Electricity 1 Flashcards
State Coulomb’s Law.
The force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them.
Define Electric Field.
An electric field is the region around a charged object in which its electric force acts.
Define Electric Field Strength.
The electric field strength at a point in an electric field is the force acting on a test charge q placed at the point divide by the magnitude of the charge.
E = F/Q (E=Electric Field Strength, F=Force and Q=Charge)
Define Potential Difference.
The potential difference from A to B is the work done per unit charge to move a charge from point A to point B.
Define the unit of potential difference.
1 volt is equal to 1 Joule over 1 Coulomb
Explain how to charge an insulator by friction.
A plastic rod is rubbed with a cloth.
Electrons leave the rod and transfer to the cloth.
As the rod has lost electrons it is now positively charged.
The cloth has gained electrons so it is negatively charged.
Explain how to charge a metal sphere by induction.
Bring a negatively charged rod up to a sphere on insulated stand.
Charge separation occurs with positive charges attracted to the rod and negative charges repelled.
The metal sphere is then grounded and the electrons now run to the earth.
Remove the earth connection and then the rod leaving the sphere positively charged.
Discuss charge distribution in 3D objects.
Charge resides on the outer surface of an object and surface charge density is greatest at the sharp corners of the object.
What are the uses of the principle that charge resides on the outside of a conductor?
Co-axial cable
Electrostatic Shielding
Faraday Cage
What is a proof plane?
A metal disc attached to an insulator body. When it touches a charged body it takes some of the charge from the body.
Explain how point discharge occurs.
Charges accumulate at a point. Air is ionised around the point like charges are repelled and unlike charges are attracted creating a electrostatic wind.
Define electrostatic wind.
The movement of like charges repelled by point discharge.
Explain how a lightning conductor works and the two situations that may occur.
A lightning conductor is sharp spike at the top of a building that is grounded with a high concentration of positive charge at the spike.
When a negatively charged cloud passes overhead the following occurs:
Situation 1: À huge number of electrons travel from the cloud to the spike forcing the air to be a conductor and entering the earth. Observed as a flash of lightning, no damage done to the building.
Situation 2: The positives at the spike ionise surrounding air, a good conductor. Electrons travel through the ionised air and into the ground. The cloud has been neutralised, nothing observed, no damage done to the building.
Is a test charge positive or negative? What is an electric field line?
Positive;
The path along which a test charge moves.
Name five properties of electric field lines.
Start at positive charge end at negative.
Don’t intersect.
Perpendicular to surface of the conductor.
No electric field lines inside and empty hollow charge conductor.
Aluminium acts as electrostatic shielding blocking the electric field
What is the principle of the faraday cage?
That there is no electric field inside an empty hollow charged conductor.
Explain two uses of electric fields.
-Aircraft have tyres made of conducting materials to remove charge and prevent fires.
-Electrostatic Precipitator removes dust particles from industrial chimneys by ionising them and attracting them to charged plates.
Define capacitance. Give formula.
Is the ratio of the charge q on the object to the voltage of the object.
C = q/V
Define the unit of capacitance
1 farad = 1 coulomb / 1 volt
What is the insulator separating the capacitors called?
Dielectric
What is the formula for capacitance on a parallel plate capacitor? What unit is area measured in?
C = e0 x A / d
C = Capacitance, e0 = permittivity, A = overlap area, d = distance between plates
Area is measured in metres squared.
Give two uses of a capacitor.
-Flash guns in a camera
-Rectifier
What is the net charge on a capacitor?
0
List the factors that affect capacitance.
Overlap area
Distance apart
Permittivity of dielectric
