Electric CurrentElectric Current

Electricity Lesson 1Electricity Lesson 1

Learning Objectives

To establish what you already understand about electricity.

To know what is meant by an electric current.

To know how to calculate the charge flow in a circuit.

To be able to define the coulomb.

The Plan...

To check what you remember from GCSE.

Build some circuits to check/change your ideas.

Discuss what is meant by electric current.

Practice some calculations.

Electricity Random Fact

Electrons only move through a wire at a speed of about 1mm/sec.

Practical Work!

Draw circuit diagram and then build a circuit to test your ideas – risk assessment.

Work individually – you have to in the exam!

Tips:- Perfect the art of fault finding – replace each

component one at a time. Start with the simplest circuit and build on

that.

Electricity

What is electricity?

Electric Current

The electric current is the rate of flow of charge in a wire or component.

unit is the ampere (A)

Due to the passage of charge particles referred to as charge carriers.

In metals the charge carriers are electrons. In liquids & gases the charge carriers are

ions.

The Coulomb

The unit of charge is the coulomb (C), which is defined as the charge flow in one second when the current is one ampere.

The symbol for charge is Q. The symbol for the unit, coulomb is C.

The charge on an electron is e=1.6 × 10-19 C

Charge Flow

For a current I, the charge flow ΔQ in a time Δt is given by:-

The symbol Δ is delta, a Greek capital letter Δ, meaning “change in”.

tIQ

interval timecurrent flow charge

Current

For a current I, the charge flow ΔQ in a time Δt is given by:-

t

QI

Question

If the charge on one electron is e=1.6 × 10-19 C, how many electrons are needed to make up 1 C of charge?

Answer

If the charge on one electron is e=1.6 × 10-19 C, how many electrons are needed to make up 1 C of charge?

1819-

1025.6101.6

C 1 electrons .of no

C

Possible Trap

There are some important multipliers for current:

1 microamp (1 μA) = 1 × 10-6 A  1 milliamp (1 mA) = 1 × 10-3 A

You must use current in amps, charge in coulombs and time in seconds for calculations.

Watch out for this!

Worked Example

What is the charge passing a point if a current of 10 pA flows for 1 year?

Learning Objectives

To establish what you already understand about electricity.

To know what is meant by an electric current.

To know how to calculate the charge flow in a circuit.

To be able to define the coulomb.

End

Today’s Objectives

Describe the relationship between current and charge

Know what happens to the current at any junction in a circuit

Describe the relationship between the current entering and leaving a component

Describe the relationship between the current passing through 2 or more components in series

Spooning charge

Electric charge can be picked up and carried by a spoon, just as if it were sugar or milk. An insulated metal spoon can carry charge from the terminal of a high voltage supply across to a charge-measuring instrument – e.g. Coulomb meter.

Alternatively use a charged polythene rod to ‘spoon’ charge onto the Coulomb meter.

‘Spooning’ Charge

link to earth socket

5 kV supplyinternal 50M resistor

bare 4mm plug

044

coulomb meter

insulating handle

metal disk on 4mm plug

Calculating the number of electrons

Knowing that the charge on an electron is –1.6 ´ 10–19 C, you can calculate the number of electrons in a 'spoonful' of charge. A typical spoonful of negative charge is –2 nC. So the number of electrons is:

electrons 102.1C101.6

nC2electronon chargespoonon charge

10

19-

Outcomes

1. Charges, which you cannot see, are real enough and can be ladled around and measured, just like other more tangible physical quantities.

2. Charges are either positive or negative.

3. The electron has a very small charge. A 'charged object' is one with a slight excess or deficit of electrons.

Shuttling ball experiment

             

Discussion: Defining current, the coulomb

Current is defined as rate of change of charge. This can be done graphically. Current is the gradient of a graph of charge transferred against time.

I = dQ/dt. The idea of the gradient can be introduced by asking how the

charge transferred by the shuttling ball increases with time - it will go up in a series of steps but, given a large number of transfers, these will approximate to a constant slope. The average current is equal to its gradient. The equation I = Q/t (familiar from pre-16 science lessons) is useful but stress that this refers to an average current I and care must be taken when I is changing.

A current of one amp is equivalent to a flow of one coulomb per second.

The coulomb defined as the charge passed by a current of 1 A in 1 s, i.e. 1 C = 1 A s.

Introductory questions on charge and current

Convert 25 mA to A 2. Convert 0.50 A to mA 3. A torch bulb passes a current of 120 mA. (a) How many coulombs of charge flow through the lamp in 1

minute? (b) How many coulombs of charge flow through the lamp in 1

hour? (c) How many electrons leave the negative terminal of the cell

each second? 4. A car battery is rated as 36 A h. In principle this means it could

pass a current of 1 A for 36 h before it runs down. How much charge passes through the battery if it is completely run down?

5. An electron beam in a beam tube carries a current of 125 A. (a) What charge is delivered to the screen of the tube every

second? (b) How many electrons hit the screen each second?

Circuit rules Current rules

Current rules At any junction in a circuit the total current

leaving the junction is equal to the total current entering the junction (Kirchhoff’s current Law)

The current entering a component is the same as the current leaving the component (from KS 3 and 4)

The current passing through 2 or more components in series is the same through each component. (from KS 3 and 4)

Kirchhoff’s current law

• The current entering any junction is equal to the current leaving that junction. i1 + i4 = i2 + i3

Conclusions

The current is the charge per second : I = dQ/dt.

At any junction in a circuit the total current leaving the junction is equal to the total current entering the junction (Kirchhoff’s current Law)

The current entering a component is the same as the current leaving the component

The current passing through 2 or more components in series is the same through each component.