Lesson 5 Notes Effects of Current

Name…………………..

Class………….

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G482

Electrons, Photons and Waves Module 2.1:

Electric Current

Lesson

Content

/ Objectives /

Keywords

/ Checklist
Module 2.1: Electric Current
1. / Conventional current / (a) explain that electric current is a net flow of
charged particles;
(b) explain that electric current in a metal is due to the movement of electrons, whereas in an electrolyte the current is due to the movement of ions;
(c) explain what is meant by conventional current and electron flow; / Charge, Coulomb, Current, Ampere, electron, (elektron (greek for amber – where the effect of static charge was noticed first)) electrolyte, ions, conventional current.
2. / Charge and current / (d) select and use the equation ΔQ = IΔt;
(e) define the coulomb;
(f) describe how an ammeter may be used to measure the current in a circuit;
(g) recall and use the elementary charge e = 1.6 × 10-19 C; / Charge, Coulomb, Ampere, Ammeter, Current, Circuit
3. / Charge and current 2 / (h) describe Kirchhoff’s first law and appreciate that this is a consequence of conservation of charge; / Charge, Circuit, conservation, conventional current.
Circuit diagrams.
4. / Equation for current / (i) state what is meant by the term mean drift velocity of charge carriers;
(j) select and use the equation I = Anev;
(k) describe the difference between conductors, semiconductors and insulators in terms of the number density n. / mean drift velocity, current, area, electron charge, velocity, conductor, semiconductor, insulator, number density.
5. / G482 Module 1: 2.1 Electric Current Test / Review and assess their knowledge and understanding

Lesson 1 Notes – Conventional Current.

Objectives

(a) explain that electric current is a net flow of charged particles;

(b) explain that electric current in a metal is due to the movement of electrons, whereas in an electrolyte the current is due to the movement of ions;

Which way does electricity flow?

We say that electricity flows from the positive (+) terminal of a battery to the negative (-) terminal of the battery. We can imagine particles with positive electric charge flowing in this direction around the circuit, like the red dots in the diagram.

This flow of electric charge is called conventional current.

The particles that move in fact have negative charge! And they flow in the opposite direction!

The electron

When electricity was discovered scientists tried many experiments to find out which way the electricity was flowing around circuits, but in those early days they found it was impossible to find the direction of flow.

They knew there were two types of electric charge, positive (+) and negative (-), and they decided to say that electricity was a flow of positive charge from + to -. They knew this was a guess, but a decision had to be made! Everything known at that time could also be explained if electricity was negative charge flowing the other way, from - to +.

The electron was discovered in 1897 and it was found to have a negative charge. The guess made in the early days of electricity was wrong! Electricity in almost all conductors is really the flow of electrons (negative charge) from - to +.

By the time the electron was discovered the idea of electricity flowing from + to - (conventional current) was firmly established. Luckily it is not a problem to think of electricity in this way because positive charge flowing forwards is equivalent to negative charge flowing backwards.

To prevent confusion you should always use conventional current when trying to understand how circuits work, imagine positively charged particles flowing from + to -.

Electrolysis

+ -

In other materials, charged particles flow in both directions at the same time. Electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties. For example, an electrochemical cell may be constructed with salt water (a solution of sodium chloride) on one side of a membrane and pure water on the other. The membrane lets the positive sodium ions pass, but not the negative chlorine ions, so a net current results. (Electrolysis)

Electric currents in plasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, flowing protons constitute the electric current. To simplify this situation, the original definition of conventional current still stands.

Lesson 2 Notes – Current and Charge. ΔQ=IΔt.

Objectives

(d) select and use the equation ΔQ = IΔt;

(e) define the coulomb;

(f) describe how an ammeter may be used to measure the current in a circuit;

(g) recall and use the elementary charge e = 1.6 × 10-19 C;

'Spooning' amounts of charge

Electric charge can be picked up and carried by a spoon, just as if it were sugar or milk. Fix a metal spoon to an insulating handle, touch it onto the terminal of a high voltage supply, and carry the spoon across to a charge-measuring instrument, onto which the charge is dumped. Repeat the action: the charge measured increases by the previous amount. You can go on transferring charge like this in equal amounts several times.

With a bigger spoon more charge is transferred. With a bigger potential difference from the supply, more charge is transferred. So the amount of charge on a conductor depends on both the potential difference and on the size of the conductor.

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:

Defining current, the coulomb using the shuttling ball demo.

The ball picks up charge ΔQ from 1 plate and delivers it to the other side in a time Δt. We could draw this as below:

t/sec

If ΔQ is very small it would approximate to a line of best fit as drawn. If Q increases an amount ΔQ in a time Δt, the gradient of this will be ΔQ/Δt. The average current is equal to its gradient.

Current is the rate of change of charge, so I = ΔQ/Δt

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

Lesson 2 – Current and charge. ΔQ=IΔt.

1. Fig.1.1 shows a lightning strike between a cloud and the ground.

a) The current in the lightning strike is 7800A. The strike lasts for a time of 230ms.

i) Calculate

1 the charge flowing between the cloud and the ground

…………………………………………………………………………………………

………………………………………………………………………………………… (3)

2 the number of electrons transferred to the ground.

…………………………………………………………………………………………

………………………………………………………………………………………

………………………………………………………………………………………… (2)

Total [5]

2)a) Name an instrument used to measure

i) Electric current

………………………………………………………………………………………… (1)

ii) potential difference

………………………………………………………………………………………… (1)

b) The electric charge ΔQ passing a point in a circuit is given by the equation

ΔQ=IΔt

State what is represented by the other symbols I and Δt.

I: …………………………………………………………………………………… (1)

Δt: …………………………………………………………………………………… (1)

c) A water heater is switched on for 1500s. During this time, a charge of 7.5x103 C passes. Calculate the electric current,

…………………………………………………………………………………………

………………………………………………………………………………………… (2)

Total [6]

3)a)i) State what is meant by electric current

………………………………………………………………………………………… (1)

ii) A mobile phone is connected to a charger for 600s. The charger delivers a constant current 350mA during this interval. Calculate the total charge supplied to the mobile phone.

…………………………………………………………………………………………

………………………………………………………………………………………… (3)

Total [4]

Lesson 3 notes - Kirchhoff's First law

Objectives

(h) describe Kirchhoff’s first law and appreciate that this is a consequence of conservation of charge;

Introduction

Kirchhoff's two laws are equations based on conservation of charge and conservation of energy that can be applied to any electric circuit. The two laws can be used to work out the current in any branch of a circuit, given the emfs and resistances in the circuit.

Kirchhoff's first law

Kirchhoff's first law states that the total current entering a junction is equal to the total current leaving the junction.

Kirchhoff's first law is a statement of conservation of charge since it means that the total charge flowing into a junction in a given time is equal to the total charge leaving the junction in the same time.

The total current into a junction = the total current out of the junction.

Using the convention that currents leaving a junction are the opposite sign to currents entering the junction, the first law may be expressed as the following equation:

I1 + I2 + I3 + … = 0 where I1, I2, I3 etc represent the currents in the branches connected to the junction.

Lesson 3 Questions – Kirchoff’s first Law

1) i) Kirchoff’s first law is based on the conservation of an electrical quantity. State the law and the quantity conserved.

…………………………………………………………………………………………

………………………………………………………………………………………… (2)

ii) Determine the current I in each of the circuits below.

a)………………………………………………………………………………………

…………………………………………………………………………………………

b)………………………………………………………………………………………

…………………………………………………………………………………………

c)………………………………………………………………………………………

…………………………………………………………………………………………

Total [6]

2) Fig 2.1 shows part of an electric circuit.

fig 2.1

a) Name the component marked X

………………………………………………………………………………………… (1)

b) Determine the magnitude of the currents I1, I2 and I3.

I1 =………………………mA (1)

I2 =………………………mA (1)

I3 =………………………mA (1)

Total [4]

Lesson 4 Notes – Current Equation.

Objectives

(i) state what is meant by the term mean drift velocity of charge carriers;

(j) select and use the equation I = Anev;
(k) describe the difference between conductors, semiconductors and insulators in terms of the number density n.

How fast must free electrons move in a wire to produce a decent current?

‘Current’ means the rate at which electric charge flows past a point in a circuit. Imagine standing at point X with a stopwatch and timing the charge flowing past. (We have to imagine that all the electrons move at the same speed, v.) We'll watch what happens to the electron highlighted in red.

Suppose you start your watch and let it run for a time, t. The highlighted electron will have travelled a distance .

In fact, in time t, all of the electrons in the cylinder of length have flowed past you.

So what current has flowed? We need to work out how much charge has passed point A.
We start by thinking of the volume of the cylinder.

Volume of cylinder = A × / where A is the cross-sectional area of the wire
If concentration of electrons in the metal is n per cubic metre then:
Number of electrons in cylinder = n × A ×
If each electron carries charge Q then:
Charge carried by electrons in cylinder = n × A × × Q
But the length of the cylinder is v x t / where v is the drift velocity and t is the time we used
So:
Charge carried by electrons in cylinder = n × A × v × t × Q
This is the amount of charge which passes point A in time t. To find the current which this represents, we need to find the rate at which the charge has flowed. So we divide by the time t.
Current = charge / time = n × A × v × t × Q / t =nAvQ
So the electric current I flowing in a wire is given by
/ where n is the number of electrons per cubic metre
A is the cross sectional area of the wire
v is the drift velocity of the electrons
Q is the charge of an electron

So this becomes

I=nAve

with e being the charge on an electron of 1.6 x 10-19C

Lesson 4 questions - Current Equation

1 (a) Explain in terms of band theory what is meant by a free electron.

......

...... [2]

(b) Describe in detail the motion of free electrons in a copper wire

when there is no current in the wire

when there is a current in the wire.

Your answer should include the meanings of root-mean-square speed and drift velocity and the factors that determine them.

......

...... [7]

4.0 x 10–7 m2.

The drift velocity of free electrons in the wire is 1.4 x 10–4 m s–1.

(i) Calculate n, the number of free electrons per unit volume in copper.

n = ...... m–3 [2]

(ii) Calculate the new drift velocity when

1 the current is changed to 0.25 A in the same wire

drift velocity = ...... m s–1 [1]

2 a current of 0.75 A is carried in a copper wire of twice the diameter.

drift velocity = ...... m s–1 [1]

[Total: 13]