Physics B.5 Current and Circuits

What is potential difference?

Work done per unit charge passing between two points and is measured in volts.

Current

The rate of flow of electric charge measured in amperes. 1A = 1Cs^-1. Average current = amount of charge transferred/time taken.

Resistance

A measure of the opposition a material exerts against the flow of electrons. Similar to friction. Electrons drift slowly through a conductor when a voltage is put across the ends. The metal’s atoms interfere with the motion of the electrons, causing resistance.

How can resistance be useful?

Resistance can be useful such as within a lamp where it causes the current to lose energy, heating the wire and making it glow.

Factors that can change resistance

1. temperature

2. cross-sectional area

3. type of material

4. length

Ohm’s law (not an actual law)

The current in an ohmic conductor is proportional to the voltage across it, provided that the temperature and other physical conditions are kept constant.

Voltage-current graphs

If a graph is plotted with voltage on the y axis and current on the x axis, it can be seen that the gradient is the resistance and the intercept is 0. When the axes are drawn the other way the gradient = 1/R.

Non-ohmic conductor

• The lamp heats up as the current increases which increases the resistance.

• gradient is not related to the resistance

• resistance is defined as the ratio of V to I, not the rate of change of V with I.

Relationship between resistance and area and length

If temperature is kept constant then the resistance is proportional to area and length. So there must be a constant of proportionality: resistivity.

Electromotive force

The emf of a cell is the work done per unit charge in moving charge completely around the circuit or the energy supplied per unit charge.

Discharge of a cell

When a cell is connected to a load (resistor, bulb, etc) the current begins flowing around the circuit so the cell is said to be discharging.

What does emf equal when no current is flowing

The emf is equal to the terminal potential difference.

Terminal potential difference

emf - lost volts (p.d. across the internal resistor)

Field

An area in which you feel a force as long as you have the correct attribute to interact with that field.

Power

The rate at which work is done or energy is transferred

Power in a cell

In a perfect cell, the power is the amount of chemical energy transferred to electrical energy per unit time. power delivered = emf x I.

In a real cell, the actual power delivered will be a bit lower, since there will be some power dissipated in the internal resistance.

Power dissipated

The power dissipated in the resistor is the amount of electrical energy transferred to thermal energy per unit time. P = VI

Current and voltage in series and parallel circuits

Series:

• current - same

• voltage - splits

Parallel

• current - splits

• voltage - same

Adding cells in series

Batteries with the same polarity:

V1 + V2

Batteries with opposite po;arity

If V1> V2 then V1-V2 (++)

If V2>V1 then V2-V1 (--)

Resistance in series vs parallel

Series: R_total= R1+R2+R3 + …

Parallel: 1/R_total=1/R1 + 1/R2 + 1/R3 +…

Potential divider circuits

Simple series circuits that we use to give us different values of voltage output. If we use variable resistors we can change the voltage output.

Light dependent resistor

When light shines on an LDR it releases electrons from their atoms

• brightness increases

• current increases

• resistance decreases

Thermistor

When the temperature of a thermistor increases it releases electrons from their atoms

• temperature increases

• current increases

• resistance decreases

Potentiometer

A device that can be used instead of a variable resistor. It does the same thing but can give 0V outputs.