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Resistance (CIE IGCSE Physics)
Revision Note
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)
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Worked example
A 12 Ω resistor has a current of 0.3 A flowing through it.
Determine the potential difference across the resistor.
Answer:
Step 1: List the known quantities
- Resistance,
- Current,
Step 2: Write out the equation for Ohm's law and rearrange to make potential difference the subject
Step 3: Substitute in the known values to calculate
I-V graphs for Ohmic resistors, filament lamps & diodes
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
Exam Tip
In your IGCSE exam, you could be asked to recognise, sketch or explain the I-V graphs for a wire / fixed resistor (ohmic conductors), a filament lamp and a diode.
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