Electricity Flashcards
What is a vacuum?
No matter in a volume of space
Like charges repel
Unlike charges attract
Attraction is effected by distance
Charges flow in materials
When charges move, they produce a magnetic field, moving anticlockwise
Like charges
Repel, force is positive
Unlike charges
Attract, force is negative
What is the force equation?
Product of the charges divided by the distance squared
What is the SI unit for charge?
Coulomb
Explain a focusing cap on an X-ray tube
To keep the electrons focused on the target anode, and thus creating a sharper image. Keeping the flow of electrons repelling and confined in the area of movement towards the target.
Define electrical potential
Work done in moving a unit charge from infinity to that point
What is the SI unit for electrical potential
Volt
Define potential difference
The difference in work done moving a unit charge from one point to another
What is electromagnetic force (EMF)
The driving force across a power source (negative charge flows from a negative to positive potential)
Conduction band
Free electrons
Forbidden zones
Electrons cannot travel freely
Valence band
Electrons can freely move from the outer shell
Filled band
Complete shells
Insulators
Require more energy for electrons to enter conduction band as the forbidden zone is larger
Must increase temperature and Voltage
Semi-conductors
Requires less energy as the forbidden zone is shorter
At an increased temperature, more electrons will enter the conduction band, but at room temperature some will enter anyway
Conductors
No forbidden zone, so electrons can freely pass through bands (random movement)
What happens as charge storage increases?
Repulsive effect slows down storage
To increase, increase the size of the storage capacitator, or the potential difference
What is capacitance?
The quantity of charge which can be stored on a conductor per unit potential
What is the SI unit for capacitance?
Farad
What is a capacitor?
A device used to store electrical current, consists of two electrode conductors and a separating insulator
What does the insulating layer do in a capacitor?
Prevents contact between the two plates, thus preventing charge flow
Increases the capacitance of the capacitor
What effects capacitance?
Distance between plates
Area of plates
Uses of capacitors
Voltage smoothing (overcomes X-ray energy fluctuations)
Timing circuits (signal sent to end exposure when capacitor reaches max)
State some facts about electron flow
Direction of movement is equally random
Resistance to flow is affected by temperature due to increased collisions
What is electrical current?
Flow from negative to positive
What is convectional current?
Flow from positive to negative
What is the SI unit for current?
Ampere
How do you work out the electric charge flowing through an X-ray tube during an exposer?
milliamps x time(s)
What is the SI unit for resistance and what is resistance?
Ohms
Collisions between vibrating electrons
What factors affect resistance?
Length
Cross sectional area
Conductor or insulator?
Temperature
What is Ohm’s Law?
The current flowing through a conductor is directly proportional to the potential difference between its ends
What is the equation for Potential Difference?
V + I x R
(V=potential difference
I= current
R= resistance)
What is electrical power?
The rate of doing work
Power= voltage x current
How do you minimise power loss?
High voltage and low current
How to reduce resistance?
Copper
Increase cross sectional area
Electrical safety features
Isolation
Insulation
Earthing
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