Electricity Flashcards
What do voltmeters measure?
The difference in p.d. across a component.
Define p.d with the help of a relevant equation.
The measure of how much energy is transferred by each coulomb of charge (V = E/Q).
Define current with the help of a relevant equation.
The rate of flow of charge (I = Q/T).
Define resistance.
The measure of how much a component resists the flow of current.
How do you know if a resistor is in ohmic using an IV graph?
- Straight line through the origin (directly proportional).
- Constant gradient = constant R.
- The steeper the gradient, the lower the resistance.
Outline why metals are non-ohmic conductors with reference to the processes that take place in them.
Resistance is not constant. At higher currents, delocalised electrons collide with the ionic lattice at a greater rate, causing them to vibrate faster and temperature to increase, therefore increasing resistance.
What is a diode?
A component which only lets current flow in one direction: low resistance in forward direction, high resistance in reverse direction.
What is a superconductor?
A material which has a resistance that decreases to 0 at the critical temperature.
Define resistivity using a relevant equation.
How resistive a material is to the flow of charge as its length increases; the resistance of a cube of unit length sides (ρ = RA/L).
How can resisitivity be calculated in an experiment?
- Measure the diameter of a wire with a micrometer (in multiple places and calculate the mean) and use to calculate the cross-sectional area.
- Change L of wire (measure with metre rule - ensure wire is taut) by moving one crocodile clip. Use wire of material for which resistivity doesn’t change much with temperature e.g. nichrome.
- Calculate R from V/I for each length.
What is Kirchhoff’s 2nd law?
The sum of EMFs (p.d. rises) must equal the sum of p.d. drops in a closed loop.
How do V, I and R behave in a series circuit?
- Total p.d. is shared between all components.
- Current is the same for all components.
- Total resistance = sum of resistances.
How do V, I and R behave in a parallel circuit?
- P.d. for each branch = p.d. of source.
- Current is split between the branches.
- Adding more resistors in parallel reduces total R.
What is Kirchhoff’s 1st law?
Charge (and current) is conserved at any junction in a circuit (total I in = total I out).
How do thermistors and LDRs react to temperature/ light increases?
When temperature/ light intensity increases, resistance decreases.
Define power with the help of a relevant equation.
The rate of energy transfer (P = E/t).
What is the EMF and how is it different to terminal p.d.?
EMF is the energy supplied to each unit charge.
The rest of the circuit won’t get the full EMF because some p.d. is lost due to the internal resistance of the battery.
