P4.
3 Electric Circuits
Electric Circuits 3. Electric current is a flow of electrical
1. Circuit diagrams use standard symbols for charge.
components: 4. The size of the electric current is the rate
of flow of electrical charge.
2. For electrical charge to flow through a
5. Charge flow, current and time are linked
closed circuit the circuit must include a
by the equation:
source of potential difference.
𝑸 = 𝑰𝒕
Circuit Symbol Component Function Charge flow, Q, in Coulombs, C; current, I,
Name in amperes, A; time, t, in seconds, s.
Cell Push
charges
Series and Parallel Circuits
Battery around the
6. A current has the same value at any point
circuit.
Supplies in a single closed loop.
energy 7. Electrical components can be connected
Bulb/Lamp Lights up in series or in parallel.
8. When components are connected in
Ammeter Measures series, the current is the same through
current each component.
9. When components are connected in
Voltmeter Measures series, the potential difference of the
potential
power supply is shared between
difference
components.
Open Breaks the
switch circuit 10. When components are connected in
parallel, the potential difference across
Closed Completes
switch the circuit each branch is the same.
11. When components are connected in
Fixed Adds parallel, the total current through the
Resistor resistance to whole circuit is the sum of the currents
a circuit through the separate branches.
Variable Can be 12. The current through a component
Resistor used to vary depends on the resistance of the
current component and the potential difference
Diode Allows across the component.
current to 13. The greater the resistance of the
flow in one component the smaller the current for a
direction
given potential difference across the
only
component.
Light- Gives out
emitting light when 14. Current, potential difference or resistance
diode (LED) current flows can be calculated using the equation:
through in 𝑽 = 𝑰𝑹
one Potential difference, V, in Volts, V;
direction current, I, in amperes, A; resistance, R, in
Thermistor Resistance Ohms, Ω.
decreases 15. Voltmeters must be connected in
with higher parallel and ammeters must be
temperature connected in series.
Light- Resistance
dependent decreases
resistor with higher
(LDR) light
intensity
P4.3 Electric Circuits
16. When components are connected in 27. Power is the rate at which energy is
series, the total resistance of two transferred or work is done.
components is the sum of the resistance 28. When charge flows through a circuit, work
of each component: is done.
29. The power transfer in any circuit device is
𝑹𝒕𝒐𝒕𝒂𝒍 = 𝑹𝟏 + 𝑹𝟐
related to the potential difference across
it and the current through it, and to the
17. When components are connected in
energy changes over time.
parallel, the total resistance of two
resistors is less than the resistance of the 𝑷 = 𝑽𝑰
smallest individual resistor. Power, P, in watts, W; potential
18. Adding resistors in series increases the difference, V, in volts, V; current, I, in
total resistance because the current amperes, A.
decreases but adding resistors in parallel 𝑷 = 𝑰𝟐 𝑹
decreases the total resistance because Power, P, in watts, W; current, I, in
the current increases as there is another amperes, A; resistance, R, in ohms, Ω
branch.
Mains Electricity and the National Grid
Resistance of Components
30. Direct current (d.c.) travels in one
19. For some resistors (fixed resistors), the
direction only.
value of R remains constant but that in
31. Cells and batteries supply direct current.
others it can change as the current
32. Alternating current (a.c.) continually
changes.
reverses direction.
20. The current through an ohmic conductor
33. Mains electricity supplies alternating
(at a constant temperature) is directly
current.
proportional to the potential difference
34. Mains A.C. has a potential difference of
across the resistor. This means that the
resistance remains constant as the current 230 V and a frequency of 50 Hz.
changes. 35. The National Grid is a system of cables,
21. The resistance of components such as pylons and transformers linking power
lamps, diodes, thermistors and LDRs is not stations to consumers.
36. Electrical power is transferred from power
constant; it changes with the current
through the component. stations to consumers using the National
Grid.
22. The resistance of a filament lamp
37. Step-up transformers are used to increase
increases as the temperature of the
filament increases. the potential difference from the power
station to the transmission cables to
23. The current through a diode flows in one
direction only. The diode has a very high increase the efficiency of transfer
38. Step-down transformers are used to
resistance in the reverse direction.
24. The resistance of a thermistor decreases decrease, to a much lower value, the
as the temperature increases. potential difference for domestic use, so it
25. There are a number of applications of is safe for consumers
thermistors in circuits e.g. a thermostat 39. Power is conserved in transformers.
26. The resistance of an LDR decreases as 40. (HT) 𝑽𝟏 𝑰𝟏 = 𝑽𝟐 𝑰𝟐
light intensity increases.
Electrical Power
