Electric Charges and Fields
Class 12 · Physics
4 ideas in Class 12 Physics, explained in plain words, with games and quizzes to practise.
Electric Charge and Its Properties
Part of: Charge and Its Basic Properties
Rubbing two suitable materials transfers tiny electrons from one to the other, leaving one positively and the other negatively charged. Like charges repel and unlike charges attract. Total charge adds like ordinary numbers, is never created or destroyed in an isolated system, and always comes in whole multiples of one electron charge.
Why it matters. Every electrical device depends on moving charges in a controlled way. Mixing up conductors and insulators, or forgetting that charge is conserved, leads to wrong predictions about what happens when charged objects touch.
Formulas
- Total charge of a system: q = q1 + q2 + q3 + ... with correct signs
- Quantisation: q = n e, where n is an integer
- Electron charge magnitude: e = 1.6 x 10^-19 C
Key words
- Electric charge
- : A property of matter that causes attraction or repulsion; it comes in positive and negative kinds.
- Conductor
- : A material in which electric charge moves freely, such as metals and the human body.
- Insulator
- : A material in which charge stays where it is placed, such as plastic, glass or dry wood.
- Quantisation of charge
- : The rule that any observable charge is a whole-number multiple of one electron charge.
- Conservation of charge
- : The rule that the total charge of an isolated system never changes during any process.
- Electroscope
- : A simple device whose leaves spread apart to show the presence and amount of charge.
Try it in two minutes
- Rub a plastic pen on dry hair and pick up tiny paper bits with it.
- Ask why the bits jump only to the rubbed part of the pen.
- Touch the pen to a metal tap and ask where the charge went.
Try at home. Test a steel spoon and a plastic spoon for picking up paper bits after rubbing, and compare.
Coulomb's Law and Superposition
Part of: Coulomb's Law and Superposition
The force between two point charges grows with the product of the charges and shrinks with the square of the distance between them, acting along the line joining them. When several charges are present, the total force on any one charge is the vector sum of the separate pair-wise forces.
Why it matters. This law is the starting point for all of electrostatics, from motors to lightning. Forgetting the square, or adding forces as plain numbers instead of vectors, gives answers that are wrong by large factors.
Formulas
- Coulomb's law: F = k q1 q2 / r^2
- k = 1 / (4 pi e0) = 9 x 10^9 N m^2 / C^2
- Force on q in a field: F = q E
Key words
- Point charge
- : A charged body treated as concentrated at one spot because it is tiny compared to its separation.
- Coulomb's law
- : The rule that the force between two point charges is proportional to their product over the squared distance.
- Superposition principle
- : The rule that the total electric force is the vector sum of the separate forces from each charge.
- Electrostatic constant
- : The constant k, about 9 x 10^9, that sets the strength of the electric force in SI units.
Try it in two minutes
- Write the product of two charges and divide by the square of their distance.
- Multiply by the constant 9 x 10^9 to get the force in newtons.
- Draw arrows for each pair-wise force, then add the arrows tip to tail.
Try at home. Work out how the force changes when the distance is doubled, tripled and halved.
Electric Field and Field Lines
Part of: Electric Field and Field Lines
The electric field at a point is the force a tiny positive test charge would feel there, divided by that charge. Field lines sketch the field: they leave positive charges, enter negative ones, never cross, and lie closer together where the field is stronger.
Why it matters. Fields let us predict forces without tracking every distant charge. Misreading line diagrams, such as thinking lines cross or that a negative charge feels a force along the field, reverses the predicted motion.
Formulas
- Field from force: E = F / q
- Field of a point charge: E = k Q / r^2, away from Q if Q is positive
- Unit of field: N/C, which is the same as V/m
Key words
- Electric field
- : Force per unit positive charge at a point, telling what force any charge placed there feels.
- Test charge
- : A tiny positive charge used to probe a field without disturbing the charges that make it.
- Field lines
- : Curves whose tangent at each point gives the field direction there.
- Uniform field
- : A field with the same strength and direction everywhere, drawn with parallel equally spaced lines.
Try it in two minutes
- Divide the force on a test charge by the test charge to get the field.
- Sketch lines leaving a positive charge and entering a negative one.
- Compare line spacing at two spots to say where the field is stronger.
Try at home. Draw the field lines for two equal positive charges and mark the midpoint.
Dipole, Flux and Gauss's Law
Part of: Dipole, Flux and Gauss's Law
A dipole is a pair of equal and opposite charges whose strength is measured by its dipole moment. Electric flux counts field lines crossing a surface. Gauss's law says the total flux out of any closed surface equals the enclosed charge divided by epsilon nought, which makes the field of symmetric shapes easy to find.
Why it matters. Gauss's law turns hard summations into one-line answers for spheres, wires and sheets, and it underlies capacitors and shielding. Mixing up axial and equatorial dipole fields, or counting unenclosed charge in Gauss's law, are the classic errors.
Formulas
- Dipole moment: p = q x 2a, from minus to plus
- Torque on dipole: T = p E sin(theta)
- Gauss's law: flux = q inside / e0
- Field of infinite line: E = lambda / (2 pi e0 r)
- Field of infinite sheet: E = sigma / (2 e0)
Key words
- Electric dipole
- : Two equal and opposite charges separated by a small distance, acting as one unit.
- Dipole moment
- : Charge times separation, pointing from the negative charge to the positive one.
- Electric flux
- : A measure of the electric field passing through a surface, counted with sign.
- Gauss's law
- : Total outward flux through a closed surface equals enclosed charge over epsilon nought.
- Gaussian surface
- : An imaginary closed surface chosen for its symmetry to apply Gauss's law easily.
Try it in two minutes
- Point the dipole moment arrow from the negative charge to the positive one.
- For flux, multiply enclosed charge by 1 over epsilon nought.
- Pick a symmetric Gaussian surface and compare inside with outside.
Try at home. List which charges contribute to the flux through a balloon surface around only one of two charges.
What your child will practise
- Coulomb's Law and Superposition: Space Quick Fire (Quick Fire)
- Dipole, Flux and Gauss's Law: Candy Puzzle (Clue Puzzle)
- Dipole, Flux and Gauss's Law: Ocean Quick Fire (Quick Fire)
- Electric Charge and Its Properties: Jungle Quick Fire (Quick Fire)
- Electric Field and Field Lines: Space Puzzle (Clue Puzzle)
- Quiz: Coulomb's Law and Superposition (8 questions)
- Quiz: Dipole, Flux and Gauss's Law (8 questions)
- Quiz: Electric Charge and Its Properties (8 questions)
- Quiz: Electric Field and Field Lines (8 questions)