Electromagnetism Flashcards
What is the definition of electric field strength?
Electric field strength is the force exerted on a unit charge (by an electric field).
What would the electric field pattern be around a single point negative charge?
What would the electric field pattern be around a single point positive charge?
What is the definition of electrical potential?
The work done in moving a unit charge from infinity to a point in a field.
An electric field is an example of a conservative field. What is meant by a conservative field?
The energy required to move a charge
between two points in an electric field is
independent of the path taken
A few metals, including iron, nickel and cobalt, are classed as ferromagnetic materials. Applying an external magnetic field to one of these materials results in it becoming magnetised.
Explain how this external magnetic field causes the material to become magnetised.
The external field aligns the magnetic dipoles in the material, magnetising it.
What would the the electric field pattern between two oppositely charged parallel plates look like? (ignore end effects).
Three similar looking relationships are shown below for E, F and V.
Define what E, F and V are in terms of the charges (Q) in each relationship.
- E is the electric field strength at a distance r from point charge Q.
- F is the electrostatic force that exists between point charges Q1 and Q2, separated by a distance r. (This can be attractive or repulsive.)
- V is the electrostatic potential of an electric field at a distance r from point charge Q.
Sketch a current against time graph for a charging capacitor in a d.c.“RC” circuit.
Sketch a current against time graph for a discharging capacitor in a d.c.“RC” circuit.
(note the ‘negative’ current indicating a change in dirction of current flow)
Sketch a voltage against time graph for a discharging capacitor in a d.c.“RC” circuit.
Sketch a voltage against time graph for a charging capacitor in a d.c.“RC” circuit.
What is meant by the ‘time constant’ of a d.c. “CR” circuit?
- When the charge stored in a capacitor has increased by 63% of the difference between initial charge and full charge, one time constant has passed.
- The time constant can be calculated using the relationship τ = RC
How could you determine the time constant from a voltage-time graph such as this?
Work out what 63% of the supply voltage is, and read off the graph how long it taks to reach this value from when it started charging.
What is meant by the term capacitive reactance?
In an a.c. circuit a capacitor will oppose the flow of current (similarly to resistance).
We call this opposition to a.c. current capacitive reactance Xc