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Series circuits

You should know the characteristics of the current and potential difference in series circuits.

Current

When two or more components are connected in series, the same current flows through each component.

Potential difference

When two or more components are connected in series, the total potential difference of the supply is shared between them. This means that if you add together the voltages across each component connected in series, the total equals the voltage of the power supply.

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You should know what happens to the potential difference and current when the number of cells in a circuit is changed.

Potential difference

A typical cell produces a potential difference of 1.5 V. When two or more cells are connected in series in a circuit, the total potential difference is the sum of their potential differences. For example, if two 1.5 V cells are connected in series in the same direction, the total potential difference is 3.0 V. If two 1.5V cells are connected in series, but in opposite directions, the total potential difference is 0V, so no current will flow.

Current

When more cells are connected in series in a circuit, they produce a bigger potential difference across its components. More current flows through the components as a result.

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A potential difference, also called voltage, across an electrical component is needed to make a current flow through it. Cells or batteries often provide the potential difference needed.

Measuring potential difference

Potential difference is measured in volts, V. The potential difference across a component in a circuit is measured using a voltmeter. This must be connected in parallel with the component.

Calculating potential difference

The potential difference between two points in an electric circuit is the work done when a coulomb of charge passes between the points. You can calculate the size of a potential difference using this equation:

V = W ÷ Q

V is the potential difference in volts, V

W is the work done (energy transferred) in joules, J

Q is the charge in coulombs, C

For example, what is the potential difference if 48 J of energy is transferred when 4 C of charge passes?

Potential difference = 48 ÷ 4 = 12 V

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Electric current is a flow of electric charge. No current can flow if the circuit is broken - for example, when a switch is open.

Measuring current

Current is measured in amperes (which is often abbreviated to amps or A). The current flowing through a component in a circuit is measured using an ammeter. This must be connected in series with the component.

Calculating current

The size of an electric current is the rate of flow of electric charge. You can calculate the size of a current using this equation:

I = Q ÷ t

I is the current in amperes (amps), A

Q is the charge in coulombs, C

t is the time in seconds, s

For example, what is the current if 20 C of charge passes in 5 s?

Current = 20 ÷ 5 = 4 A

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Circuit diagrams

Two things are important for a circuit to work:

There must be a complete circuit

There must be no short circuits

To check for a complete circuit, follow a wire coming out of the battery with your finger. You should be able to go out of the battery, through the lamp, and back to the battery.

To check for a short circuit, see if you can find a way past the lamp without going through any other component. If you can, there is a short circuit and the lamp will not light.

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Electrical circuits can be represented by circuit diagrams. The various electrical components are shown by using standard symbols in circuit diagrams. Components can be connected in series, or in parallel. The characteristics of the current and potential difference (voltage) are different in series and parallel circuits.

Circuit symbols

You need to be able to draw and interpret circuit diagrams.

Standard symbols

The diagram below shows the standard circuit symbols you need to know.

Open Switch | Closed Switch | Lamp | Cell
Battery | Voltmeter | Resistor | Fuse
Ammeter | Variable resistor | Thermistor | Light dependent resistor (LDR)
Light emitting diode (LED) | Diode

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Attraction and repulsion

You should know how and why insulators can be electrically charged.

Moving charges

When you rub two different insulating materials against each other they become electrically charged. This only works for insulated objects. Electrical charges move easily to earth through conductors such as metals.

When the materials are rubbed against each other:

Negatively charged particles called electrons move from one material to the other

The material that loses electrons becomes positively charged

The material that gains electrons becomes negatively charged

Both materials gain an equal amount of charge, but the charges are opposite

Detecting charge

Two charged objects will repel each other if they have the same type of charge (they areboth positive or both negative). Two charged objects will attract each other if they have opposite charges.

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You should know the difference between series and parallel connections in circuits.

Series connections

Components that are connected one after another on the same loop of the circuit are connected in series. The current that flows across each component connected in series is the same.

Two lamps connected in series

The circuit diagram shows a circuit with two lamps connected in series. If one lamp breaks, the other lamp will not light.

Parallel connections

Components that are connected on separate loops are connected in parallel. The current is shared between each component connected in parallel.

Two lamps connected in parallel

The circuit diagram shows a circuit with two lamps connected in parallel. If one lamp breaks, the other lamp will still li

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