Basic resistor knowledge

Here are some pretty pictures to get us going...

This is basically how a rotary potentiometer (variable resistor) works - twisting the knob changes the amount of metal in the resistor that is used in the circuit, thereby changing the resistance.

Here, a variable resistor is connected in a circuit.

Important points to note:

• Typical pots (potentiometers) are rated to run at a maximum of 0.2W to 0.5W
• The fourth band on a resistor tells us how precise or tolerant the resistor is
• 'Colours of Tolerance': red is +/- 2% tolerance; gold is +/- 5% tolerance; silver 10%; no band 20%
• If there are many resistors connected in series, the total resistance of the circuit is the sum of all the resistors' resistances
• The rate of energy conversion (basically power??) is measured in watts

Important information on voltage dividers:

Working out the total resistance in a parallel circuit:

Questions on Resistance:

1. 3.7 A of current flows towards and through R
2. The current flowing through R is now 0.2 A in the same direction
3. R2 because it has the highest resistance, so the voltage/potential difference will drop the most across it
4. V = IR
   Making I the subject:
   I = V/R
   Therefore the current (I) through the circuit is
   10/(56 + 120 + 82)
   = 10/258
   = 5/129
   = 0.0388 A (3 s.f.)
   = 38.8 mA
   
   V = IR
   Therefore,
   Voltage across R1 = 0.0388 * 56
                                 = 2.170542635658915...
                                 = 2.17 V (3 s.f.)
   Similarly,
   Voltage across R2 = 0.0388 * 120
                                 = 4.651162790697675...
                                 = 4.65 V (3 s.f.)
   and
   Voltage across R3 = 0.0388 * 82
                                 = 3.178294573643411...
                                 = 3.18 V (3 s.f.)
   Check: 2.17 + 4.65 + 3.18 = 10 V - YAY!!! I GOT IT RIIIIIGGGHHT!!!
5. Voltage = 0.03 * 258 = 7.74 V
6. Current through R1 = 3/100 = 0.03 A = 30 mA
   Current through R2 = 3/47 = 0.06382978723 = 0.0638 A (3 s.f.) = 63.8 mA
   Voltages across R1 & R2 are both 3V (obviously!)