3. Energy, Power + Resistance

What are some of the main circuit symbols?

• Cell

• Battery

• Switch open

• Switch closed

• Power supply

• Filament lamp

• Ammeter

• Voltmeter

• Resistor

• Variable resistor

• Fuse

• Thermistor

• Diode

• LED

• LDR

What does potential difference mean + equation?

• Potential difference (p.d.) - energy transferred (lost) per unit charge that flows through the component

• V = W / Q (potential difference = work done / charge)

What does electromotive force mean + equation?

• Electromotive force (e.m.f.) - energy supplied (gained) per unit charge that flows through a source

• E = W / Q (electromotive force = work done / charge)

How is kinetic energy related to potential difference?

Kinetic energy = work done = eV (elementary charge x potential difference)

What is the equation for the kinetic energy of a charged particle?

eV = ½ mv² (elementary charge x potential difference = ½ x mass x velocity²)

What does resistance mean + equation?

• Resistance - how hard it is for current to flow (Ω)

• V = IR

What is Ohm’s law?

For an ohmic conductor at a constant temperature, current is directly proportional to p.d

How to find the I-V characteristic of a component?

1. Put a variable power supply, ammeter and the component in series

2. Put a voltmeter in parallel across the component

3. Record the current + vary the p.d using the power supply

4. Plot a graph of p.d (x-axis) against current (y-axis)

What does the I-V graph of a ohmic conductor look like + why?

Straight line → current is directly proportional to p.d (constant resistance)

e.g Fixed resistor

What does the I-V graph of a filament bulb look like + why?

Straight line at start and then curves downwards → high current → increased temperature → increased resistance

What does the I-V graph of a diode look like + why?

No current until threshold reached then curves upwards quickly → diodes only let current flow in one direciton

What does the I-V graph of a thermistor look like + why?

Curves upwards → current increases → temperature increases → decreased resistance

What happens to resistance as light intensity increases for a LDR?

As light intensity increases, resistance decreases

What does resistivity mean + equation?

• Resistivity - property of a material representing how hard it is for current to flow

• R = 𝜌L/A (resistance = resistivity x length / area)

How to determine resistivity?

1. Use a micrometer to measure the diameter of a wire at multiple points

2. Average the diameter

3. Calculate cross-sectional area

4. Setup a circuit with a battery, potentiometer, ammeter, voltmeter and changeable length of wire with a metre rule

5. Measure p.d + current

6. Repeat with different lengths of wire

7. Calculate resistance using R = V / I for each length of wire

8. Plot resistance (y-axis) against length (x-axis)

9. Gradient gives you ρ/A

How does resistivity change with temperature for conductors + semi-conductors?

• Conductors → increased collisions with ions → increased resistivity

• Semi-Conductors → more charge carriers released → decreased resistivity

What does power mean + main equation?

• Power - rate of energy transfer (W)

• P = W / t (Power = work done / time)

What are the three power equations?

• P = IV

• P = I²R

• P = V² / R

What is the equation for work done?

W = IVt (Work done = current x potential difference x time)

What does a kilowatt-hour mean?

• Kilowatt-hour - energy used by 1000W acting for an hour (kWh)

• 1 kWh = 3.6 × 10⁶ J (1000 × 60 × 60)

What does internal resistance mean + equation?

• Internal resistance - resistance inside a battery

• E = I(R + r) (e.m.f. = current (resistance + internal Resistance))

What does terminal voltage mean + equation?

• Terminal voltage - actual p.d across circuit accounting for internal resistance

• V_terminal = E - Ir

What is the e.m.f. in parallel circuits with the same voltage cell in each branch?

Total e.m.f = voltage of 1 cell

How to determine internal resistance + e.m.f.?

1. Setup a circuit with a battery, variable resistor, ammeter + voltmeter across the battery

2. Set resistor to highest resistance

3. Record current + voltage + open switch

4. Change resistance + repeat

5. Plot voltage (y-axis) against current (x-axis)

6. Gradient is the -internal resistance + y-intercept is the e.m.f.