Energy, Power & Resistance

Open Switch

Closed Switch

Cell

Voltmeter

Ammeter

Thermistor

Battery

Diode

Resistor

LDR

LED

Capacitor

Variable Resistor

Lamp

Fuse

What are circuit symbols used for?

To represent components in electrical circuits

How should circuit symbols be drawn?

Clearly and carefully with correct directions

How should wires be drawn in circuit diagrams?

As straight lines

How should junctions be drawn?

At 90° angles

What does an open switch symbol represent?

A break in the circuit

What does a closed switch symbol represent?

A complete circuit

What is a cell?

A single source of emf

What is a battery?

Two or more cells in series

What does a voltmeter measure?

Potential difference

How is a voltmeter connected?

In parallel

What does an ammeter measure?

Current

How is an ammeter connected?

In series

What does a resistor do?

Limits the current in a circuit

What does a variable resistor do?

Allows resistance to be changed

What is an LDR?

A resistor whose resistance depends on light intensity

What is a thermistor?

A resistor whose resistance depends on temperature

What does an LED do?

Emits light when current flows through it

What does a diode do?

Allows current to flow in one direction only

What does a capacitor do?

Stores electrical charge

What does a fuse do?

Breaks the circuit if current is too large

What does a lamp symbol represent?

A filament bulb

What is potential difference?

Energy transferred per unit charge

What is the equation for potential difference?

V = W/Q

What are the units of potential difference?

Volt V

What does 1 volt mean?

1 joule of energy transferred per coulomb

What happens to charge carriers across components?

They lose energy

What is electromotive force?

Energy supplied per unit charge

What is the equation for emf?

ε = W/Q

What energy transfer occurs in a cell?

Chemical to electrical

What are the units of emf?

Volt V

What is an electron gun?

A device that produces a beam of electrons

What is the cathode in an electron gun?

A heated metal filament

What process releases electrons from the cathode?

Thermionic emission

Why is the electron gun in a vacuum?

To prevent collisions with air particles

What accelerates electrons towards the anode?

A large potential difference

Why does the anode have a small hole?

To form a narrow electron beam

What energy transformation occurs during acceleration?

Electrical potential energy to kinetic energy

What equation links electron speed and p.d.?

eV = ½mv²

What assumptions are made in eV = ½mv²?

Negligible initial KE and no energy losses

What is resistance?

Opposition to the flow of charge

What is the equation for resistance?

R = V/I

What are the units of resistance?

Ohm Ω

What does 1 ohm mean?

1 volt per ampere

How is resistance measured experimentally?

Using voltmeter and ammeter readings

How are voltmeters connected in resistance experiments?

In parallel

How are ammeters connected in resistance experiments?

In series

How is resistance found from an I–V graph?

Inverse of the gradient

State Ohm’s law.

Current is proportional to p.d. at constant temperature

What condition is required for Ohm’s law?

Constant temperature

What is an ohmic component?

A component that obeys Ohm’s law

What does an ohmic I–V graph look like?

Straight line through the origin

What is a non-ohmic component?

A component that does not obey Ohm’s law

Why do metals become non-ohmic at high current?

Increased heating increases resistance

Why does heating increase resistance in metals?

More lattice vibrations cause more collisions

What is a fixed resistor?

A component with constant resistance

How do wires behave in circuits?

Like low-resistance fixed resistors

Why is a filament lamp non-ohmic?

Resistance increases as filament heats

How does the I–V graph of a filament lamp curve?

Gradient decreases as current increases

What is a diode made from?

Semiconductor material

How does a diode conduct?

In one direction only

What happens in reverse bias?

No current flows

What is the resistance in reverse bias?

Effectively infinite

What is the threshold voltage of a diode?

Minimum p.d. needed to conduct

Why does resistance fall after threshold?

Increased charge carrier density

What does NTC stand for?

Negative temperature coefficient

What happens to resistance in an NTC thermistor as temperature increases?

It decreases

Why does resistance decrease in an NTC thermistor?

More charge carriers are released

What are thermistors used for?

Temperature sensing circuits

Is a thermistor ohmic?

No

What happens to LDR resistance as light intensity increases?

It decreases

Why does LDR resistance decrease in bright light?

More charge carriers are released

What are LDRs used for?

Light sensing circuits

What is resistivity?

A material property linking resistance length and area

What is the equation for resistivity?

ρ = RA/L

What are the units of resistivity?

Ohm metre Ω m

How does resistivity depend on shape?

It does not

How does resistivity depend on material?

It depends only on the material and temperature

How does resistivity change with temperature in metals?

It increases

Why does resistivity increase in metals with temperature?

Increased ion vibrations cause more collisions

How does resistivity change with temperature in semiconductors?

It decreases

Why does resistivity decrease in semiconductors?

Charge carrier density increases

How is resistivity measured experimentally?

By measuring R L and A

How is cross-sectional area measured?

Using vernier calipers or micrometer

Why take multiple diameter readings?

To reduce uncertainty

What is electrical power?

The rate of energy transfer