Transcript revision electricity

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Electric Field – the space around a charge where electric forces can
be detected.
Field lines show the direction a positive charge would move in. The closer the
lines the stronger the force.
Volts per
division = 2 V
Volts per
division = 5 mV
Volts per
division = 20 V
Peak Voltage = 1.5 x 2
=3V
Peak voltage = 3 x 5
= 15 mV
( 0.015 V)
Peak voltage = 1.5 x 20
= 30 V
Electricity
Q = It
V = IR
Series
Parallel
I1 = I2 = I3
I = I + I2
VT = V1 + V2 + V3
V1 = V2 = V3
RT = R1 + R2 + R3
1 / RT = 1 / R1 + 1 / R2
Short cut if resistors are
identical, divide by the
number of resistors
E = Pt
P = IV
P = I2R
12 V
LED maximum
current 30 mA
and forward
voltage 3 V.
1. What would happen if the LED was the other way around?
2. Why is there a resistor in the circuit?
3. What size of resistor would you need if the LED is to work
correctly?
1
2
3
4
5
6
7
Energy = 2kJ
Time = 5 s
Power = ?
Current = 2 A
Voltage = 2V
Resistance = ?
Current = 15 A
Time = 120 s
Charge = ?
Voltage = 12 V
Current = 0.5 A
Resistance
Charge = 13 C
Time = 2 minutes
Current = ?
Current = 25 mA
Power = 1000W
Voltage = ?
Power = 1000 W
Time = 150 s
Energy = ?
10 
8. Find the resistance (a)
10 
X
20 Ω
Y
12 
10 
10 
4
(c)
(b)
(d)
Y
10 Ω
20 Ω
1. P = E÷ t = 2000 ÷ 5 = 400 W 2. R = V ÷ I = 2 ÷ 2 = 1 Ω
3. Q = It = 15 x 120 = 1800 C 4. R = V ÷ I = 12 ÷ 0.5 = 24 
5. I = Q ÷ t = 13 ÷ 120 = 0.11 A
6. V = P ÷ I = 1000 ÷ 0.025 = 40 000 V
7. E = Pt = 1000 x 150 = 150 000J
8. (a) Rt = R1 + R2 = 10 + 10 = 20 
(b) 10 ÷ 2 = 5 
(c) 1/RT = 1/R1 +1/R2 = ¼ + 1/12 = 0.25 + 0.08333= 0.333
RT = 1/0.333 = 3 
(d) 1/RT = 1/R1 +1/R2 = 1/20 + 1/10 = 0.05 + 0.1= 0.15
RT = 1/0.15 = 6.7 
add to series part 6.7 + 20 = 26.7 Ω
3
12 V
3
1.5
1.5
5
7
1. What is this meter measuring?
2. What is the current going through the bulb?
3. If both resistors are identical, what current is going through
each?
4. How would you measure the voltage across a bulb?
5. What would be the voltage across each resistor?
~
A
V

M
Activity 24 Light Controlled Switch to switch on street
lights when it gets dark
Block diagram
Block
diagram
Light
sensor
Transistor switch
LED
Circuit diagram
+5 V
0V
The variable resistor is used to set the brightness at which the
light is switched on.
+5 V
0V
Explanation
When light level
LDR .
, resistance LDR
The transistor conducts.
The LED lights.
, voltage across