4.2 - Energy, Power and resistance

What is the name of component whose circuit symbol is a rectangle with 2 arrows pointing at it?

Light dependent resistor (LDR)

Should voltmeters be connected in series or parallel?

Parallel

What is P.D?

The work done per unit charge; the difference in potential between 2 points in a circuit.

What is the difference between voltage and emf?

Voltage is the energy transferred per coulomb across a load resistance. Emf is the total amount of work done by the battery per coulomb.

Give the equation relating energy transferred to voltage

W = VQ,Where W = work done (energy transferred), V = P.D, Q = charge

True or false: The kinetic energy of an electron is equal to its charge multiplied by the voltage.

True. Energy transferred = eV = 1/2mv^2

What is resistance?

The potential difference required for each unit of current that flows through a resistor. V=IR

What is the definition of 1 Ohm?

When a resistor is subject to a voltage of 1V and allows a current of 1A through, its resistance is 1 Ohm.

What is meant by an ohmic conductor?

A conductor that obeys Ohm's law

What is Ohm's Law?

The current through an ohmic conductor is directly proportional to the potential difference across it. Only true when temperature remains constant.

What does the gradient of a current-potential difference graph represent?

1/R, Gradient = change in current/change in potential difference.

Why does the current increasing on a filament lamp cause an increase in the resistance?

Flow of current causes collisions between electrons and metal lattice, These collisions increase temperature of the lattice, As temperature increases, more collisions occur, causing resistance to increase

What is a diode?

A diode is an appliance that only allows current to flow in one direction

What is an LDR?

A semi-conductor that is sensitive to light. As light intensity increases, resistance decreases

How does a thermistor work?

Similar to an LDR but, as the temperature increases, the resistance decreases
(thermistors have a negative temperature coefficient).

What is resistivity?

The resistivity is a property of a metal which shows how easy or difficult it is for current to flow in the material (at a specific temperature)

p=RA/L

Where p=resistivity, R=resistance, A=cross-sectional area and L=length

Describe an experiment to determine the resistivity of a metal?

Measure diameter of wire using micrometer

Use this to calculate area

Vary the length and record the voltage and current for each length

Use V=IR to work out resistance

Plot a graph of resistance against length

Gradient = resistivity / area

Rearrange to work out resistivity

True or false: The resistivity of both metals and semiconductors increases with temperature.

False.

The resistivity of a metal increases with temperature because as the metal ions heat up they vibrate more and bang into electrons and slow them down.

Some semi-conductors get less resistive as temperature increases, because supplying energy actually causes more charge carriers to be released meaning current can flow easier

Is a kilowatt-hour (kWh) a unit of power, potential difference or energy?

Energy because its a unit of power multiplied by time

What is power?

The rate at which energy is transferred