Topic 5: Electricity and Magnetism Flashcards
Give the law of conservation of charge.
Charge can neither be created or destroyed. i.e. if two objects are rubbed together and electrons move, both objects will gain equal but opposite charge.
Define an electric field.
A region of space where a charged object experiences a force due to its charge.
Describe field lines.
1) The closer the field lines, the stronger the field.2) The direction of the field shows the force that would act on a positive test charge placed at that point.
Define a test charge.
One that does not disturb the electric field it is placed in.
What direction are the arrows for a positive and negative point charge?
Arrows for a positive point charge go out while arrows for a negative point charge go in.
Give Colulomb’s Law.
The force between two point charges is:- proportional to the product of their charges- inversely proportional to the square of their separation
Define the electric field strength.
The force per unit charge experienced by a small positive test charge placed in the field.
What is a uniform electric field created with?
Between two parallel, opposite (but equally) charged plates.
Define the potential difference between two points.
The work done (or energy transferred) when one unit of charge moves between two points.
Define the electronvolt.
The energy an electron would gain by being accelerated by a potential difference of 1 volt. 1eV = 1.6 x10⁻¹⁹J
Define current.
The rate of flow of electric charge. Measured in amperes (A) or Cs⁻¹.
Describe conventional current.
The opposite direction to electron flow (i.e. from the positive terminal to the negative terminal).
Define drift velocity.
The speed at which electrons move due to the current. I = nAvq
What causes drift velocity?
Within a conductor, electrons have high random speeds. An electric field will exert a force on the electrons causing them to accelerate in a direction opposite to the direction of the field. So the net velocity (drift velocity) of the electrons is in the opposite direction to the electric field.
Describe a series circuit.
- The current is the same around the circuit.
- The sum of the potential differences across the components adds up to the supply voltage.
Describe a parallel circuit.
- The current splits up between the branches.
- The potential difference across each branch is the same.
How do we measure current in a circuit?
An ammeter is used. It must be connected in series with the component that is having the current flowing through it measured. An ideal ammeter would have a resistance of zero so it does not affect the current in the circuit which it is connected.
How do we measure potential difference in a circuit?
A voltmeter is used. It must be connected in parallel with the component that is having the potential difference across it measured. An ideal voltmeter would have an infinite resistance so it does not draw any current from the circuit to which it is connected.
Define resistance.
The ratio of potential difference across the component to current flowing through the component. “A measure of the ease of current flow”.
Equations for resistance in series and parallel circuits.
Series:Rtotal = R1 + R2 + …Parallel: 1/Rtotal = 1/R1 + 1/R2 + …
Define resistivity (p).
Electrical resistivity is a property of a substance that gives a measure of how the substance opposes the flow of electric current. Measured in Ωm.
What are the three types of sensors that can be used in potential divider circuits?
1) Light dependent resistors (LDRs)2) Thermistors3) Strain gauges
Define electrical power.
The amount of energy used or supplied per unit time.
Kirchhoff’s Laws
1) At any junction in a circuit, the total current entering the junction equals the total current leaving (conservation of charge).2) Around any closed path of a circuit, the total of all the potential differences is zero (conservation of energy).
Describe primary cells.
Primary cells cannot be recharged, they are an irreversible chemical reaction and once the chemicals have been used up the cell no longer produces electrical energy. Examples include Zn-C and alkaline batteries.
Describe secondary cells.
Secondary cells are designed to be recharged. They undergo a reversible chemical reaction. Reversing the current through the cell by attaching it to an external DC power source will reverse the chemical reaction, producing more reactants. Examples include Pb-acid car batteries, Ni-Cd and Lithium ion batteries.
What is capacity given in?
Amp-hours (Ah). 1Ah is equal to 3600C.*The greater the capacity of the cell, the greater current it can deliever for a given period of time.
Define EMF.
It is the energy supplied per unit charge by the power supply to the circuit.*What the battery is trying to put out.
What is the shape of the graph of terminal potential difference against time?
Remains at emf for most of the time and then there is a rapid final drop in terminal pd due to an increase in internal resistance.
Define terminal potential difference.
The potential difference measured across the terminals of a battery when it is part of a circuit and a current is flowing. It is a measure of the energy available to the external circuit.*What the circuit gets!
How can a bar magnet be drawn?
Lines coming out of North and going into the South.
How is the magnetic field drawn going into and out of the page?
Into the page is crosses. Out of the page is dots. Think ‘o’ for out.
Describe the rule used to determine moving charges.
RH slap rule for positive charges.LH slap rule for negative charges.Thumb is the velocity of the charge/direction. Fingers is the magnetic field and the palm is the force.
Define internal resistance.
The resistance of the components inside the cell/battery. It causes an energy loss in the cell.
