# Physics Unit 5 A2 OCR A revision

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• Created by: Abbi
• Created on: 09-06-10 14:11

Physics Revision Unit 5

Electric and Magnetic Fields

Electric Fields:

Oppositely charged particles attract

Charged object in electric field – experience force

Coulomb’s Law: F = (kQ1Q2)/r2 where k = 1/4pie epsilom

Force on Q1 is equal and opposite to force on Q2

F = EQ

E in direction the positive charge would move in NC-1

Electric Field Strength is the force per unit charge

In a radial field E is inversely proportional to r2

Area under an E vs r graph is the electric potential

Field strength is the same everywhere in a uniform field

E=V/d E in Vm-1

Charged particle in uniform field – experiences a constant force parallel to the electric field lines

Positively charged- force in same direction as field lines

Particle accelerates at constant rate- Newton’s 2nd

Particle’s velocity has component at right angles to the field lines – particle will have uniform velocity – Newton’s 1st

Combined effect of constant acceleration and constant velocity at right angles is a parabola

Similarities between gravitational and electric fields:

· Both force per unit something

· Both inverse square laws

· Point mass/charge creates radial field

Differences:

· G force attractive. E force either attractive or repulsive

· Objects can be shielded from electric fields, not G fields

· Medium affects size of electric forces not G forces

Magnetic Fields:

Region where a force is exerted on magnetic materials

Field lines

North to south

Closer lines = stronger field

When current flows in wire – magnetic field around wire

Direction of magnetic field around a current-carrying wire can be worked out using the right-hand rule. Thumb current, fingers field.

Single coil or solenoid field

Current carrying wire in external magnetic field – fields interact

Form stretched catapult effect where flux lines are closer together

Cause force

Current parallel to flux lines – no force

Direction of the force is always perpendicular to both the current direction and the magnetic field – Fleming’s left hand rule. First = field, second = current, thumb = force/motion

F = Bil

Magnetic field strength – the force on one meter of wire carrying a current of one amp at right angles to the magnetic field

Also called flux density and measured in teslas, T

1 tesla = Wb/m2

Vector quantity

F = BIl sinθ

Charged Particles in Magnetic Fields:

Electric current – flow of negatively charged electrons

Affected by magnetic fields so current carrying wire experiences a force

I = qv/l

F = Bqv

Force on a moving charge in a magnetic field is always perpendicular to its direction of travel

Condition for circular motion

Effect used in particle accelerators, which use magnetic fields to accelerate particles to very high energies along circular paths

The radius of curvature of a path of a charged particle moving through a magnetic field gives you information about the particle’s charge and mass meaning you can identify different particles by studying how they