Flow of Charge

Physics – 12.1.1 Charge, current, potential difference Resistance

Electrical current as the rate of flow of charge.

Recall and use of formulae (shown above)

From GCSE physics we already know that;

So simply in this circuit the resistance of the bulb is 12. This is a measure of how much the bulb resists the flow of electrons.

GCSE Background

We also should know be aware of the example that when an electrical storm discharges a static charge as lightning we can talk about the energy converted in the process as;

Energy converted = Charge x Potential Difference

E = QV

Hence another to think of this is;

This process whereby ions exchange electrons through a molten liquid or dissolved solid is also a way in which a “current” flows. As charge carriers are moving. Hence we can say that energy converted is;

E = QV

or

E = VIt

GCSE Background - Electrolysis

NB: Check out animations online from ADL!

When electrons move through a wire we call it an electrical current. It has a simple definition which leads to 1A = 2 x10-7Nm or 1A = 1Cs-1

Units of Electrical Current

When electrons move through a wire we call it an electrical current. The electrons move as there is a potential difference.

The larger the p.d. the higher the current flow or Coulombs per second.

1A = 1Cs-1

A simple graph of this process would be where a steady current has flowed for 20s seconds;

The number of Coulombs of charge that have flowed is 100C

Current Flow Basics

The area under the graph can be found by simple calculus and integration;

Current Flow Calculus

ItItItItQ

dtIQ

IdtdQ

sodt

dQI

t

QI

t

QI

ItQ

tt

t

0

0

0

,0

NB: calculus not required for exam but good simple example for Mathematicians!

Potential Difference (Theory)

If we think of potential difference in terms of energy transfer;

1) To bring two like charges near each other work must be done.

2) To separate two opposite charges, work must be done.

3) Whenever work gets done, energy changes form.

Monkey example; Imagine two positive spheres in space and a monkey. Imagine that a small monkey does some work on one of the positive charges. He pushes the small charge towards the big charge;

+ +

+ +Work

This is all about a separation of charge between two points. The closer he brings it, the more potential energy it has since the two charges want to repel

When he releases the charge, work gets done on the charge. This is a change in energy;

Electrical potential energy to Kinetic energy.

If the monkey brought the charge closer to the other object, it would have more potential energy.

If he brought 2 or 3 charges instead of one, then he would have had to do more work.

Since the potential energy can change when the amount of charge you are moving changes, it is helpful to describe the potential energy per unit of charge. This is known as electrical potential or potential difference.

Potential Difference (Theory)

V = potential difference in volts, VW = Energy or work done in Joules, Jq = charge in coulombs, C

This is all about a separation of charge between two points.

To understand this we should remember that the charge on one electron is;

1e = 1.6 x 10-19C

Inversely to this the number of electrons that makes up 1 Coulomb of charge is

1C = 6.25 x 1018 electrons

The volt (V) defines the work done per coulomb of charge transferred between two points where;

1V = 1 JC-1

We can also derive a smaller unit of energy (J) for an electron which is the electron volt. Sub 3 into 1 i.e. x each side directly and multiply out the units

1eV = 1.6 x 10-19J

Potential Difference (Definition)

NB: Remember this definition for Unit 1 as well!

1

2

3

4

Potential Difference (chemical cells)

One way of separating charges is a chemical cell;

1) Two chemical pastes are separated

2) An anode is formed at one end (positive) & a cathode at the other (negative)

3) When a wire is connected charges try and equalise pushing electrons through the wire.

4) Energy is lost inside the cell in the process. (Internal Resistance)

5) This can also be done with liquids i.e. a Daniell cell as shown on the right but obviously not a lot of good for transportation in an ipod!

The Basics

Detailed Example….Galvanic cell (also called voltaic cell) uses chemical reaction to produce electrical energy (flow of electrons).

CuSO4(aq)ZnSO4(aq)

Key points

1. Metal electrodes can be placed in a solution of same metal and neg ions. Either more metal ions are produced or metal in solution plates the electrode.

2. Copper from solution ends up on anode. Zinc ends up in solution in this case.

3. The salt bridge provides a ready supply of neg ions but blocks the metallic ions.

4. If solutions are 1 molar the e.m.f produced in total is 1.10V

5. You can measure this by using a voltmeter and a larger resistance in the circuit.

http://www.mpoweruk.com/chemistries.htm

Flow of Charge in Circuits

Previously we have looked at equations of Power, Resistance & Flow of Charge;

P = VI P = E/t V = IR P = I2R P = V2/R Q = It E = VIt E = VQ

We can conduct some simple experiments and mathematical substitutions to prove and verify these formulae. Form a group of 3-4 and work it out for yourselves. Be ready to present to the rest of the class.

12V D.C. Supply

Voltmeter(s)

Ammeter(s)

3x 12V 21W bulbs

Stopwatch

Hint: Try connecting the bulbs in different configurations whilst monitoring the voltage and current. Then map out the connections on a sheet of blank A3 paper & graph paper as you wish with results, images and calculations. You may also want to try out the formulae for resistors in series or parallel from further on in the course!

Flow of Charge in Circuits

P = VI

P=E/t

V=IR

P = I2R

P=V2/R

Q=It

E = VIt

E = VQ

Ideas for Circuits….