Current Electricity Concepts in AQA A-Level Physics

Electric current

The rate of flow of charge. It is measured in amperes (amps).

Potential difference

The work done moving a unit charge between 2 points in a circuit. V = W / Q

Resistance

How difficult it is for current to flow through an appliance. A component has a resistance of 1Ω if 1A flows through it when a p.d of 1V is applied across it. R = V / I

Ohmic conductor

A conductor that obeys Ohm's law, meaning that current is directly proportional to potential difference providing physical conditions (such as temperature) remain constant.

Measuring current in a circuit

You can measure the current in a circuit with an ammeter connected in series with the component.

Measuring potential difference

Using a voltmeter, connected in parallel across the component being measured.

Gradient of a current-potential difference graph

Rate of change of current with respect to voltage. This is not the same as 1/R.

Graph of an ohmic conductor

Yes, as the line has a constant gradient and passes through the origin. This shows that voltage is directly proportional to current.

Appliance with higher resistance

B. A higher voltage is required in B for the same change in current in comparison to A.

Common appliance represented by curve

A filament lamp. As the current increases the resistance also increases. A big increase in the voltage produces only a small increase in current.

Current increase in filament lamp

As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude. This impedes the movement of electrons through the lamp as they collide with the ions (resistance has increased).

Diode

A semiconductor device that allows current to flow in one direction only.

Diode

A appliance that only allows current to flow in one direction.

Voltmeters

You should assume they have infinite resistance.

Ammeter

This assumption means that there would be 0 potential difference across the ammeter and no energy is lost across it; it does not affect the circuit.

Light Dependent Resistor (LDR)

A semiconductor that is sensitive to light. As the light intensity increases, its resistance decreases.

Thermistor

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

Resistivity

The resistance of a 1m cylinder with a cross sectional area of 1m2. Resistivity is an inherent property of a material. ρ = RA/L

Experiment to determine resistivity of a metal

1. Measure the diameter of the wire with a micrometer. Then calculate the cross-sectional area = 𝜋(d/2)2. 2. Set up the circuit as shown. 3. Vary the wire length and record the voltage and current for each length. 4. Use R=V/I to work out the resistance. 5. Plot a graph of resistance against wire length. 6. The gradient = resistivity ÷ cross-sectional area. 7. So resistivity = gradient x cross-sectional area.

Superconductor

A material that has a resistivity of zero at or below a critical temperature. The critical temperature is an inherent property of the material.

Total resistance in a series circuit

RTotal= R1 + R2 + R3 + ... Add the individual resistances of each component.

Voltage in parallel circuit with 6 cells

5V.

Voltage in series circuit with 6 cells

If 6 cells, each of voltage 5V, are arranged in series what voltage is provided to the circuit?

Voltage in series circuit

30V (5V x 6)

Current in series circuit

The current through all of the components is the same so the current does not vary.

Current in parallel components

No, each branch of a parallel circuit can have different currents through them according to Kirchhoff's first law.

Kirchhoff's first law

All of the current going into a junction is equal to the current leaving the junction.

Kirchhoff's second law

For any path (loop) of a circuit, the sum of all of the potential differences must equal to total emf of the circuit.

Emf in series circuit with two cells negative to negative

They will cancel out. The total emf would be equal to: ε total= ε1 - ε2.

Power

The rate of energy transfer.

Power equation

Power = Current x Voltage, P = IV

Purpose of a potential divider

To provide variable potential difference, or to provide a constant specific potential difference.

Voltage across 7Ω resistor in series circuit

Total resistance = 6 + 7 = 13Ω; (7/13) x 12 = 6.5V.

Emf

Electromotive force: the electrical energy transferred by a power supply per unit charge.

Rearranged equation of Ɛ = I(R + r)

Ɛ = IR + Ir; So for a graph of V against I, V = -rI + Ɛ; Gradient = -r and y-intercept = Ɛ.

Applications of superconductors

Power cables, which would reduce energy loss through heating to zero during transmission; Strong magnetic fields, which would not require a constant power source.

Maglev trains

Use strong magnetic fields from superconductors, where there would be no friction between the train and rail.

Total resistance in parallel arrangement

1 / RT = ⅓ + 1/7 + 1/9; 1 / RT = 37 / 63; RT= 63 / 37 = 1.7Ω.