| Intermediate Technology Education | Energy and Power Technology | Pre-Design | Topic 3 | Activity 9 |


To investigate the electrical relationships of EMF, current and electrical resistance.
  • Distinguish between direct current and alternating current electricity
  • Define electrical resistance and state the unit of measure
  • Identify the symbols used to represent electrical potential difference, electrical current, and electrical resistance.
  • Identify the relationship between EMF, current and electrical resistance
  • Make electrical calculations using Ohm's Law
  • Define electrical power and state the unit of measure
  • Make electrical calculations using the power law
  • Determine Electric Energy Consumption


The previous activity Electric Potential Energy introduced the fundamental concepts (ideas) about electricity. 

EMF (electromotive force) was introduced as the force that causes electrons to move.  It is the electric force, or potential difference, that occurs when there is a difference in charge between two points (excess electrons in one place, not enough in another). 

Electric current was introduced as the coordinated movement of electrons from one point to another. If two points with an electric potential difference are connected by a path that can conduct electrons, the electrons will move from the point of excess electrons to the point of too few electrons until there is no difference between them.  This movement of electrons is called electric current.

The activity also introduced the units used to measure the quantity of EMF and electric current.  It turns out that a specific number of electrons (6.24 x 1018) is used as the basis for each unit of measure. 

  • EMF is measured in volts.  1 volt is 6.24 x 1018 electrons of potential energy.
  • Current is measured in amperes or amps.  1 amp is 1 volt of electrons moving past a point in 1 second, or 6.24 x 1018 electrons per second.

This activity will look at several other electricity concepts, and how they are related to one another.  You will use all these concepts when you work with electric circuits, make electrical measurements, and perform electrical calculations.

Topics covered

  • Direct current and alternating current electricity
  • Electrical resistance property
  • Unit of measure for electrical resistance—the ohm
  • Ohm's Law
  • Electrical power
  • The power law

Direct Current and Alternating Current

Electricity is electron flow from one point to another.  If the electron flow is always in the same direction, we call that direct current electricity.  Batteries, for example, produce direct current (DC)electricity. 

If, on the other hand, the electrons flow first in one direction, and then back in the opposite direction in a regular cycle, we call that alternating current (AC).  The electricity available in the wall receptacles in your house is alternating current.  The current flows in one direction, and then back the other way 120 times a second.  Each back and forth flow is call a cycle.  120/2 is 60, so it is a 60 times a second cycle.  As with all such things, the number of cycles in AC electricity is named after a scientist who made an important contribution to our knowledge.  Cycles be second is called hertz (Hz) after Heinrich Hertz (1857-1894).

Electrical Resistance Property

Electrical resistance is a property of materials that affects how easily electrons can move from one atom to another.  There are 3 general categories of materials, based on their resistance to electron flow

  • Conductors. Some materials, particularly metals, allow atoms to move freely.  These conductors vary in how easily electrons can move from atom to atom.  Gold is one of the best conductors, for example.  Copper is also an excellent conductor and is widely used for electrical wiring.
  • Insulators. Other materials, particularly rubber and most plastics, do not allow their electrons to move from atom to atom.  These insulators are often used to encapsulate electrical components to prevent undesirable electron flow.
  • Semiconductors.  These are engineered materials that will allow electron flow under some conditions but not other conditions.  For example, a diode acts like a one way valve, allowing electrons to flow in one direction but not the other.

Unit of Measure for Electrical Resistance—The Ohm

Electrical resistance is measured in ohms, named after Georg Ohm (1689-1854).  By definition

1 ohm = a voltage drop of 1 volt across an element with 1 amp of current

Or put another way, if 1 amp of current travels through a wire and the voltage drops by 1 volt, the wire has 1 ohm of resistance.

Ohm's Law

What Ohm discovered was the fundamental relationship between EMF, electric current and electrical resistance.  The relationship can be described as

EMF (electromotive force) = Current times Resistance

Some of you may have noticed that this is essentially a mathematical relationship.  An easier way to write it is to replace the word with symbols that represent them.  The conventional symbols are

  • E represents EMF
  • I represents electrical current
  • R represents electrical resistance

So the equation becomes

E = IR    (IR is the same as I x R)

You also know now that each of these has a unit of measure

  • E (EMF) is measured in volts, and the symbol for volts is V
  • I (current) is measured in amps, and the symbol for amps is A
  • R (resistance is measured in ohms, and the symbol for ohms is Ω (the Greek letter Omega)

Sometimes people use the symbol for volts instead of the one for EMF, so the equation can also be written 

V = IR

Ohm's Law is very important to anyone working with electricity or electronics.  If you know any two quantities, you can easily calculate the other one.  For example, if you knew that the current was 0.5 amps, and the resistance was 50Ω, you could easily calculate the EMF in volts.

E = IR

E = 0.5A x  50Ω

E = 25V

Even more important, knowing this relationship lets you design a circuit that will work safely.

Electrical Power

When an electromotive force causes an electric current to flow through a circuit (closed path), some of the energy contained in the electrons is consumed by the work that they do on the lights, motors or other devices in the circuit.  Work done in an electric circuit occurs when the energy transforms to a different form. Examples are the rotary motion of a motor, the heat and light given off by a light bulb, or the picture coming from your TV screen.  If you increase the EMF (voltage) more work can get done.  If you increase the current (amps) more work can get done.

Recall from the Work, Energy and Power activity that power is the rate at which work gets done.  The same is true for electrical power.  In the case of electricity, we say that electrical power is the rate at which electrical energy is transferred by an electric circuit. 

Electrical power is represented by the letter P.

Electrical power is defined as

Electrical Power = EMF X Current

or, substituting the symbols

P = EI

and because some people use V (voltage) for E (EMF), it also is written as

P = VI

The next figure, taken from the previous activity Electric Potential Energy, shows the concept of electric power.

10 Amps at 12 Volts 

Figure Electric Power is the number of Watts times the Number of Amps 

Unit for Electrical Power—the Watt

The quantity of electrical power is measured in watts, after James Watt (1736-1819). 

1 watt of electrical power = 1 volt causing 1 ampere of current in 1 second

Recall that energy is measured in joules. Using 1 watt of electrical power is equivalent to consuming 1 joule of energy.

Suppose you have an electric heater in your room.  If it operates at 220 volts and consumes 10 amps of electricity, what is its power rating?  Using the formula, we can easily find out

P = VI

P = 220V x 10A

P = 2200W (W is the same as watts) 

Electric Energy Consumption

Energy is power times time.  As noted above the unit for power is watts.  As previously defined, energy is measured in joules.

E = watts × seconds

E = joules / second × seconds

E = joules

If you look at your monthly electric bill, you will notice that it does not use the term joules to indicate how much electrical energy you consumed during the past month.  It uses kilowatt hours to describe electrical energy consumption.  A kilowatt is 1000 watts.  A kilowatt hour is 1000 watts consumed in one hour.  If you want to know how many joules of energy that represents, you can put it into the formula above.  Remember that 1 hour has 60 minutes and each minute has 60 seconds.

E = 1000 watts × (60 × 60) seconds

E = 1000 joules / second × 360 seconds

E = 360,000 joules.

Of course 1 watt hour is 1 watt of electrical energy consumed in one hour.  Expressed as joules, this amount of energy is

E = 1 watt × (60 × 60) seconds

E = 1 joule / second × 360 seconds

E = 360 joules.

To find the number of joules of electrical energy consumed in your house each month, you can multiply the number of kilowatt hours by 360,000.   Why do you think that the electric company uses kilowatt hours rather that joules on their bill?

Summary of Electrical Terminology

  • Electromotive force (EMF—symbol = E) is the number of electrons cause potential charge difference.  It is measured in volts (symbol = V).
  • Electric current is the number of electrons that flow past a point in a circuit (closed path for electron flow).  It is measured in amperes (amps—symbol = A).
  • Electric resistance is resistance of a material to the movement of electrons.  It is measured in ohms (symbol = Ω).
  • EMF, current and resistance have the relationship (Ohm's Law)
    • E = IR, also written as V = IR
  • Electric power (symbol = P) is the total number of electrons that move past a point in a circuit in a second.  It is measured in watts (symbol = W).  It is written as EMF multiplied by current
    • P = EI, also written as P = VI
  • Electrical energy is measured in joules.  360,000 joules is equivalent to 1000 kilowatt hours of electrical energy consumption.

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