue on next slide)

oppositely charged

Simple ExperimentSimple Experiment• The ball moves to touch

one plate and becomes charged as the same as the plate. Why ?

• Since like charged objects repel, the ball is then forced to the opposite plate.

• It therefore continues to move to and fro between the plates. (Why ? Please explain.)

Negatively charged

-

(continue on next slide)

swing

Simple ExperimentSimple Experiment• The ball transfer

charge from one plate to another each time it moves across the gap.

• The meter registers a tiny current because electrons pass through it from generator to replace the electrons carried from negative plate by the ball.(continue on next slide)

-

Simple ExperimentSimple Experiment

• If the plates are moved closer together, the ball bounces to and fro more rapidly.

• This makes the reading increase because the ball is ‘ferrying’ charges across the gap at a greater rate than before.

• Hence, the current through the meter is due to the flow of charge from generator to the plates.

-

Electric currentElectric current andandChargeCharge

• The concept of charges (both +ve and -ve) had been illustrated in the topic: “Electric Charge”

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Electric currentElectric currentandand ChargeCharge

• The charge carried by an electron was first measured exactly by Robert Millikan in 1915.

• He worked out that– an electron carried 1.6 x 10 -19 C– 6.2 x 1018 electrons consists 1 C of charge.

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Mr Ampere stated that

i.e. I = Q / t

Electric currentElectric current andandChargeCharge

• For steady current in a circuit, we also rewrite previous formula as:

• SI Unit of current is ampere, A.

• Also, 1 A = 1C / 1s

Charge passed = Current x Time(coulombs) (ampere) (second)

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Some circuit symbols:Some circuit symbols:Some circuit symbols:

cell

connecting wire

switch

A V

ammeter voltmeter

bulb bulb (older symbol)

ExampleExampleExample

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--

-

--

-If 25 C of charge pass through a wire for 100 s, calculate the current produced.

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since Q = ItI = 25 / 100

= 0.25 A

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switch

bulb

conventionalcurrent flow

battery

+ -

Flowing directionFlowing direction ofofelectric current

Conventionally, we always assumed that electric current is flowing from positive terminal to negative terminal.

(continue on next slide)

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switch

bulb

battery+ -

Flowing directionFlowing direction ofofelectric current

(continue on next slide)

Actually, electric current is the flowing of electrons which always pass from negative to the positive terminals in the circuit.

current

current

-

-

electron

Flowing direction ofelectric current

Flowing directionFlowing direction ofofelectric current

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An AmmeterAn AmmeterAn Ammeter• The size of current

can be measured by an ammeter.

• Meters used to measure currents in mA (milliampere) and µµµµA (microampere) are called milliammeterand microammeterrespectively.

(continue on next slide) A

Reading= 1.0 A

An AmmeterAn AmmeterAn Ammeter

• In experiment, it is important to remember that when connecting an ammeter to the cell ( or a battery), the red (+) terminal of ammeter is always joined to the +veterminal of the cell (or battery).

• Similarly the black (-) terminal of ammeter always joins to the -ve terminal of the cell.

Using of AmmeterUsing of AmmeterUsing of Ammeter• Ammeter must connect in series in the

circuit.• Ammeter has very low resistance. (How?)

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A

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A(a) (b)

+ +

ExampleExampleExample

• What is the reading of the ammeter shown ?

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Ans. 2.4 A (How? Please explain.)

GCE OGCE O--LevelLevelPast Examination PaperPast Examination Paper

Science (Physics)

All rights go to University of Cambridge Examinations Syndicate and other sources

GCE O Nov 1994

15. The diagram shows an ammeter.

What is the reading on this meter ?A 0.67 A B 0.72 AC 0.74 A D 6.70 A C

Nov 199610. The circuit below shows a cell, a resistor and

three meters.A charge of 6 coulombsflows through the resistor in 2 seconds.(a) Calculate the current

in the resistor. Statethe unit. [1]Since Q = It

I = Q/t= 6 / 2= 3 A

(continue in next slide)

10(b)(i) Name an instrumentwhich could be used

to measure the currentin the resistor. [1]

(Cont. …) Q.10 Nov 1996

(ii) Which meter, X, Y orZ measures thiscurrent? [1]

ammeter.

Meter Y

6. The diagram below shows a circuit in which an ammeter and a voltmeter are being used to find the resistance of a lamp.

(a) Which meterP or Q is theammeter?[1]

GCE ‘O’ LEVEL Nov 1995

P

Hint:ammeter : connects

in series

(continue on next slide)