Episode 108: Resistance

The idea of resistance should be familiar (although perhaps not secure) from pre-16 science course, so there is no point pretending that this is an entirely new concept. A better approach is to draw out what they know. The aim of this first episode is to provide a quantitative definition for resistance

( R = V / I) which reinforces the qualitative notion that more resistance means less current. In addition, we will look at Ohm’s law, which is not the same thing as the definition of resistance.

Electrical circuit to measure resistance

Summary

  • Demonstration: The meaning of resistance (10 minutes)
  • Discussion: Defining resistance (10 minutes)
  • Worked Example: Calculating resistance (5 minutes)
  • Student Questions: Simple calculations (10 minutes)
  • Student Experiment: Characteristics of metal wire (40 minutes)
  • Discussion: Ohm’s law. (10 minutes)

Demonstration: The meaning of resistance
Illustrate the idea of resistance with a quick demonstration. It should be clear from the demonstration that, as more resistors are added (in series) the current (and brightness of the lamp) fall whilst the voltage (electrical push) remains constant. Lead them to the idea that resistance determines the number of ‘volts per amp’ needed to maintain the current.

Episode 108-1: Increasing resistance decreases current (Word, 27 KB)

Discussion: Defining resistance
Now define resistance: R = V/I pointing out that this is the ratio of the pd across a component to the current flowing through it (i.e. literally ‘volts per amp’). Define the ohm (W) (again point out that 1 ohm is 1 volt per amp).

1 W = 1 V A-1

Point out that kilo-ohms (kW) and mega-ohms (MW) are commonly used:

1 kW = 1000 W; 1 MW = 1000 kW = 1 000 000 W

Worked example: Calculating resistance
Calculate the resistance of a lamp when a pd of 10 V makes a current of 2 mA flow through it. (This will give practice in handling powers of 10.)

R = V/I = 10 / 2 ´ 10-3 = 5 ´ 103 = 5000 W = 5 kW

Student questions: Simple calculations
With weak groups it may be worth spending a few minutes letting them calculate resistances from R = V/Iwhen currents are given in amps, milli-amps and micro-amps. This will save errors later when they measure their own currents and use the results to calculate resistance.

Episode 108-2: Introductory questions on resistance (Word, 22 KB)

Student experiment: Characteristics of metal wire
This episode concludes by measuring the voltage/current characteristic for a metal (constantan) wire. (This could be included with the other characteristics in the next episode but if it is done prior to those then Ohm’s Law can be used to interpret later results - leading to the ideas of ohmic and non-ohmic behaviour).

Episode 108-3: Electrical characteristics of a metal wire (Word, 63 KB)

Discussion: Ohm’s law
Most electrical engineers identify the equation V = IR with Ohm’s Law but this won’t do for post-16 examinations! Historically, Ohm showed that the resistance of a metal under constant physical conditions (particularly temperature) is constant. The experiment above should have demonstrated this by generating a straight line graph that passes through the origin: if I is directly proportional to V (or the other way around) then Ohm’s law is obeyed. Any conductor (metallic or otherwise) that behaves in this way is described as an ‘ohmic conductor’.

It might well be worth spending some time reinforcing the meaning of ‘directly proportional’ and emphasising that the graphical characteristic is a straight line graph that passes through the origin.

Episode 108-4: Electrical characteristics of a resistor (Word, 25 KB)

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Episode 108: Resistance (Word, 102 KB)