Resistance (Cambridge (CIE) IGCSE Physics)

Ohm's law

Resistance

• Resistance is defined as:

The opposition to current

• Resistance occurs because the free electrons flowing in the circuit (current) collide with the metal ions in the wire

• These collisions slow down the electrons, or, in other words, resist their flow

  • The higher the resistance of a circuit, the lower the current

  • This means that good conductors have a low resistance and insulators have a high resistance

• The resistance of a circuit can be increased by adding resistors (or variable resistors) to it

• Every electrical component has a resistance, even wires

  • In exam questions, the resistance of the wires and batteries are assumed to be negligible

The effect of resistance on the current in a circuit

       When a circuit has a high resistance, a lower current will flow, and vice versa

Ohm's law

• Current, , potential difference, , and resistance, , all affect one another

• Changing any one of these in a circuit, changes all of them

• Current and resistance are inversely proportional 

  • If the resistance is doubled, current will halve

• This relationship is described by the following equation, known as Ohm's law

• Where

  • = resistance, measured in ohms (Ω)

  • = potential difference, measured in volts (V)

  • = current, measured in amperes or amps (A)

Consequences of Ohm's law

• Resistors are used in circuits to control either:

  • the current in branches of the circuit (through certain components)

  • the potential difference across certain components

• This is due to the consequences of Ohm's Law

  • The current in an electrical conductor decreases as its resistance increases (for a constant p.d.)

  • The p.d. across an electrical conductor increases as its resistance increases (for a constant current)

Current-voltage graphs

Extended tier only

• The relationship between current and potential difference of a component can be shown on a current-voltage (I-V) graph

• When the relationship between current and potential difference is linear:

  • the I-V graph is a straight line which passes through the origin

  • the resistance is constant

  • these are known as ohmic resistors

• When the relationship between current and voltage is non-linear:

  • the I-V graph that is not a straight line

  • the resistance is not constant

  • these are known as non-ohmic resistors

Current-voltage (I-V) graph for a resistor and a filament lamp

Linear IV graphs are straight lines through the origin, indicating a constant resistance. Non-linear IV graphs are curved, indicating a variable resistance

• Components with linear I-V graphs (ohmic resistors) include:

  • fixed resistors (at constant temperature)

  • wires (at constant temperature)

• Components with non-linear I-V graphs (non-ohmic resistors) include:
  

  • filament lamps

  • diodes

  • LDRs

  • thermistors

I-V graph for ohmic conductors

• The relationship between current and voltage for a wire or fixed resistor is linear, or directly proportional, which means

  • the IV graph is a straight line, so voltage and current increase (or decrease) by the same amount

  • the slope of the graph is constant, so resistance is constant

I-V graph for a a wire of fixed resistor

The current is directly proportional to the potential difference (voltage) as the graph is a straight line through the origin

I-V graph for a filament lamp

• The relationship between current and voltage for a filament lamp is non-linear, or not directly proportional, which means

  • the IV graph is not a straight line, so voltage and current do not increase (or decrease) by the same amount

  • the slope of the graph is not constant, so resistance changes

• The IV graph for a filament lamp shows as voltage increases

  • the current increases at a proportionally slower rate

  • the resistance increases; the flatter the slope, the higher the resistance

I-V graph for a filament lamp

As current flows through a filament lamp, the lamp heats up; resistance increases with temperature, causing the S-shaped curve

• As current through a filament lamp increases, the resistance increases because:

  • the higher current causes the temperature of the filament to increase

  • the higher temperature causes the atoms in the metal lattice of the filament to vibrate more

  • this causes an increase in resistance as it becomes more difficult for free electrons (the current) to pass through

  • since resistance opposes the current, this causes it to increase at a slower rate

I-V graph for a diode

• A diode allows current to flow in one direction only

  • This is called forward bias

• In the reverse direction, the diode has very high resistance, and therefore no current flows

  • This is called reverse bias

• When the current is in the direction of the arrowhead symbol, this is forward bias

  • On the IV graph, this is shown by a sharp increase in voltage and current on the right side of the graph

  • This shows the resistance is very low

• When the diode is switched around, this is reverse bias

  • On the IV graph, this is shown by a zero reading of current or voltage on the left side of the graph

  • This shows the resistance is very high

I-V graph for a diode

The current is zero at all potential differences in the negative quadrants because current only flows one way through a diode; this gives the diode I-V graph its distinct shape