Electricity and Magnetism Flashcards
Conservation of Charge
charge can neither be created nor destroyed
i.e. both objects will gain equal but opposite charges if rubbed together
Electrical Conductors/ Insulators
a material that allows a flow of charge through it is called an electrical conductor
if charge cannot flow through a material it is called an electrical insulator
Electric Field Definition
a region of space where a charged object experiences a force due to its charge
What do electric field lines represent?
the closer the field lines, the stronger the field
the direction of the field shows the force that would act on a positive test charge placed at that point
Coulombs Law
the force between two point charges is:
- proportional to the product of their charges
- inversely proportional to the square of their separation
Electric Field Strength (E) Definition
the force per unit charge experienced by a small positive test charge placed in the field
Potential Difference between two points Definition
the work done (or energy transferred) (W) when one unit of charge (q) moves between two points.”
Electronvolt
one electronvolt is the energy an electron would gain by being accelerated by a potential difference of 1V
Current Definition
the rate of flow of electric charge
Electron Flow
electrons flow from the negative terminal of the power source around the circuit to the positive terminal
Conventional Current
current is said to flow in opposite direction (positive to negative)
Drift Velocity
is the speed at which electrons move due to the current
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 of the electrons is in the opposite direction of the electric field
Series Circuit
the current is the same around the circuit
the sum of the voltages (potential differences) across the components adds up to the supply voltage
Parallel Circuits
the current splits up between the branches
the voltage (potential difference) across each branch is the same
Measuring Current
an ammeter is used
it must be connected in series with the component that is having the current flowing through it
an ideal ammeter has a resistance of zero so it does not effect the current in the circuit in which it is connected
Measuring Potential Difference
a voltmeter is used
it must be connected in parallel with the component that is having the potential difference measured
an ideal voltmeter would have an infinite resistance so it does not draw any current from the circuit to which it is connected
Resistance Definition
“the ratio of potential difference across the component to current flowing through the component.”
i.e. a high resistance needs a large potential difference for a current to flow
Resistor Definition
a component whose resistance is constant over a range of potential differences (and currents)
Ohm’s Law
“the current flowing through a piece of metal is proportional to the potential difference across it providing the temperature remains constant.”
Resistivity
is a property of a substance that gives a measure of how the substance opposes the flow of electric current
Potential Dividers
two resistors in a series circuit will divide up the voltage between them in proportion to their resistances
Potentiometer
a single resistor that can be used to divide up the voltage
the output voltage to a device connected in parallel can be varied by adjusting the proportion of the potentiometer the device is connected to
Light Dependent Resistors
as the light intensity decreases the resistance of the LDR increases
Using light to turn something on:
* connected in series
- light intensity increases
- R of LDR decreases
- V across bulb increases
- bulb turns on
Using darkness to switch something on:
* connected in parallel
- light intensity decreases
- R of LDR increases
- V across LDR increases
- V across bulb increases
- bulb turns on
