# A2 Physics Definitions - Unit 5

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Definition or statement from specification

Definition or statement with page references

Electric field

Electric fields are created by electric charges.  (in an electric field a test charge will experience a force)

Define Electric Field Strength

The electric field strength is the force per unit charge (experienced by the test charge in the electric field).

Describe how electric field Lines represent electric fields

Electric field lines show the direction that a positive test charge would move.  They go away from positive charges and towards negative charges.  If the field lines are close together the Electric field is stronger.  Parallel field lines show the Electric field is uniform (see p91)

Select and use Coulombs Law

The force between two charges Q and q is given by

Where r is the distance between the centres of the two charges. Є is given on the data sheet. P92

Select  equation for Electric Field Strength of a point charge

The electric field around a charge Q is radial spreading out in all directions.  The field strength (E=F/q) at distance r from charge Q is given by

Select  equation for uniform Electric Field Strength between parallel charged plates

For parallel plates the electric field is uniform between the plates by considering work done moving between the plates as W=Fd and as W=qV it can be shown that the electric field strength depends only upon the potential difference V and the plate separation d.  P92.

Explain the effect of a uniform electric field on charged particles

A charged particle q experiences a force F=Eq along the direction of the field lines.

Describe similarities and differences between gravitational fields of point masses and electric fields of point charges.

Both fields fall off as

Gravitational fields are always attractive ie towards the mass.

Electric field lines can be towards a positive charge or away from a negative charge

Electric fields act on charges, gravitational fields act on masses.

Describe magnetic field patterns of a long straight current carrying wire and a long solenoid.

Long wire with current flowing is fig 2 p96.  Circles around the wire.  Direction of field lines from right hand thumb rule

Long solenoid Fig 4 p97.  Note straight parallel uniform field lines down centre show the field is constant in the solenoid (then decreasing at the ends).  An iron core in a solenoid concentrates the field lines increasing the flux density in the iron core to make an electromagnet.

Note the field lines are in the direction that a magnetic north pole would move ie away from north pole and towards south pole.

State and use Flemings left hand rule to determine the force on a current conductor placed at right angles to a magnetic field

P100 Fig1 and 2.  Left hand.  First Finger field direction (from N to S).  Second finger current direction (from + to -).  Thumb force direction.  Note the force is at right angles to the current and