Resistance — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions

Section 1 of 9Resistance

1Resistance

Resistance the opposition to current.

2Cause

• Cause free electrons collide with the metal ions in the wire, which slows them and resists their flow.

3Effect

• Effect the higher the resistance of a circuit, the lower the current.

4Conductors and insulators

• Conductors and insulators good conductors have low resistance and insulators have high resistance.

5Control

• Control adding resistors or variable resistors increases the resistance of a circuit.

6Assumption

• Assumption wires and cells are normally assumed to have negligible resistance.

Section 2 of 9Resistance equation

Equation

• R = V ÷ I, with R in ohms (Ω), V in volts and I in amperes.

Rearranged forms

• V = I × R and I = V ÷ R.

Inverse relationship

• Inverse relationship for a constant p.d., doubling the resistance halves the current.

Direct relationship

• Direct relationship for a constant current, the p.d. across a conductor increases as its resistance increases.

Use of resistors

• Use of resistors resistors control the current in a branch or the p.d. across a component.

Section 3 of 9Current–voltage graphs

Ohmic conductor

• Ohmic conductor the I–V graph is a straight line through the origin,
  • so the current is directly proportional to the p.d. and the resistance is constant.
• Fixed resistors and wires at constant temperature are ohmic.

Non-ohmic conductor

• Non-ohmic conductor the I–V graph is curved, so the resistance changes.
• Filament lamps, diodes, LDRs and thermistors are non-ohmic.

Section 4 of 9Filament lamp

Graph

• Graph the current increases more slowly as the p.d. increases, giving an S-shaped curve.

Explanation

• Explanation the higher current heats the filament, so the metal ions vibrate more and electrons collide with them more often.

Result

• Result the resistance increases as the temperature rises.

Section 5 of 9Diode

Forward bias

• Forward bias current flows in the direction of the arrowhead, the resistance is very low and the current rises sharply.

Reverse bias

• Reverse bias the diode has very high resistance, so no current flows.

Use

• Use a diode allows current in one direction only.

Section 6 of 9Resistance of a wire

Cause

• Cause electrons transfer energy to the metal ions when they collide, which raises the temperature.

Length

• Length a longer wire has more resistance because each electron collides with more ions.
• R ∝ L, so doubling the length doubles the resistance.

Cross-sectional area

• Cross-sectional area a thicker wire has less resistance because more electrons can flow through it.
• R ∝ 1 ÷ A, so doubling the area halves the resistance.

Section 7 of 9More from the specification

1

Coulomb forces between like charges that repel

• Coulomb forces between like charges that repel like charges repel one another along divergent electrostatic field lines in current-carrying conductors.

2

Resistance and potential difference

• Electrical resistance is the ratio of potential difference across a conductor to the electric current flowing through it (R = V / I), measured in ohms (Ω).

3

Electric charge and static charging of polythene

• Friction transfers negative electrons when a polythene rod is rubbed with a cloth, leaving the rod negatively charged while equal positive charges remain on the cloth.

4

Electrical power and energy dissipation in resistors

• The rate of electrical energy transfer in a resistor is calculated by power equations P = IV = I²R, converting electrical potential energy into thermal energy.

5

Conservation and measurement of charge

• Electric charge is measured in coulombs (C) using the relationship Q = It;
  • in any closed electrical circuit, total charge remains strictly conserved.

6

Resistance and potential difference

• Electrical resistance is the ratio of potential difference across a conductor to the electric current flowing through it (R = V / I), measured in ohms (Ω).

7

Electric charge and static charging of polythene

• Friction transfers negative electrons when a polythene rod is rubbed with a cloth, leaving the rod negatively charged while equal positive charges remain on the cloth.

8

Electrical power and energy dissipation in resistors

• The rate of electrical energy transfer in a resistor is calculated by power equations P = IV = I²R, converting electrical potential energy into thermal energy.

9

Conservation and measurement of charge

• Electric charge is measured in coulombs (C) using the relationship Q = It;
  • in any closed electrical circuit, total charge remains strictly conserved.

10

Resistance and potential difference

• Electrical resistance is the ratio of potential difference across a conductor to the electric current flowing through it (R = V / I), measured in ohms (Ω).

11

Electric charge and static charging of polythene

• Friction transfers negative electrons when a polythene rod is rubbed with a cloth, leaving the rod negatively charged while equal positive charges remain on the cloth.

12

Electrical power and energy dissipation in resistors

• The rate of electrical energy transfer in a resistor is calculated by power equations P = IV = I²R, converting electrical potential energy into thermal energy.

13

Conservation and measurement of charge

• Electric charge is measured in coulombs (C) using the relationship Q = It;
  • in any closed electrical circuit, total charge remains strictly conserved.

14

Gold-leaf electroscope

• A device that detects charge by the rising of a gold leaf.

15

Terminal

• A connection point of a cell, marked + or -.

16

Voltage

• A measure of the electrical energy supplied or changed in a circuit.

17

Potential difference

• The work done by a unit charge passing through a component.

18

Electric field

• A region where a charge experiences a force.

19

Alternating current

• A current that continuously changes direction.

20

Field line

• A line showing the direction of the force on a north pole.

21

Electromotive force

• The electrical work done by a source in moving a unit charge around a complete circuit.

22

Electrical energy

• Energy transferred by a current, E = V × I × t.

23

Charge

• A quantity measured in coulombs that is moved round a circuit.

24

Polythene

• Material that becomes negative when rubbed with cloth.

25

Electroscope

• Device for detecting charge using a gold leaf.

26

Acetate

• Material that becomes positive when rubbed with cloth.

Section 8 of 9Key terms

Term	Meaning
Resistance	The opposition to current, R = V ÷ I.
Ohm	The unit of resistance.
Ohmic conductor	A conductor whose I–V graph is a straight line through the origin.
Non-ohmic conductor	A conductor whose resistance changes with current or temperature.
Diode	A component that allows current in one direction only.
Filament lamp	A lamp with a thin wire that heats up and whose resistance rises with temperature.
Cross-sectional area	The area of a wire seen when it is cut across.

Section 9 of 9Exam technique

1

Saying a filament lamp obeys Ohm's law — its resistance rises with temperature.

2

Saying a thicker wire has more resistance — a thicker wire has less resistance.

3

Saying a diode conducts in both directions — it conducts only in forward bias.

4

Confusing the gradient of an I–V graph with resistance — a steeper line means lower resistance.

5

Saying resistance is used up in a resistor — resistance only limits the current.