Electricity

What is electric current?

The rate of flow of charge: electrons in metals, ions in electrolytes

What unit is current measured in?

Amps (A)

What is the difference between conventional current and electron flow?

Conventional current flows from the positive to the negative terminal; electrons actually flow from negative to positive

What is direct current (DC)?

Current that flows in one direction only

What unit is charge measured in?

Coulombs (C)

What is the elementary charge, e?

The magnitude of the charge on an electron or proton: proton +e, electron −e

How do you calculate charge?

Charge Q (C) = current I (A) × time t (s)

What is potential difference?

The work done per unit charge moving between two points, measured in volts

How is resistance defined?

Resistance R (Ω) = potential difference V (V) ÷ current I (A)

What unit is resistance measured in?

Ohms (Ω)

What is Ohm's law?

For an ohmic conductor, current is directly proportional to potential difference, provided temperature and other physical conditions stay constant

How does an LDR's resistance change with light?

Resistance decreases as light intensity increases

How does an NTC thermistor's resistance change with temperature?

Resistance decreases as temperature increases

How do you calculate power from current and potential difference?

P (W) = I (A) × V (V)

How do you calculate power from current and resistance?

P (W) = I² (A²) × R (Ω)

How do you calculate power from potential difference and resistance?

P (W) = V² (V²) ÷ R (Ω)

How do you calculate work done in a circuit from p.d., current and time?

W (J) = V (V) × I (A) × t (s)

How do you calculate work done in a circuit from p.d. and charge?

W (J) = V (V) × Q (C)

What is Kirchhoff's first law?

Current going into a junction equals current coming out (conservation of charge)

What is Kirchhoff's second law?

In any closed loop, the total p.d. across the components equals the p.d. supplied to the loop (conservation of energy)

How do you find total resistance in series?

Add up all the resistances: R_T = R₁ + R₂ + …

How do you find total resistance in parallel?

1/R_T = 1/R₁ + 1/R₂ + …

How does potential difference vary for cells connected in series?

The total p.d. is the sum of the individual cell p.d.s (cells connected the same way round)

How does potential difference vary for identical cells connected in parallel?

The overall p.d. stays the same as for one cell

What is a potential divider circuit used for?

To split a supply voltage between components, giving a chosen output voltage

What is a potentiometer?

A variable resistor used as a potential divider, to give an adjustable output voltage

Why are LDRs and thermistors used in potential dividers?

So the output voltage changes automatically with light level or temperature