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Activity: Simple circuits

In this activity you will come to understand how a simple cell creates potential difference. New terms (electromotive force and internal resistance) are added to existing vocabulary (potential difference) and you will be able sketch how terminal potential difference varies with time for a real battery. Ohm's law will be applied to a simple circuit and you will be able to derive an equation for electrical power.

Need to know


Doing this at home?

All the circuits can be built using Paul Falstad’s brilliant circuit simulator. You still have to make the right connections but won’t cause a fire if you get it wrong.

Cells

An electric cell is a device that converts chemical energy into electrical PE and a battery is made several of them joined together. What is basically happening is the atomic charges are being rearranged.

  • Is there a difference in potential between points A and B in the mixed up atoms?
  • Is there a difference in potential between A and B in the arranged charges?
  • Explain why work has to be done to rearrange the charges.
  • Can the arranged charges be used to do work on a + charge?

Have a look at this simulation:

  • Notice how the electrons flow from the - to + side of the battery
  • You can see how the electrons interact with the atomic lattice so that they drift through the conductor rather then accelerating.
  • Why does work have to be done to take electrons from the + to - side of the battery.
  • Where does the energy come from to move the electrons closer together?
  • Click "show inside of battery" and all is revealed.

So fairies move the charges. This isn't really the answer but it might as well be as far as we are concerned in this course. If you want to know how chemical energy is converted to electrical PE in a battery you will have to study chemistry.

Conventional current

The previous simulation is a bit confusing because it shows the movement of electrons rather than conventional current which would move from + to - this is the way that + charges would move. Current flows from high potential to low potential.

Fairies or chemicals?

The principle behind this cell can be understood even with basic chemistry.

When metals are put into acid they react to produce hydrogen leaving excess electrons on the metal.
Zinc reacts faster then copper so if both metals are put into acid the zinc gets more electrons than the copper.

  • Which metal has a higher potential?
  • If a conductor was connected between the metal plates which way would current flow?
  • measure the pd between the terminals of the lemon cell set up in the lab.

Symbol


The symbol represents the different potential of the two side of the cell. The big side is the high potential so current always flows from the big side to the little.

EMF

The confusingly titled 'electromotive force' is in fact:

The energy converted from chemical to electrical PE per unit charge.

The unit is J C-1 or volts.

Internal resistance

In reality, cells aren't perfect. some thermal energy is released.

Internal resistance is the resistance of the inside of a battery.

  • Current flowing through the internal resistance will result in energy loss, where does this energy go?
  • If a cell is connected to a low resistance wire what happens? (this is called short circuiting, don't try it)
  • Can you explain what would happen to the chemical energy in a cell if a cell of zero internal resistance was connected to a zero resistance wire?

Terminal potential difference

The potential difference across the terminals of the battery is the work done per unit charge in taking a small + charge from the - terminal to the + terminal.

Unit: volt

  • Why is the terminal pd less than the EMF when current flows but the same when no current flows?

Discharge

After some time all of the chemical energy will have been converted to electrical and the battery will be of no use.

  • Modern batteries are designed so that the the EMF stays the same for the life of the battery. Why is this desirable?

Circuits

The simplest circuit

In this exercise Paul Falstad's circuit simulator will be used to simulate the simple circuits covered in the course. Note that the default symbol used for resistance is not the same as the one used by the IB, if you want the IB symbol choose "European resistors" from the options.

The simplest circuit consists of a cell and a resistor connected by wires.

  • Open the circuit simulator and choose "blank circuit" from the "circuits" menu.
  • Right click anywhere on the window and you will get a list of options. Select "voltage source 2 terminal" from the "inputs/outputs". The place it by click and drag.
  • To change the properties of the component hover over it with the cursor (it will turn white), right click then choose edit from the list.

  • Add a resistor and connect it to the battery with wires (all options in the right click list).
  • To measure the properties of a component hover over it with the cursor, all details of that component will then appear

Equation for simplest circuit

Using the letters on the diagram answer the following:

  • When a unit charge flows how much chemical energy is converted to electrical PE?
  • According to Ohm's law what is the potential difference across the resistor?
  • If energy is conserved we can say that ε = IR, explain why.
  • Use the simulation to show this is the case.

The not-so-simple circuit (with internal resistance)

Real cells have internal resistance, this can easily be represented by adding a resistor in series with the cell. Set the internal resistance to 1 Ω.

  • Vary the load resistor and observe how the PD across it changes. You should see that when the load is big the PD is about the same as the EMF of the cell.
  • Observe how the power dissipated in the internal resistance changes as the load is varied.

Equation for not so simple circuit

Use the letters on the diagram to answer the following

  • How much chemical energy is converted to electrical when unit charge flows?
  • How much heat energy is produced per unit charge inside the battery?
  • How much heat energy is produced per unit charge in R?
  • Apply the law of conservation of energy to get ε = IR + Ir

To help visualise what is happening in the circuit you can think of going up and down in terms of PE (like going up and down stairs).

  • This diagram shows a combination of 5 V cells each with zero internal resistance. Point A is at a potential of 0V, what is the potential at points B, C, D, E, F?
  • What is the PD between E and F?
  • What is the current in the circuit?

Build the circuit below with a 5 V battery.

What is the PD across the 5 kΩ resistor?

  V (4 sf)

Place your cursor on either side of the resistor and read the potential, PD is the difference.

3.621 - 0.001965

 

Total Score:

Summary

Assess yourself

Now know


For students with access to StudyIB:

Circuits 1

Circuits 2

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