Electric circuit

power, circuit elements and series-parallel connections

Electrical Power

• P = VQ/t = VI (Why ?)

• Unit: Watt (W) = VA

• kW, MW, GW

Electrical power on an ohmic device

• P = VI

• V = RI (Ohm’s Law)

• P = V2/R = I2R

Q1: On a light bulb, you see “60W, 120V”.

A) 60J electric energy is consumed every second by the light bulb at 120V;

B) 60J heat (invisible) is produced every second;

C) The radiation power (visible light) is 60W;

D) 60W is total radiation power (including visible light, infrared heat etc).

E) Both A) and D).

Q2

Two light bulbs. One is of “60W, 120V” and the other “100W, 120V”.

A) 60W light bulb has a bigger resistance;B) 60W light bulb has a smaller resistance;C) The resistance of 60W light bulb is equal to

that of 120W light bulb.

Rank in order, from largest to smallest, the powers Pa to Pd dissipated in resistors a to d.

1. Pb > Pa = Pc = Pd 2. Pb = Pc > Pa > Pc 3. Pb = Pd > Pa > Pc 4. Pb > Pc > Pa > Pd 5. Pb > Pd > Pa > Pc

Q3

Circuit elements

Elements with negligible resistance---Wires.

We assume that there is no potential difference across a wire.

Fuses and switch

Kirchhoff's rules

• At any junction point, the sum of all currents entering the junction must equal the sum of all currents leaving the junction.

( Junction law).

Along any loop, the sum of the potential difference of each component is equal to zero (Loop law).

Q4

Q5

As more resistors R are added to a series circuit, the current drawn from the battery

1. increases.

2. remains the same.

3. decreases.

R3R2R1

Combinations of resistors• Resistors in series:

• V = V1+V2+V3 = IR1+ IR2+ IR3

• Req = V/I = R1+ R2+ R3

V

R1 R2 R3

Terminal Voltage• Ideal battery: is the work is done by an

electrochemical reaction in order to separate charges.

• Terminal voltage Vab < due to internal resistance Ri in real batteries.

• The internal resistance is in series with the load resistance R, i.e., the resistance of the external electric circuit.

iab RIV

a b

R

Ri

battery

q

Wchem

Q6 A light bulb having a resistance R is

connected to a battery. If the light bulb is replaced with another light bulb having a larger resistance, the terminal voltage Vab of the battery

1. increases with increasing R

2. decreases with increasing R

3. remains the same.a b

R

Ri

Q7 As more resistors R are added to a

parallel circuit, the total current drawn from the battery

1. increases.

2. remains the same.

3. decreases. R3

R2

R1

I

I1

I2

I3

Combinations of resistors• Resistors in parallel

• I = I1+I2+I3 = V/ R1 + V/ R2+ V/R3

• I = V/Req

• 1/Req = 1/ R1+ 1/ R2+ 1/R3

R3

V

R2

R1

The equivalent resistance for a group of parallel resistors is

1. less than any resistor in the group.2. equal to the smallest resistance in the group.3. equal to the average resistance of the group.4. equal to the largest resistance in the group.5. larger than any resistor in the group.

Q8

...1111

321

RRRReq